keyctl

title: keyctl(2)

text:
# keyctl(2)

## NAME
keyctl - manipulate the kernel's key management facility

## SYNOPSIS

```
#include <linux/keyctl.h>     /* Definition of KEY* constants */
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>
```

```
long syscall(SYS_keyctl, int operation, unsigned long arg2,
             unsigned long arg3, unsigned long arg4,
             unsigned long arg5);
```

Note: glibc provides no wrapper for keyctl(), necessitating the use of syscall(2).

## DESCRIPTION
keyctl() allows user-space programs to perform key manipulation.

The operation performed by keyctl() is determined by the value of the operation argument. Each of these operations is wrapped by the libkeyutils library (provided by the keyutils package) into individual functions (noted below) to permit the compiler to check types.

The permitted values for operation are:

KEYCTL_GET_KEYRING_ID (since Linux 2.6.10)

Map a special key ID to a real key ID for this process.

This operation looks up the special key whose ID is provided in arg2 (cast to key_serial_t). If the special key is found, the ID of the corresponding real key is returned as the function result. The following values may be specified in arg2:

KEY_SPEC_THREAD_KEYRING

This specifies the calling thread's thread-specific keyring. See thread-keyring(7).

KEY_SPEC_PROCESS_KEYRING

This specifies the caller's process-specific keyring. See process-keyring(7).

KEY_SPEC_SESSION_KEYRING

This specifies the caller's session-specific keyring. See session-keyring(7).

KEY_SPEC_USER_KEYRING

This specifies the caller's UID-specific keyring. See user-keyring(7).

KEY_SPEC_USER_SESSION_KEYRING

This specifies the caller's UID-session keyring. See user-session-keyring(7).

KEY_SPEC_REQKEY_AUTH_KEY (since Linux 2.6.16)

This specifies the authorization key created by request_key(2) and passed to the process it spawns to generate a key. This key is available only in a request-key(8)-style program that was passed an authorization key by the kernel and ceases to be available once the requested key has been instantiated; see request_key(2).

KEY_SPEC_REQUESTOR_KEYRING (since Linux 2.6.29)

This specifies the key ID for the request_key(2) destination keyring. This keyring is available only in a request-key(8)-style program that was passed an authorization key by the kernel and ceases to be available once the requested key has been instantiated; see request_key(2).

The behavior if the key specified in arg2 does not exist depends on the value of arg3 (cast to int). If arg3 contains a nonzero value, then—if it is appropriate to do so (e.g., when looking up the user, user-session, or session key)—a new key is created and its real key ID returned as the function result. Otherwise, the operation fails with the error ENOKEY.

If a valid key ID is specified in arg2, and the key exists, then this operation simply returns the key ID. If the key does not exist, the call fails with error ENOKEY.

The caller must have search permission on a keyring in order for it to be found.

The arguments arg4 and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_get_keyring_ID(3).

KEYCTL_JOIN_SESSION_KEYRING (since Linux 2.6.10)

Replace the session keyring this process subscribes to with a new session keyring.

If arg2 is NULL, an anonymous keyring with the description "_ses" is created and the process is subscribed to that keyring as its session keyring, displacing the previous session keyring.

Otherwise, arg2 (cast to char *) is treated as the description (name) of a keyring, and the behavior is as follows:

- If a keyring with a matching description exists, the process will attempt to subscribe to that keyring as its session keyring if possible; if that is not possible, an error is returned. In order to subscribe to the keyring, the caller must have search permission on the keyring.

- If a keyring with a matching description does not exist, then a new keyring with the specified description is created, and the process is subscribed to that keyring as its session keyring.

The arguments arg3, arg4, and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_join_session_keyring(3).

KEYCTL_UPDATE (since Linux 2.6.10)

Update a key's data payload.

The arg2 argument (cast to key_serial_t) specifies the ID of the key to be updated. The arg3 argument (cast to void *) points to the new payload and arg4 (cast to size_t) contains the new payload size in bytes.

The caller must have write permission on the key specified and the key type must support updating.

A negatively instantiated key (see the description of KEYCTL_REJECT) can be positively instantiated with this operation.

The arg5 argument is ignored.

This operation is exposed by libkeyutils via the function keyctl_update(3).

KEYCTL_REVOKE (since Linux 2.6.10)

Revoke the key with the ID provided in arg2 (cast to key_serial_t). The key is scheduled for garbage collection; it will no longer be findable, and will be unavailable for further operations. Further attempts to use the key will fail with the error EKEYREVOKED.

The caller must have write or setattr permission on the key.

The arguments arg3, arg4, and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_revoke(3).

KEYCTL_CHOWN (since Linux 2.6.10)

Change the ownership (user and group ID) of a key.

The arg2 argument (cast to key_serial_t) contains the key ID. The arg3 argument (cast to uid_t) contains the new user ID (or -1 in case the user ID shouldn't be changed). The arg4 argument (cast to gid_t) contains the new group ID (or -1 in case the group ID shouldn't be changed).

The key must grant the caller setattr permission.

For the UID to be changed, or for the GID to be changed to a group the caller is not a member of, the caller must have the CAP_SYS_ADMIN capability (see capabilities(7)).

If the UID is to be changed, the new user must have sufficient quota to accept the key. The quota deduction will be removed from the old user to the new user should the UID be changed.

The arg5 argument is ignored.

This operation is exposed by libkeyutils via the function keyctl_chown(3).

KEYCTL_SETPERM (since Linux 2.6.10)

Change the permissions of the key with the ID provided in the arg2 argument (cast to key_serial_t) to the permissions provided in the arg3 argument (cast to key_perm_t).

If the caller doesn't have the CAP_SYS_ADMIN capability, it can change permissions only for the keys it owns. (More precisely: the caller's filesystem UID must match the UID of the key.)

The key must grant setattr permission to the caller regardless of the caller's capabilities.

The permissions in arg3 specify masks of available operations for each of the following user categories:

possessor (since Linux 2.6.14)

This is the permission granted to a process that possesses the key (has it attached searchably to one of the process's keyrings); see keyrings(7).

user

This is the permission granted to a process whose filesystem UID matches the UID of the key.

group

This is the permission granted to a process whose filesystem GID or any of its supplementary GIDs matches the GID of the key.

other

This is the permission granted to other processes that do not match the user and group categories.

The user, group, and other categories are exclusive: if a process matches the user category, it will not receive permissions granted in the group category; if a process matches the user or group category, then it will not receive permissions granted in the other category.

The possessor category grants permissions that are cumulative with the grants from the user, group, or other category.

Each permission mask is eight bits in size, with only six bits currently used. The available permissions are:

view

This permission allows reading attributes of a key.

This permission is required for the KEYCTL_DESCRIBE operation.

The permission bits for each category are KEY_POS_VIEW, KEY_USR_VIEW, KEY_GRP_VIEW, and KEY_OTH_VIEW.

read

This permission allows reading a key's payload.

This permission is required for the KEYCTL_READ operation.

The permission bits for each category are KEY_POS_READ, KEY_USR_READ, KEY_GRP_READ, and KEY_OTH_READ.

write

This permission allows update or instantiation of a key's payload. For a keyring, it allows keys to be linked and unlinked from the keyring,

This permission is required for the KEYCTL_UPDATE, KEYCTL_REVOKE, KEYCTL_CLEAR, KEYCTL_LINK, and KEYCTL_UNLINK operations.

The permission bits for each category are KEY_POS_WRITE, KEY_USR_WRITE, KEY_GRP_WRITE, and KEY_OTH_WRITE.

search

This permission allows keyrings to be searched and keys to be found. Searches can recurse only into nested keyrings that have search permission set.

This permission is required for the KEYCTL_GET_KEYRING_ID, KEYCTL_JOIN_SESSION_KEYRING, KEYCTL_SEARCH, and KEYCTL_INVALIDATE operations.

The permission bits for each category are KEY_POS_SEARCH, KEY_USR_SEARCH, KEY_GRP_SEARCH, and KEY_OTH_SEARCH.

link

This permission allows a key or keyring to be linked to.

This permission is required for the KEYCTL_LINK and KEYCTL_SESSION_TO_PARENT operations.

The permission bits for each category are KEY_POS_LINK, KEY_USR_LINK, KEY_GRP_LINK, and KEY_OTH_LINK.

setattr (since Linux 2.6.15).

This permission allows a key's UID, GID, and permissions mask to be changed.

This permission is required for the KEYCTL_REVOKE, KEYCTL_CHOWN, and KEYCTL_SETPERM operations.

The permission bits for each category are KEY_POS_SETATTR, KEY_USR_SETATTR, KEY_GRP_SETATTR, and KEY_OTH_SETATTR.

As a convenience, the following macros are defined as masks for all of the permission bits in each of the user categories: KEY_POS_ALL, KEY_USR_ALL, KEY_GRP_ALL, and KEY_OTH_ALL.

The arg4 and arg5 arguments are ignored.

This operation is exposed by libkeyutils via the function keyctl_setperm(3).

KEYCTL_DESCRIBE (since Linux 2.6.10)

Obtain a string describing the attributes of a specified key.

The ID of the key to be described is specified in arg2 (cast to key_serial_t). The descriptive string is returned in the buffer pointed to by arg3 (cast to char *); arg4 (cast to size_t) specifies the size of that buffer in bytes.

The key must grant the caller view permission.

The returned string is null-terminated and contains the following information about the key:

type;uid;gid;perm;description

In the above, type and description are strings, uid and gid are decimal strings, and perm is a hexadecimal permissions mask. The descriptive string is written with the following format:

%s;%d;%d;%08x;%s

Note: the intention is that the descriptive string should be extensible in future kernel versions. In particular, the description field will not contain semicolons; it should be parsed by working backwards from the end of the string to find the last semicolon. This allows future semicolon-delimited fields to be inserted in the descriptive string in the future.

Writing to the buffer is attempted only when arg3 is non-NULL and the specified buffer size is large enough to accept the descriptive string (including the terminating null byte). In order to determine whether the buffer size was too small, check to see if the return value of the operation is greater than arg4.

The arg5 argument is ignored.

This operation is exposed by libkeyutils via the function keyctl_describe(3).

KEYCTL_CLEAR

Clear the contents of (i.e., unlink all keys from) a keyring.

The ID of the key (which must be of keyring type) is provided in arg2 (cast to key_serial_t).

The caller must have write permission on the keyring.

The arguments arg3, arg4, and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_clear(3).

KEYCTL_LINK (since Linux 2.6.10)

Create a link from a keyring to a key.

The key to be linked is specified in arg2 (cast to key_serial_t); the keyring is specified in arg3 (cast to key_serial_t).

If a key with the same type and description is already linked in the keyring, then that key is displaced from the keyring.

Before creating the link, the kernel checks the nesting of the keyrings and returns appropriate errors if the link would produce a cycle or if the nesting of keyrings would be too deep (The limit on the nesting of keyrings is determined by the kernel constant KEYRING_SEARCH_MAX_DEPTH, defined with the value 6, and is necessary to prevent overflows on the kernel stack when recursively searching keyrings).

The caller must have link permission on the key being added and write permission on the keyring.

The arguments arg4 and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_link(3).

KEYCTL_UNLINK (since Linux 2.6.10)

Unlink a key from a keyring.

The ID of the key to be unlinked is specified in arg2 (cast to key_serial_t); the ID of the keyring from which it is to be unlinked is specified in arg3 (cast to key_serial_t).

If the key is not currently linked into the keyring, an error results.

The caller must have write permission on the keyring from which the key is being removed.

If the last link to a key is removed, then that key will be scheduled for destruction.

The arguments arg4 and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_unlink(3).

KEYCTL_SEARCH (since Linux 2.6.10)

Search for a key in a keyring tree, returning its ID and optionally linking it to a specified keyring.

The tree to be searched is specified by passing the ID of the head keyring in arg2 (cast to key_serial_t). The search is performed breadth-first and recursively.

The arg3 and arg4 arguments specify the key to be searched for: arg3 (cast as char *) contains the key type (a null-terminated character string up to 32 bytes in size, including the terminating null byte), and arg4 (cast as char *) contains the description of the key (a null-terminated character string up to 4096 bytes in size, including the terminating null byte).

The source keyring must grant search permission to the caller. When performing the recursive search, only keyrings that grant the caller search permission will be searched. Only keys with for which the caller has search permission can be found.

If the key is found, its ID is returned as the function result.

If the key is found and arg5 (cast to key_serial_t) is nonzero, then, subject to the same constraints and rules as KEYCTL_LINK, the key is linked into the keyring whose ID is specified in arg5. If the destination keyring specified in arg5 already contains a link to a key that has the same type and description, then that link will be displaced by a link to the key found by this operation.

Instead of valid existing keyring IDs, the source (arg2) and destination (arg5) keyrings can be one of the special keyring IDs listed under KEYCTL_GET_KEYRING_ID.

This operation is exposed by libkeyutils via the function keyctl_search(3).

KEYCTL_READ (since Linux 2.6.10)

Read the payload data of a key.

The ID of the key whose payload is to be read is specified in arg2 (cast to key_serial_t). This can be the ID of an existing key, or any of the special key IDs listed for KEYCTL_GET_KEYRING_ID.

The payload is placed in the buffer pointed by arg3 (cast to char *); the size of that buffer must be specified in arg4 (cast to size_t).

The returned data will be processed for presentation according to the key type. For example, a keyring will return an array of key_serial_t entries representing the IDs of all the keys that are linked to it. The user key type will return its data as is. If a key type does not implement this function, the operation fails with the error EOPNOTSUPP.

If arg3 is not NULL, as much of the payload data as will fit is copied into the buffer. On a successful return, the return value is always the total size of the payload data. To determine whether the buffer was of sufficient size, check to see that the return value is less than or equal to the value supplied in arg4.

The key must either grant the caller read permission, or grant the caller search permission when searched for from the process keyrings (i.e., the key is possessed).

The arg5 argument is ignored.

This operation is exposed by libkeyutils via the function keyctl_read(3).

KEYCTL_INSTANTIATE (since Linux 2.6.10)

(Positively) instantiate an uninstantiated key with a specified payload.

The ID of the key to be instantiated is provided in arg2 (cast to key_serial_t).

The key payload is specified in the buffer pointed to by arg3 (cast to void *); the size of that buffer is specified in arg4 (cast to size_t).

The payload may be a NULL pointer and the buffer size may be 0 if this is supported by the key type (e.g., it is a keyring).

The operation may be fail if the payload data is in the wrong format or is otherwise invalid.

If arg5 (cast to key_serial_t) is nonzero, then, subject to the same constraints and rules as KEYCTL_LINK, the instantiated key is linked into the keyring whose ID specified in arg5.

The caller must have the appropriate authorization key, and once the uninstantiated key has been instantiated, the authorization key is revoked. In other words, this operation is available only from a request-key(8)-style program. See request_key(2) for an explanation of uninstantiated keys and key instantiation.

This operation is exposed by libkeyutils via the function keyctl_instantiate(3).

KEYCTL_NEGATE (since Linux 2.6.10)

Negatively instantiate an uninstantiated key.

This operation is equivalent to the call:

keyctl(KEYCTL_REJECT, arg2, arg3, ENOKEY, arg4);

The arg5 argument is ignored.

This operation is exposed by libkeyutils via the function keyctl_negate(3).

KEYCTL_SET_REQKEY_KEYRING (since Linux 2.6.13)

Set the default keyring to which implicitly requested keys will be linked for this thread, and return the previous setting. Implicit key requests are those made by internal kernel components, such as can occur when, for example, opening files on an AFS or NFS filesystem. Setting the default keyring also has an effect when requesting a key from user space; see request_key(2) for details.

The arg2 argument (cast to int) should contain one of the following values, to specify the new default keyring:

KEY_REQKEY_DEFL_NO_CHANGE

Don't change the default keyring. This can be used to discover the current default keyring (without changing it).

KEY_REQKEY_DEFL_DEFAULT

This selects the default behaviour, which is to use the thread-specific keyring if there is one, otherwise the process-specific keyring if there is one, otherwise the session keyring if there is one, otherwise the UID-specific session keyring, otherwise the user-specific keyring.

KEY_REQKEY_DEFL_THREAD_KEYRING

Use the thread-specific keyring (thread-keyring(7)) as the new default keyring.

KEY_REQKEY_DEFL_PROCESS_KEYRING

Use the process-specific keyring (process-keyring(7)) as the new default keyring.

KEY_REQKEY_DEFL_SESSION_KEYRING

Use the session-specific keyring (session-keyring(7)) as the new default keyring.

KEY_REQKEY_DEFL_USER_KEYRING

Use the UID-specific keyring (user-keyring(7)) as the new default keyring.

KEY_REQKEY_DEFL_USER_SESSION_KEYRING

Use the UID-specific session keyring (user-session-keyring(7)) as the new default keyring.

KEY_REQKEY_DEFL_REQUESTOR_KEYRING (since Linux 2.6.29)

Use the requestor keyring.

All other values are invalid.

The arguments arg3, arg4, and arg5 are ignored.

The setting controlled by this operation is inherited by the child of fork(2) and preserved across execve(2).

This operation is exposed by libkeyutils via the function keyctl_set_reqkey_keyring(3).

KEYCTL_SET_TIMEOUT (since Linux 2.6.16)

Set a timeout on a key.

The ID of the key is specified in arg2 (cast to key_serial_t). The timeout value, in seconds from the current time, is specified in arg3 (cast to unsigned int). The timeout is measured against the realtime clock.

Specifying the timeout value as 0 clears any existing timeout on the key.

The /proc/keys file displays the remaining time until each key will expire. (This is the only method of discovering the timeout on a key.)

The caller must either have the setattr permission on the key or hold an instantiation authorization token for the key (see request_key(2)).

The key and any links to the key will be automatically garbage collected after the timeout expires. Subsequent attempts to access the key will then fail with the error EKEYEXPIRED.

This operation cannot be used to set timeouts on revoked, expired, or negatively instantiated keys.

The arguments arg4 and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_set_timeout(3).

KEYCTL_ASSUME_AUTHORITY (since Linux 2.6.16)

Assume (or divest) the authority for the calling thread to instantiate a key.

The arg2 argument (cast to key_serial_t) specifies either a nonzero key ID to assume authority, or the value 0 to divest authority.

If arg2 is nonzero, then it specifies the ID of an uninstantiated key for which authority is to be assumed. That key can then be instantiated using one of KEYCTL_INSTANTIATE, KEYCTL_INSTANTIATE_IOV, KEYCTL_REJECT, or KEYCTL_NEGATE. Once the key has been instantiated, the thread is automatically divested of authority to instantiate the key.

Authority over a key can be assumed only if the calling thread has present in its keyrings the authorization key that is associated with the specified key. (In other words, the KEYCTL_ASSUME_AUTHORITY operation is available only from a request-key(8)-style program; see request_key(2) for an explanation of how this operation is used.) The caller must have search permission on the authorization key.

If the specified key has a matching authorization key, then the ID of that key is returned. The authorization key can be read (KEYCTL_READ) to obtain the callout information passed to request_key(2).

If the ID given in arg2 is 0, then the currently assumed authority is cleared (divested), and the value 0 is returned.

The KEYCTL_ASSUME_AUTHORITY mechanism allows a program such as request-key(8) to assume the necessary authority to instantiate a new uninstantiated key that was created as a consequence of a call to request_key(2). For further information, see request_key(2) and the kernel source file Documentation/security/keys-request-key.txt.

The arguments arg3, arg4, and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_assume_authority(3).

KEYCTL_GET_SECURITY (since Linux 2.6.26)

Get the LSM (Linux Security Module) security label of the specified key.

The ID of the key whose security label is to be fetched is specified in arg2 (cast to key_serial_t). The security label (terminated by a null byte) will be placed in the buffer pointed to by arg3 argument (cast to char *); the size of the buffer must be provided in arg4 (cast to size_t).

If arg3 is specified as NULL or the buffer size specified in arg4 is too small, the full size of the security label string (including the terminating null byte) is returned as the function result, and nothing is copied to the buffer.

The caller must have view permission on the specified key.

The returned security label string will be rendered in a form appropriate to the LSM in force. For example, with SELinux, it may look like:

unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

If no LSM is currently in force, then an empty string is placed in the buffer.

The arg5 argument is ignored.

This operation is exposed by libkeyutils via the functions keyctl_get_security(3) and keyctl_get_security_alloc(3).

KEYCTL_SESSION_TO_PARENT (since Linux 2.6.32)

Replace the session keyring to which the parent of the calling process subscribes with the session keyring of the calling process.

The keyring will be replaced in the parent process at the point where the parent next transitions from kernel space to user space.

The keyring must exist and must grant the caller link permission. The parent process must be single-threaded and have the same effective ownership as this process and must not be set-user-ID or set-group-ID. The UID of the parent process's existing session keyring (f it has one), as well as the UID of the caller's session keyring much match the caller's effective UID.

The fact that it is the parent process that is affected by this operation allows a program such as the shell to start a child process that uses this operation to change the shell's session keyring. (This is what the keyctl(1) new_session command does.)

The arguments arg2, arg3, arg4, and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_session_to_parent(3).

KEYCTL_REJECT (since Linux 2.6.39)

Mark a key as negatively instantiated and set an expiration timer on the key. This operation provides a superset of the functionality of the earlier KEYCTL_NEGATE operation.

The ID of the key that is to be negatively instantiated is specified in arg2 (cast to key_serial_t). The arg3 (cast to unsigned int) argument specifies the lifetime of the key, in seconds. The arg4 argument (cast to unsigned int) specifies the error to be returned when a search hits this key; typically, this is one of EKEYREJECTED, EKEYREVOKED, or EKEYEXPIRED.

If arg5 (cast to key_serial_t) is nonzero, then, subject to the same constraints and rules as KEYCTL_LINK, the negatively instantiated key is linked into the keyring whose ID is specified in arg5.

The caller must have the appropriate authorization key. In other words, this operation is available only from a request-key(8)-style program. See request_key(2).

The caller must have the appropriate authorization key, and once the uninstantiated key has been instantiated, the authorization key is revoked. In other words, this operation is available only from a request-key(8)-style program. See request_key(2) for an explanation of uninstantiated keys and key instantiation.

This operation is exposed by libkeyutils via the function keyctl_reject(3).

KEYCTL_INSTANTIATE_IOV (since Linux 2.6.39)

Instantiate an uninstantiated key with a payload specified via a vector of buffers.

This operation is the same as KEYCTL_INSTANTIATE, but the payload data is specified as an array of iovec structures:

```
struct iovec {

    void  *iov_base;    /* Starting address of buffer */

    size_t iov_len;     /* Size of buffer (in bytes) */
};

```

The pointer to the payload vector is specified in arg3 (cast as const struct iovec *). The number of items in the vector is specified in arg4 (cast as unsigned int).

The arg2 (key ID) and arg5 (keyring ID) are interpreted as for KEYCTL_INSTANTIATE.

This operation is exposed by libkeyutils via the function keyctl_instantiate_iov(3).

KEYCTL_INVALIDATE (since Linux 3.5)

Mark a key as invalid.

The ID of the key to be invalidated is specified in arg2 (cast to key_serial_t).

To invalidate a key, the caller must have search permission on the key.

This operation marks the key as invalid and schedules immediate garbage collection. The garbage collector removes the invalidated key from all keyrings and deletes the key when its reference count reaches zero. After this operation, the key will be ignored by all searches, even if it is not yet deleted.

Keys that are marked invalid become invisible to normal key operations immediately, though they are still visible in /proc/keys (marked with an 'i' flag) until they are actually removed.

The arguments arg3, arg4, and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_invalidate(3).

KEYCTL_GET_PERSISTENT (since Linux 3.13)

Get the persistent keyring (persistent-keyring(7)) for a specified user and link it to a specified keyring.

The user ID is specified in arg2 (cast to uid_t). If the value -1 is specified, the caller's real user ID is used. The ID of the destination keyring is specified in arg3 (cast to key_serial_t).

The caller must have the CAP_SETUID capability in its user namespace in order to fetch the persistent keyring for a user ID that does not match either the real or effective user ID of the caller.

If the call is successful, a link to the persistent keyring is added to the keyring whose ID was specified in arg3.

The caller must have write permission on the keyring.

The persistent keyring will be created by the kernel if it does not yet exist.

Each time the KEYCTL_GET_PERSISTENT operation is performed, the persistent keyring will have its expiration timeout reset to the value in:

```
/proc/sys/kernel/keys/persistent_keyring_expiry

```

Should the timeout be reached, the persistent keyring will be removed and everything it pins can then be garbage collected.

Persistent keyrings were added to Linux in kernel version 3.13.

The arguments arg4 and arg5 are ignored.

This operation is exposed by libkeyutils via the function keyctl_get_persistent(3).

KEYCTL_DH_COMPUTE (since Linux 4.7)

Compute a Diffie-Hellman shared secret or public key, optionally applying key derivation function (KDF) to the result.

The arg2 argument is a pointer to a set of parameters containing serial numbers for three "user" keys used in the Diffie-Hellman calculation, packaged in a structure of the following form:

```
struct keyctl_dh_params {

    int32_t private; /* The local private key */

    int32_t prime; /* The prime, known to both parties */

    int32_t base;  /* The base integer: either a shared

                      generator or the remote public key */
};

```

Each of the three keys specified in this structure must grant the caller read permission. The payloads of these keys are used to calculate the Diffie-Hellman result as:

base ^ private mod prime

If the base is the shared generator, the result is the local public key. If the base is the remote public key, the result is the shared secret.

The arg3 argument (cast to char *) points to a buffer where the result of the calculation is placed. The size of that buffer is specified in arg4 (cast to size_t).

The buffer must be large enough to accommodate the output data, otherwise an error is returned. If arg4 is specified zero, in which case the buffer is not used and the operation returns the minimum required buffer size (i.e., the length of the prime).

Diffie-Hellman computations can be performed in user space, but require a multiple-precision integer (MPI) library. Moving the implementation into the kernel gives access to the kernel MPI implementation, and allows access to secure or acceleration hardware.

Adding support for DH computation to the keyctl() system call was considered a good fit due to the DH algorithm's use for deriving shared keys; it also allows the type of the key to determine which DH implementation (software or hardware) is appropriate.

If the arg5 argument is NULL, then the DH result itself is returned. Otherwise (since Linux 4.12), it is a pointer to a structure which specifies parameters of the KDF operation to be applied:

```
struct keyctl_kdf_params {

    char *hashname;     /* Hash algorithm name */

    char *otherinfo;    /* SP800-56A OtherInfo */

    __u32 otherinfolen; /* Length of otherinfo data */

    __u32 __spare[8];   /* Reserved */
};

```

The hashname field is a null-terminated string which specifies a hash name (available in the kernel's crypto API; the list of the hashes available is rather tricky to observe; please refer to the "Kernel Crypto API Architecture" documentation for the information regarding how hash names are constructed and your kernel's source and configuration regarding what ciphers and templates with type CRYPTO_ALG_TYPE_SHASH are available) to be applied to DH result in KDF operation.

The otherinfo field is an OtherInfo data as described in SP800-56A section 5.8.1.2 and is algorithm-specific. This data is concatenated with the result of DH operation and is provided as an input to the KDF operation. Its size is provided in the otherinfolen field and is limited by KEYCTL_KDF_MAX_OI_LEN constant that defined in security/keys/internal.h to a value of 64.

The __spare field is currently unused. It was ignored until Linux 4.13 (but still should be user-addressable since it is copied to the kernel), and should contain zeros since Linux 4.13.

The KDF implementation complies with SP800-56A as well as with SP800-108 (the counter KDF).

This operation is exposed by libkeyutils (from version 1.5.10 onwards) via the functions keyctl_dh_compute(3) and keyctl_dh_compute_alloc(3).

KEYCTL_RESTRICT_KEYRING (since Linux 4.12)

Apply a key-linking restriction to the keyring with the ID provided in arg2 (cast to key_serial_t). The caller must have setattr permission on the key. If arg3 is NULL, any attempt to add a key to the keyring is blocked; otherwise it contains a pointer to a string with a key type name and arg4 contains a pointer to string that describes the type-specific restriction. As of Linux 4.12, only the type "asymmetric" has restrictions defined:

builtin_trusted

Allows only keys that are signed by a key linked to the built-in keyring (".builtin_trusted_keys").

builtin_and_secondary_trusted

Allows only keys that are signed by a key linked to the secondary keyring (".secondary_trusted_keys") or, by extension, a key in a built-in keyring, as the latter is linked to the former.

key_or_keyring:key

key_or_keyring:key:chain

If key specifies the ID of a key of type "asymmetric", then only keys that are signed by this key are allowed.

If key specifies the ID of a keyring, then only keys that are signed by a key linked to this keyring are allowed.

If ":chain" is specified, keys that are signed by a keys linked to the destination keyring (that is, the keyring with the ID specified in the arg2 argument) are also allowed.

Note that a restriction can be configured only once for the specified keyring; once a restriction is set, it can't be overridden.

The argument arg5 is ignored.

## RETURN VALUE
For a successful call, the return value depends on the operation:

KEYCTL_GET_KEYRING_ID

The ID of the requested keyring.

KEYCTL_JOIN_SESSION_KEYRING

The ID of the joined session keyring.

KEYCTL_DESCRIBE

The size of the description (including the terminating null byte), irrespective of the provided buffer size.

KEYCTL_SEARCH

The ID of the key that was found.

KEYCTL_READ

The amount of data that is available in the key, irrespective of the provided buffer size.

KEYCTL_SET_REQKEY_KEYRING

The ID of the previous default keyring to which implicitly requested keys were linked (one of KEY_REQKEY_DEFL_USER_*).

KEYCTL_ASSUME_AUTHORITY

Either 0, if the ID given was 0, or the ID of the authorization key matching the specified key, if a nonzero key ID was provided.

KEYCTL_GET_SECURITY

The size of the LSM security label string (including the terminating null byte), irrespective of the provided buffer size.

KEYCTL_GET_PERSISTENT

The ID of the persistent keyring.

KEYCTL_DH_COMPUTE

The number of bytes copied to the buffer, or, if arg4 is 0, the required buffer size.

All other operations

Zero.

On error, -1 is returned, and errno is set to indicate the error.

## ERRORS
EACCES

The requested operation wasn't permitted.

EAGAIN

operation was KEYCTL_DH_COMPUTE and there was an error during crypto module initialization.

EDEADLK

operation was KEYCTL_LINK and the requested link would result in a cycle.

EDEADLK

operation was KEYCTL_RESTRICT_KEYRING and the requested keyring restriction would result in a cycle.

EDQUOT

The key quota for the caller's user would be exceeded by creating a key or linking it to the keyring.

EEXIST

operation was KEYCTL_RESTRICT_KEYRING and keyring provided in arg2 argument already has a restriction set.

EFAULT

operation was KEYCTL_DH_COMPUTE and one of the following has failed:

- copying of the struct keyctl_dh_params, provided in the arg2 argument, from user space;

- copying of the struct keyctl_kdf_params, provided in the non-NULL arg5 argument, from user space (in case kernel supports performing KDF operation on DH operation result);

- copying of data pointed by the hashname field of the struct keyctl_kdf_params from user space;

- copying of data pointed by the otherinfo field of the struct keyctl_kdf_params from user space if the otherinfolen field was nonzero;

- copying of the result to user space.

EINVAL

operation was KEYCTL_SETPERM and an invalid permission bit was specified in arg3.

EINVAL

operation was KEYCTL_SEARCH and the size of the description in arg4 (including the terminating null byte) exceeded 4096 bytes.

EINVAL

size of the string (including the terminating null byte) specified in arg3 (the key type) or arg4 (the key description) exceeded the limit (32 bytes and 4096 bytes respectively).

EINVAL (Linux kernels before 4.12)

operation was KEYCTL_DH_COMPUTE, argument arg5 was non-NULL.

EINVAL

operation was KEYCTL_DH_COMPUTE And the digest size of the hashing algorithm supplied is zero.

EINVAL

operation was KEYCTL_DH_COMPUTE and the buffer size provided is not enough to hold the result. Provide 0 as a buffer size in order to obtain the minimum buffer size.

EINVAL

operation was KEYCTL_DH_COMPUTE and the hash name provided in the hashname field of the struct keyctl_kdf_params pointed by arg5 argument is too big (the limit is implementation-specific and varies between kernel versions, but it is deemed big enough for all valid algorithm names).

EINVAL

operation was KEYCTL_DH_COMPUTE and the __spare field of the struct keyctl_kdf_params provided in the arg5 argument contains nonzero values.

EKEYEXPIRED

An expired key was found or specified.

EKEYREJECTED

A rejected key was found or specified.

EKEYREVOKED

A revoked key was found or specified.

ELOOP

operation was KEYCTL_LINK and the requested link would cause the maximum nesting depth for keyrings to be exceeded.

EMSGSIZE

operation was KEYCTL_DH_COMPUTE and the buffer length exceeds KEYCTL_KDF_MAX_OUTPUT_LEN (which is 1024 currently) or the otherinfolen field of the struct keyctl_kdf_parms passed in arg5 exceeds KEYCTL_KDF_MAX_OI_LEN (which is 64 currently).

ENFILE (Linux kernels before 3.13)

operation was KEYCTL_LINK and the keyring is full. (Before Linux 3.13, the available space for storing keyring links was limited to a single page of memory; since Linux 3.13, there is no fixed limit.)

ENOENT

operation was KEYCTL_UNLINK and the key to be unlinked isn't linked to the keyring.

ENOENT

operation was KEYCTL_DH_COMPUTE and the hashing algorithm specified in the hashname field of the struct keyctl_kdf_params pointed by arg5 argument hasn't been found.

ENOENT

operation was KEYCTL_RESTRICT_KEYRING and the type provided in arg3 argument doesn't support setting key linking restrictions.

ENOKEY

No matching key was found or an invalid key was specified.

ENOKEY

The value KEYCTL_GET_KEYRING_ID was specified in operation, the key specified in arg2 did not exist, and arg3 was zero (meaning don't create the key if it didn't exist).

ENOMEM

One of kernel memory allocation routines failed during the execution of the syscall.

ENOTDIR

A key of keyring type was expected but the ID of a key with a different type was provided.

EOPNOTSUPP

operation was KEYCTL_READ and the key type does not support reading (e.g., the type is "login").

EOPNOTSUPP

operation was KEYCTL_UPDATE and the key type does not support updating.

EOPNOTSUPP

operation was KEYCTL_RESTRICT_KEYRING, the type provided in arg3 argument was "asymmetric", and the key specified in the restriction specification provided in arg4 has type other than "asymmetric" or "keyring".

EPERM

operation was KEYCTL_GET_PERSISTENT, arg2 specified a UID other than the calling thread's real or effective UID, and the caller did not have the CAP_SETUID capability.

EPERM

operation was KEYCTL_SESSION_TO_PARENT and either: all of the UIDs (GIDs) of the parent process do not match the effective UID (GID) of the calling process; the UID of the parent's existing session keyring or the UID of the caller's session keyring did not match the effective UID of the caller; the parent process is not single-thread; or the parent process is init(1) or a kernel thread.

ETIMEDOUT

operation was KEYCTL_DH_COMPUTE and the initialization of crypto modules has timed out.

## VERSIONS
This system call first appeared in Linux 2.6.10.

## CONFORMING TO
This system call is a nonstandard Linux extension.

## NOTES
A wrapper is provided in the libkeyutils library. When employing the wrapper in that library, link with -lkeyutils. However, rather than using this system call directly, you probably want to use the various library functions mentioned in the descriptions of individual operations above.

## EXAMPLES
The program below provide subset of the functionality of the request-key(8) program provided by the keyutils package. For informational purposes, the program records various information in a log file.

As described in request_key(2), the request-key(8) program is invoked with command-line arguments that describe a key that is to be instantiated. The example program fetches and logs these arguments. The program assumes authority to instantiate the requested key, and then instantiates that key.

The following shell session demonstrates the use of this program. In the session, we compile the program and then use it to temporarily replace the standard request-key(8) program. (Note that temporarily disabling the standard request-key(8) program may not be safe on some systems.) While our example program is installed, we use the example program shown in request_key(2) to request a key.

```
$ cc -o key_instantiate key_instantiate.c -lkeyutils
$ sudo mv /sbin/request-key /sbin/request-key.backup
$ sudo cp key_instantiate /sbin/request-key
$ ./t_request_key user mykey somepayloaddata
Key ID is 20d035bf
$ sudo mv /sbin/request-key.backup /sbin/request-key
```

Looking at the log file created by this program, we can see the command-line arguments supplied to our example program:

```
$ cat /tmp/key_instantiate.log
Time: Mon Nov  7 13:06:47 2016
Command line arguments:

  argv[0]:            /sbin/request-key

  operation:          create

  key_to_instantiate: 20d035bf

  UID:                1000

  GID:                1000

  thread_keyring:     0

  process_keyring:    0

  session_keyring:    256e6a6
Key description:      user;1000;1000;3f010000;mykey
Auth key payload:     somepayloaddata
Destination keyring:  256e6a6
Auth key description: .request_key_auth;1000;1000;0b010000;20d035bf
```

The last few lines of the above output show that the example program was able to fetch:

- the description of the key to be instantiated, which included the name of the key (mykey);

- the payload of the authorization key, which consisted of the data (somepayloaddata) passed to request_key(2);

- the destination keyring that was specified in the call to request_key(2); and

- the description of the authorization key, where we can see that the name of the authorization key matches the ID of the key that is to be instantiated (20d035bf).

The example program in request_key(2) specified the destination keyring as KEY_SPEC_SESSION_KEYRING. By examining the contents of /proc/keys, we can see that this was translated to the ID of the destination keyring (0256e6a6) shown in the log output above; we can also see the newly created key with the name mykey and ID 20d035bf.

```
$ cat /proc/keys | egrep 'mykey|256e6a6'
0256e6a6 I--Q---  194 perm 3f030000  1000  1000 keyring  _ses: 3
20d035bf I--Q---    1 perm 3f010000  1000  1000 user     mykey: 16
```

### Program source

```
/* key_instantiate.c */
#include <sys/types.h>
#include <keyutils.h>
#include <time.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#ifndef KEY_SPEC_REQUESTOR_KEYRING
#define KEY_SPEC_REQUESTOR_KEYRING      -8
#endif
int
main(int argc, char *argv[])
{

    FILE *fp;

    time_t t;

    char *operation;

    key_serial_t key_to_instantiate, dest_keyring;

    key_serial_t thread_keyring, process_keyring, session_keyring;

    uid_t uid;

    gid_t gid;

    char dbuf[256];

    char auth_key_payload[256];

    int akp_size;       /* Size of auth_key_payload */

    int auth_key;

    fp = fopen("/tmp/key_instantiate.log", "w");

    if (fp == NULL)

        exit(EXIT_FAILURE);

    setbuf(fp, NULL);

    t = time(NULL);

    fprintf(fp, "Time: %s\n", ctime(&t));

    /*

     * The kernel passes a fixed set of arguments to the program

     * that it execs; fetch them.

     */

    operation = argv[1];

    key_to_instantiate = atoi(argv[2]);

    uid = atoi(argv[3]);

    gid = atoi(argv[4]);

    thread_keyring = atoi(argv[5]);

    process_keyring = atoi(argv[6]);

    session_keyring = atoi(argv[7]);

    fprintf(fp, "Command line arguments:\n");

    fprintf(fp, "  argv[0]:            %s\n", argv[0]);

    fprintf(fp, "  operation:          %s\n", operation);

    fprintf(fp, "  key_to_instantiate: %jx\n",

            (uintmax_t) key_to_instantiate);

    fprintf(fp, "  UID:                %jd\n", (intmax_t) uid);

    fprintf(fp, "  GID:                %jd\n", (intmax_t) gid);

    fprintf(fp, "  thread_keyring:     %jx\n",

            (uintmax_t) thread_keyring);

    fprintf(fp, "  process_keyring:    %jx\n",

            (uintmax_t) process_keyring);

    fprintf(fp, "  session_keyring:    %jx\n",

            (uintmax_t) session_keyring);

    fprintf(fp, "\n");

    /*

     * Assume the authority to instantiate the key named in argv[2].

     */

    if (keyctl(KEYCTL_ASSUME_AUTHORITY, key_to_instantiate) == -1) {

        fprintf(fp, "KEYCTL_ASSUME_AUTHORITY failed: %s\n",

                strerror(errno));

        exit(EXIT_FAILURE);

    }

    /*

     * Fetch the description of the key that is to be instantiated.

     */

    if (keyctl(KEYCTL_DESCRIBE, key_to_instantiate,

                dbuf, sizeof(dbuf)) == -1) {

        fprintf(fp, "KEYCTL_DESCRIBE failed: %s\n", strerror(errno));

        exit(EXIT_FAILURE);

    }

    fprintf(fp, "Key description:      %s\n", dbuf);

    /*

     * Fetch the payload of the authorization key, which is

     * actually the callout data given to request_key().

     */

    akp_size = keyctl(KEYCTL_READ, KEY_SPEC_REQKEY_AUTH_KEY,

                      auth_key_payload, sizeof(auth_key_payload));

    if (akp_size == -1) {

        fprintf(fp, "KEYCTL_READ failed: %s\n", strerror(errno));

        exit(EXIT_FAILURE);

    }

    auth_key_payload[akp_size] = '\0';

    fprintf(fp, "Auth key payload:     %s\n", auth_key_payload);

    /*

     * For interest, get the ID of the authorization key and

     * display it.

     */

    auth_key = keyctl(KEYCTL_GET_KEYRING_ID,

            KEY_SPEC_REQKEY_AUTH_KEY);

    if (auth_key == -1) {

        fprintf(fp, "KEYCTL_GET_KEYRING_ID failed: %s\n",

                strerror(errno));

        exit(EXIT_FAILURE);

    }

    fprintf(fp, "Auth key ID:          %jx\n", (uintmax_t) auth_key);

    /*

     * Fetch key ID for the request_key(2) destination keyring.

     */

    dest_keyring = keyctl(KEYCTL_GET_KEYRING_ID,

                          KEY_SPEC_REQUESTOR_KEYRING);

    if (dest_keyring == -1) {

        fprintf(fp, "KEYCTL_GET_KEYRING_ID failed: %s\n",

                strerror(errno));

        exit(EXIT_FAILURE);

    }

    fprintf(fp, "Destination keyring:  %jx\n", (uintmax_t) dest_keyring);

    /*

     * Fetch the description of the authorization key. This

     * allows us to see the key type, UID, GID, permissions,

     * and description (name) of the key. Among other things,

     * we will see that the name of the key is a hexadecimal

     * string representing the ID of the key to be instantiated.

     */

    if (keyctl(KEYCTL_DESCRIBE, KEY_SPEC_REQKEY_AUTH_KEY,

                dbuf, sizeof(dbuf)) == -1) {

        fprintf(fp, "KEYCTL_DESCRIBE failed: %s\n", strerror(errno));

        exit(EXIT_FAILURE);

    }

    fprintf(fp, "Auth key description: %s\n", dbuf);

    /*

     * Instantiate the key using the callout data that was supplied

     * in the payload of the authorization key.

     */

    if (keyctl(KEYCTL_INSTANTIATE, key_to_instantiate,

               auth_key_payload, akp_size + 1, dest_keyring) == -1) {

        fprintf(fp, "KEYCTL_INSTANTIATE failed: %s\n",

                strerror(errno));

        exit(EXIT_FAILURE);

    }

    exit(EXIT_SUCCESS);
}
```

## SEE ALSO
keyctl(1), add_key(2), request_key(2), keyctl(3), keyctl_assume_authority(3), keyctl_chown(3), keyctl_clear(3), keyctl_describe(3), keyctl_describe_alloc(3), keyctl_dh_compute(3), keyctl_dh_compute_alloc(3), keyctl_get_keyring_ID(3), keyctl_get_persistent(3), keyctl_get_security(3), keyctl_get_security_alloc(3), keyctl_instantiate(3), keyctl_instantiate_iov(3), keyctl_invalidate(3), keyctl_join_session_keyring(3), keyctl_link(3), keyctl_negate(3), keyctl_read(3), keyctl_read_alloc(3), keyctl_reject(3), keyctl_revoke(3), keyctl_search(3), keyctl_session_to_parent(3), keyctl_set_reqkey_keyring(3), keyctl_set_timeout(3), keyctl_setperm(3), keyctl_unlink(3), keyctl_update(3), recursive_key_scan(3), recursive_session_key_scan(3), capabilities(7), credentials(7), keyrings(7), keyutils(7), persistent-keyring(7), process-keyring(7), session-keyring(7), thread-keyring(7), user-keyring(7), user_namespaces(7), user-session-keyring(7), request-key(8)

The kernel source files under Documentation/security/keys/ (or, before Linux 4.13, in the file Documentation/security/keys.txt).


kill

package: man-pages

title: kill(2)

summary: send signal to a process

text:
# kill(2)

## NAME
kill - send signal to a process

## SYNOPSIS

```
#include <signal.h>
```

```
int kill(pid_t pid, int sig);
```

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

kill():

```
    _POSIX_C_SOURCE
```

## DESCRIPTION
The kill() system call can be used to send any signal to any process group or process.

If pid is positive, then signal sig is sent to the process with the ID specified by pid.

If pid equals 0, then sig is sent to every process in the process group of the calling process.

If pid equals -1, then sig is sent to every process for which the calling process has permission to send signals, except for process 1 (init), but see below.

If pid is less than -1, then sig is sent to every process in the process group whose ID is -pid.

If sig is 0, then no signal is sent, but existence and permission checks are still performed; this can be used to check for the existence of a process ID or process group ID that the caller is permitted to signal.

For a process to have permission to send a signal, it must either be privileged (under Linux: have the CAP_KILL capability in the user namespace of the target process), or the real or effective user ID of the sending process must equal the real or saved set-user-ID of the target process. In the case of SIGCONT, it suffices when the sending and receiving processes belong to the same session. (Historically, the rules were different; see NOTES.)

## RETURN VALUE
On success (at least one signal was sent), zero is returned. On error, -1 is returned, and errno is set to indicate the error.

## ERRORS
EINVAL

An invalid signal was specified.

EPERM

The calling process does not have permission to send the signal to any of the target processes.

ESRCH

The target process or process group does not exist. Note that an existing process might be a zombie, a process that has terminated execution, but has not yet been wait(2)ed for.

## CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

## NOTES
The only signals that can be sent to process ID 1, the init process, are those for which init has explicitly installed signal handlers. This is done to assure the system is not brought down accidentally.

POSIX.1 requires that kill(-1,sig) send sig to all processes that the calling process may send signals to, except possibly for some implementation-defined system processes. Linux allows a process to signal itself, but on Linux the call kill(-1,sig) does not signal the calling process.

POSIX.1 requires that if a process sends a signal to itself, and the sending thread does not have the signal blocked, and no other thread has it unblocked or is waiting for it in sigwait(3), at least one unblocked signal must be delivered to the sending thread before the kill() returns.

### Linux notes
Across different kernel versions, Linux has enforced different rules for the permissions required for an unprivileged process to send a signal to another process. In kernels 1.0 to 1.2.2, a signal could be sent if the effective user ID of the sender matched effective user ID of the target, or the real user ID of the sender matched the real user ID of the target. From kernel 1.2.3 until 1.3.77, a signal could be sent if the effective user ID of the sender matched either the real or effective user ID of the target. The current rules, which conform to POSIX.1, were adopted in kernel 1.3.78.

## BUGS
In 2.6 kernels up to and including 2.6.7, there was a bug that meant that when sending signals to a process group, kill() failed with the error EPERM if the caller did not have permission to send the signal to any (rather than all) of the members of the process group. Notwithstanding this error return, the signal was still delivered to all of the processes for which the caller had permission to signal.

## SEE ALSO
kill(1), _exit(2), pidfd_send_signal(2), signal(2), tkill(2), exit(3), killpg(3), sigqueue(3), capabilities(7), credentials(7), signal(7)


link

title: link(2)

summary: make a new name for a file

text:
# link(2)

## NAME
link, linkat - make a new name for a file

## SYNOPSIS

```
#include <unistd.h>
```

```
int link(const char *oldpath, const char *newpath);
```

```
#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>
```

```
int linkat(int olddirfd, const char *oldpath,
           int newdirfd, const char *newpath, int flags);
```

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

linkat():

```
    Since glibc 2.10:

        _POSIX_C_SOURCE >= 200809L

    Before glibc 2.10:

        _ATFILE_SOURCE
```

## DESCRIPTION
link() creates a new link (also known as a hard link) to an existing file.

If newpath exists, it will not be overwritten.

This new name may be used exactly as the old one for any operation; both names refer to the same file (and so have the same permissions and ownership) and it is impossible to tell which name was the "original".

### linkat()
The linkat() system call operates in exactly the same way as link(), except for the differences described here.

If the pathname given in oldpath is relative, then it is interpreted relative to the directory referred to by the file descriptor olddirfd (rather than relative to the current working directory of the calling process, as is done by link() for a relative pathname).

If oldpath is relative and olddirfd is the special value AT_FDCWD, then oldpath is interpreted relative to the current working directory of the calling process (like link()).

If oldpath is absolute, then olddirfd is ignored.

The interpretation of newpath is as for oldpath, except that a relative pathname is interpreted relative to the directory referred to by the file descriptor newdirfd.

The following values can be bitwise ORed in flags:

AT_EMPTY_PATH (since Linux 2.6.39)

If oldpath is an empty string, create a link to the file referenced by olddirfd (which may have been obtained using the open(2) O_PATH flag). In this case, olddirfd can refer to any type of file except a directory. This will generally not work if the file has a link count of zero (files created with O_TMPFILE and without O_EXCL are an exception). The caller must have the CAP_DAC_READ_SEARCH capability in order to use this flag. This flag is Linux-specific; define _GNU_SOURCE to obtain its definition.

AT_SYMLINK_FOLLOW (since Linux 2.6.18)

By default, linkat(), does not dereference oldpath if it is a symbolic link (like link()). The flag AT_SYMLINK_FOLLOW can be specified in flags to cause oldpath to be dereferenced if it is a symbolic link. If procfs is mounted, this can be used as an alternative to AT_EMPTY_PATH, like this:

```
linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,

       newname, AT_SYMLINK_FOLLOW);

```

Before kernel 2.6.18, the flags argument was unused, and had to be specified as 0.

See openat(2) for an explanation of the need for linkat().

## RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.

## ERRORS
EACCES

Write access to the directory containing newpath is denied, or search permission is denied for one of the directories in the path prefix of oldpath or newpath. (See also path_resolution(7).)

EDQUOT

The user's quota of disk blocks on the filesystem has been exhausted.

EEXIST

newpath already exists.

EFAULT

oldpath or newpath points outside your accessible address space.

EIO

An I/O error occurred.

ELOOP

Too many symbolic links were encountered in resolving oldpath or newpath.

EMLINK

The file referred to by oldpath already has the maximum number of links to it. For example, on an ext4(5) filesystem that does not employ the dir_index feature, the limit on the number of hard links to a file is 65,000; on btrfs(5), the limit is 65,535 links.

ENAMETOOLONG

oldpath or newpath was too long.

ENOENT

A directory component in oldpath or newpath does not exist or is a dangling symbolic link.

ENOMEM

Insufficient kernel memory was available.

ENOSPC

The device containing the file has no room for the new directory entry.

ENOTDIR

A component used as a directory in oldpath or newpath is not, in fact, a directory.

EPERM

oldpath is a directory.

EPERM

The filesystem containing oldpath and newpath does not support the creation of hard links.

EPERM (since Linux 3.6)

The caller does not have permission to create a hard link to this file (see the description of /proc/sys/fs/protected_hardlinks in proc(5)).

EPERM

oldpath is marked immutable or append-only. (See ioctl_iflags(2).)

EROFS

The file is on a read-only filesystem.

EXDEV

oldpath and newpath are not on the same mounted filesystem. (Linux permits a filesystem to be mounted at multiple points, but link() does not work across different mount points, even if the same filesystem is mounted on both.)

The following additional errors can occur for linkat():

EBADF

olddirfd or newdirfd is not a valid file descriptor.

EINVAL

An invalid flag value was specified in flags.

ENOENT

AT_EMPTY_PATH was specified in flags, but the caller did not have the CAP_DAC_READ_SEARCH capability.

ENOENT

An attempt was made to link to the /proc/self/fd/NN file corresponding to a file descriptor created with

```
open(path, O_TMPFILE | O_EXCL, mode);

```

See open(2).

ENOENT

An attempt was made to link to a /proc/self/fd/NN file corresponding to a file that has been deleted.

ENOENT

oldpath is a relative pathname and olddirfd refers to a directory that has been deleted, or newpath is a relative pathname and newdirfd refers to a directory that has been deleted.

ENOTDIR

oldpath is relative and olddirfd is a file descriptor referring to a file other than a directory; or similar for newpath and newdirfd

EPERM

AT_EMPTY_PATH was specified in flags, oldpath is an empty string, and olddirfd refers to a directory.

## VERSIONS
linkat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.

## CONFORMING TO
link(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.

linkat(): POSIX.1-2008.

## NOTES
Hard links, as created by link(), cannot span filesystems. Use symlink(2) if this is required.

POSIX.1-2001 says that link() should dereference oldpath if it is a symbolic link. However, since kernel 2.0, Linux does not do so: if oldpath is a symbolic link, then newpath is created as a (hard) link to the same symbolic link file (i.e., newpath becomes a symbolic link to the same file that oldpath refers to). Some other implementations behave in the same manner as Linux. POSIX.1-2008 changes the specification of link(), making it implementation-dependent whether or not oldpath is dereferenced if it is a symbolic link. For precise control over the treatment of symbolic links when creating a link, use linkat().

### Glibc notes
On older kernels where linkat() is unavailable, the glibc wrapper function falls back to the use of link(), unless the AT_SYMLINK_FOLLOW is specified. When oldpath and newpath are relative pathnames, glibc constructs pathnames based on the symbolic links in /proc/self/fd that correspond to the olddirfd and newdirfd arguments.

## BUGS
On NFS filesystems, the return code may be wrong in case the NFS server performs the link creation and dies before it can say so. Use stat(2) to find out if the link got created.

## SEE ALSO
ln(1), open(2), rename(2), stat(2), symlink(2), unlink(2), path_resolution(7), symlink(7)


listen

title: listen(2)

summary: listen for connections on a socket

text:
# listen(2)

## NAME
listen - listen for connections on a socket

## SYNOPSIS

```
#include <sys/socket.h>
```

```
int listen(int sockfd, int backlog);
```

## DESCRIPTION
listen() marks the socket referred to by sockfd as a passive socket, that is, as a socket that will be used to accept incoming connection requests using accept(2).

The sockfd argument is a file descriptor that refers to a socket of type SOCK_STREAM or SOCK_SEQPACKET.

The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow. If a connection request arrives when the queue is full, the client may receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that a later reattempt at connection succeeds.

## RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.

## ERRORS
EADDRINUSE

Another socket is already listening on the same port.

EADDRINUSE

(Internet domain sockets) The socket referred to by sockfd had not previously been bound to an address and, upon attempting to bind it to an ephemeral port, it was determined that all port numbers in the ephemeral port range are currently in use. See the discussion of /proc/sys/net/ipv4/ip_local_port_range in ip(7).

EBADF

The argument sockfd is not a valid file descriptor.

ENOTSOCK

The file descriptor sockfd does not refer to a socket.

EOPNOTSUPP

The socket is not of a type that supports the listen() operation.

## CONFORMING TO
POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in 4.2BSD).

## NOTES
To accept connections, the following steps are performed:

1.

A socket is created with socket(2).

2.

The socket is bound to a local address using bind(2), so that other sockets may be connect(2)ed to it.

3.

A willingness to accept incoming connections and a queue limit for incoming connections are specified with listen().

4.

Connections are accepted with accept(2).

The behavior of the backlog argument on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completely established sockets waiting to be accepted, instead of the number of incomplete connection requests. The maximum length of the queue for incomplete sockets can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncookies are enabled there is no logical maximum length and this setting is ignored. See tcp(7) for more information.

If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently capped to that value. Since Linux 5.4, the default in this file is 4096; in earlier kernels, the default value is 128. In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.

## EXAMPLES
See bind(2).

## SEE ALSO
accept(2), bind(2), connect(2), socket(2), socket(7)


listxattr

title: listxattr(2)

summary: list extended attribute names

text:
# listxattr(2)

## NAME
listxattr, llistxattr, flistxattr - list extended attribute names

## SYNOPSIS

```
#include <sys/xattr.h>
```

```
ssize_t listxattr(const char *path, char *list, size_t size);
ssize_t llistxattr(const char *path, char *list, size_t size);
ssize_t flistxattr(int fd, char *list, size_t size);
```

## DESCRIPTION
Extended attributes are name:value pairs associated with inodes (files, directories, symbolic links, etc.). They are extensions to the normal attributes which are associated with all inodes in the system (i.e., the stat(2) data). A complete overview of extended attributes concepts can be found in xattr(7).

listxattr() retrieves the list of extended attribute names associated with the given path in the filesystem. The retrieved list is placed in list, a caller-allocated buffer whose size (in bytes) is specified in the argument size. The list is the set of (null-terminated) names, one after the other. Names of extended attributes to which the calling process does not have access may be omitted from the list. The length of the attribute name list is returned.

llistxattr() is identical to listxattr(), except in the case of a symbolic link, where the list of names of extended attributes associated with the link itself is retrieved, not the file that it refers to.

flistxattr() is identical to listxattr(), only the open file referred to by fd (as returned by open(2)) is interrogated in place of path.

A single extended attribute name is a null-terminated string. The name includes a namespace prefix; there may be several, disjoint namespaces associated with an individual inode.

If size is specified as zero, these calls return the current size of the list of extended attribute names (and leave list unchanged). This can be used to determine the size of the buffer that should be supplied in a subsequent call. (But, bear in mind that there is a possibility that the set of extended attributes may change between the two calls, so that it is still necessary to check the return status from the second call.)

### Example
The list of names is returned as an unordered array of null-terminated character strings (attribute names are separated by null bytes ('\0')), like this:

```
user.name1\0system.name1\0user.name2\0
```

Filesystems that implement POSIX ACLs using extended attributes might return a list like this:

```
system.posix_acl_access\0system.posix_acl_default\0
```

## RETURN VALUE
On success, a nonnegative number is returned indicating the size of the extended attribute name list. On failure, -1 is returned and errno is set to indicate the error.

## ERRORS
E2BIG

The size of the list of extended attribute names is larger than the maximum size allowed; the list cannot be retrieved. This can happen on filesystems that support an unlimited number of extended attributes per file such as XFS, for example. See BUGS.

ENOTSUP

Extended attributes are not supported by the filesystem, or are disabled.

ERANGE

The size of the list buffer is too small to hold the result.

In addition, the errors documented in stat(2) can also occur.

## VERSIONS
These system calls have been available on Linux since kernel 2.4; glibc support is provided since version 2.3.

## CONFORMING TO
These system calls are Linux-specific.

## BUGS
As noted in xattr(7), the VFS imposes a limit of 64 kB on the size of the extended attribute name list returned by listxattr(7). If the total size of attribute names attached to a file exceeds this limit, it is no longer possible to retrieve the list of attribute names.

## EXAMPLES
The following program demonstrates the usage of listxattr() and getxattr(2). For the file whose pathname is provided as a command-line argument, it lists all extended file attributes and their values.

To keep the code simple, the program assumes that attribute keys and values are constant during the execution of the program. A production program should expect and handle changes during execution of the program. For example, the number of bytes required for attribute keys might increase between the two calls to listxattr(). An application could handle this possibility using a loop that retries the call (perhaps up to a predetermined maximum number of attempts) with a larger buffer each time it fails with the error ERANGE. Calls to getxattr(2) could be handled similarly.

The following output was recorded by first creating a file, setting some extended file attributes, and then listing the attributes with the example program.

### Example output

```
$ touch /tmp/foo
$ setfattr -n user.fred -v chocolate /tmp/foo
$ setfattr -n user.frieda -v bar /tmp/foo
$ setfattr -n user.empty /tmp/foo
$ ./listxattr /tmp/foo
user.fred: chocolate
user.frieda: bar
user.empty: <no value>
```

### Program source (listxattr.c)

```
#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/xattr.h>
int
main(int argc, char *argv[])
{

    ssize_t buflen, keylen, vallen;

    char *buf, *key, *val;

    if (argc != 2) {

        fprintf(stderr, "Usage: %s path\n", argv[0]);

        exit(EXIT_FAILURE);

    }

    /*

     * Determine the length of the buffer needed.

     */

    buflen = listxattr(argv[1], NULL, 0);

    if (buflen == -1) {

        perror("listxattr");

        exit(EXIT_FAILURE);

    }

    if (buflen == 0) {

        printf("%s has no attributes.\n", argv[1]);

        exit(EXIT_SUCCESS);

    }

    /*

     * Allocate the buffer.

     */

    buf = malloc(buflen);

    if (buf == NULL) {

        perror("malloc");

        exit(EXIT_FAILURE);

    }

    /*

     * Copy the list of attribute keys to the buffer.

     */

    buflen = listxattr(argv[1], buf, buflen);

    if (buflen == -1) {

        perror("listxattr");

        exit(EXIT_FAILURE);

    }

    /*

     * Loop over the list of zero terminated strings with the

     * attribute keys. Use the remaining buffer length to determine

     * the end of the list.

     */

    key = buf;

    while (buflen > 0) {

        /*

         * Output attribute key.

         */

        printf("%s: ", key);

        /*

         * Determine length of the value.

         */

        vallen = getxattr(argv[1], key, NULL, 0);

        if (vallen == -1)

            perror("getxattr");

        if (vallen > 0) {

            /*

             * Allocate value buffer.

             * One extra byte is needed to append 0x00.

             */

            val = malloc(vallen + 1);

            if (val == NULL) {

                perror("malloc");

                exit(EXIT_FAILURE);

            }

            /*

             * Copy value to buffer.

             */

            vallen = getxattr(argv[1], key, val, vallen);

            if (vallen == -1)

                perror("getxattr");

            else {

                /*

                 * Output attribute value.

                 */

                val[vallen] = 0;

                printf("%s", val);

            }

            free(val);

        } else if (vallen == 0)

            printf("<no value>");

        printf("\n");

        /*

         * Forward to next attribute key.

         */

        keylen = strlen(key) + 1;

        buflen -= keylen;

        key += keylen;

    }

    free(buf);

    exit(EXIT_SUCCESS);
}
```

## SEE ALSO
getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2), setxattr(2), stat(2), symlink(7), xattr(7)


llseek

title: llseek(2)

summary: reposition read/write file offset

text:
# llseek(2)

## NAME
_llseek - reposition read/write file offset

## SYNOPSIS

```
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>
```

```
int syscall(SYS__llseek, unsigned int fd, unsigned long offset_high,
            unsigned long offset_low, loff_t *result,
            unsigned int whence);
```

Note: glibc provides no wrapper for _llseek(), necessitating the use of syscall(2).

## DESCRIPTION
Note: for information about the llseek(3) library function, see lseek64(3).

The _llseek() system call repositions the offset of the open file description associated with the file descriptor fd to the value

(offset_high << 32) | offset_low

This new offset is a byte offset relative to the beginning of the file, the current file offset, or the end of the file, depending on whether whence is SEEK_SET, SEEK_CUR, or SEEK_END, respectively.

The new file offset is returned in the argument result. The type loff_t is a 64-bit signed type.

This system call exists on various 32-bit platforms to support seeking to large file offsets.

## RETURN VALUE
Upon successful completion, _llseek() returns 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.

## ERRORS
EBADF

fd is not an open file descriptor.

EFAULT

Problem with copying results to user space.

EINVAL

whence is invalid.

## CONFORMING TO
This function is Linux-specific, and should not be used in programs intended to be portable.

## NOTES
You probably want to use the lseek(2) wrapper function instead.

## SEE ALSO
lseek(2), open(2), lseek64(3)


title: lookup_dcookie(2)

summary: return a directory entry's path

text:
# lookup_dcookie(2)

## NAME
lookup_dcookie - return a directory entry's path

## SYNOPSIS

```
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>
```

```
int syscall(SYS_lookup_dcookie, uint64_t cookie, char *buffer,
            size_t len);
```

Note: glibc provides no wrapper for lookup_dcookie(), necessitating the use of syscall(2).

## DESCRIPTION
Look up the full path of the directory entry specified by the value cookie. The cookie is an opaque identifier uniquely identifying a particular directory entry. The buffer given is filled in with the full path of the directory entry.

For lookup_dcookie() to return successfully, the kernel must still hold a cookie reference to the directory entry.

## RETURN VALUE
On success, lookup_dcookie() returns the length of the path string copied into the buffer. On error, -1 is returned, and errno is set to indicate the error.

## ERRORS
EFAULT

The buffer was not valid.

EINVAL

The kernel has no registered cookie/directory entry mappings at the time of lookup, or the cookie does not refer to a valid directory entry.

ENAMETOOLONG

The name could not fit in the buffer.

ENOMEM

The kernel could not allocate memory for the temporary buffer holding the path.

EPERM

The process does not have the capability CAP_SYS_ADMIN required to look up cookie values.

ERANGE

The buffer was not large enough to hold the path of the directory entry.

## VERSIONS
Available since Linux 2.5.43. The ENAMETOOLONG error return was added in 2.5.70.

## CONFORMING TO
lookup_dcookie() is Linux-specific.

## NOTES
lookup_dcookie() is a special-purpose system call, currently used only by the oprofile(1) profiler. It relies on a kernel driver to register cookies for directory entries.

The path returned may be suffixed by the string " (deleted)" if the directory entry has been removed.

## SEE ALSO
oprofile(1)


lseek

title: lseek(2)

summary: reposition read/write file offset

text:
# lseek(2)

## NAME
lseek - reposition read/write file offset

## SYNOPSIS

```
#include <unistd.h>
```

```
off_t lseek(int fd, off_t offset, int whence);
```

## DESCRIPTION
lseek() repositions the file offset of the open file description associated with the file descriptor fd to the argument offset according to the directive whence as follows:

SEEK_SET

The file offset is set to offset bytes.

SEEK_CUR

The file offset is set to its current location plus offset bytes.

SEEK_END

The file offset is set to the size of the file plus offset bytes.

lseek() allows the file offset to be set beyond the end of the file (but this does not change the size of the file). If data is later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes ('\0') until data is actually written into the gap.

### Seeking file data and holes
Since version 3.1, Linux supports the following additional values for whence:

SEEK_DATA

Adjust the file offset to the next location in the file greater than or equal to offset containing data. If offset points to data, then the file offset is set to offset.

SEEK_HOLE

Adjust the file offset to the next hole in the file greater than or equal to offset. If offset points into the middle of a hole, then the file offset is set to offset. If there is no hole past offset, then the file offset is adjusted to the end of the file (i.e., there is an implicit hole at the end of any file).

In both of the above cases, lseek() fails if offset points past the end of the file.

These operations allow applications to map holes in a sparsely allocated file. This can be useful for applications such as file backup tools, which can save space when creating backups and preserve holes, if they have a mechanism for discovering holes.

For the purposes of these operations, a hole is a sequence of zeros that (normally) has not been allocated in the underlying file storage. However, a filesystem is not obliged to report holes, so these operations are not a guaranteed mechanism for mapping the storage space actually allocated to a file. (Furthermore, a sequence of zeros that actually has been written to the underlying storage may not be reported as a hole.) In the simplest implementation, a filesystem can support the operations by making SEEK_HOLE always return the offset of the end of the file, and making SEEK_DATA always return offset (i.e., even if the location referred to by offset is a hole, it can be considered to consist of data that is a sequence of zeros).

The _GNU_SOURCE feature test macro must be defined in order to obtain the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.

The SEEK_HOLE and SEEK_DATA operations are supported for the following filesystems:

- Btrfs (since Linux 3.1)

- OCFS (since Linux 3.2)

- XFS (since Linux 3.5)

- ext4 (since Linux 3.8)

- tmpfs(5) (since Linux 3.8)

- NFS (since Linux 3.18)

- FUSE (since Linux 4.5)

- GFS2 (since Linux 4.15)

## RETURN VALUE
Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. On error, the value (off_t) -1 is returned and errno is set to indicate the error.

## ERRORS
EBADF

fd is not an open file descriptor.

EINVAL

whence is not valid. Or: the resulting file offset would be negative, or beyond the end of a seekable device.

ENXIO

whence is SEEK_DATA or SEEK_HOLE, and offset is beyond the end of the file, or whence is SEEK_DATA and offset is within a hole at the end of the file.

EOVERFLOW

The resulting file offset cannot be represented in an off_t.

ESPIPE

fd is associated with a pipe, socket, or FIFO.

## CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in the next POSIX revision (Issue 8).

## NOTES
See open(2) for a discussion of the relationship between file descriptors, open file descriptions, and files.

If the O_APPEND file status flag is set on the open file description, then a write(2) always moves the file offset to the end of the file, regardless of the use of lseek().

The off_t data type is a signed integer data type specified by POSIX.1.

Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

On Linux, using lseek() on a terminal device fails with the error ESPIPE.

## SEE ALSO
dup(2), fallocate(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)


madvise

title: madvise(2)

summary: give advice about use of memory

text:
# madvise(2)

## NAME
madvise - give advice about use of memory

## SYNOPSIS

```
#include <sys/mman.h>
```

```
int madvise(void *addr, size_t length, int advice);
```

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

madvise():

```
    Since glibc 2.19:

        _DEFAULT_SOURCE

    Up to and including glibc 2.19:

        _BSD_SOURCE
```

## DESCRIPTION
The madvise() system call is used to give advice or directions to the kernel about the address range beginning at address addr and with size length bytes In most cases, the goal of such advice is to improve system or application performance.

Initially, the system call supported a set of "conventional" advice values, which are also available on several other implementations. (Note, though, that madvise() is not specified in POSIX.) Subsequently, a number of Linux-specific advice values have been added.

### Conventional advice values
The advice values listed below allow an application to tell the kernel how it expects to use some mapped or shared memory areas, so that the kernel can choose appropriate read-ahead and caching techniques. These advice values do not influence the semantics of the application (except in the case of MADV_DONTNEED), but may influence its performance. All of the advice values listed here have analogs in the POSIX-specified posix_madvise(3) function, and the values have the same meanings, with the exception of MADV_DONTNEED.

The advice is indicated in the advice argument, which is one of the following:

MADV_NORMAL

No special treatment. This is the default.

MADV_RANDOM

Expect page references in random order. (Hence, read ahead may be less useful than normally.)

MADV_SEQUENTIAL

Expect page references in sequential order. (Hence, pages in the given range can be aggressively read ahead, and may be freed soon after they are accessed.)

MADV_WILLNEED

Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.)

MADV_DONTNEED

Do not expect access in the near future. (For the time being, the application is finished with the given range, so the kernel can free resources associated with it.)

After a successful MADV_DONTNEED operation, the semantics of memory access in the specified region are changed: subsequent accesses of pages in the range will succeed, but will result in either repopulating the memory contents from the up-to-date contents of the underlying mapped file (for shared file mappings, shared anonymous mappings, and shmem-based techniques such as System V shared memory segments) or zero-fill-on-demand pages for anonymous private mappings.

Note that, when applied to shared mappings, MADV_DONTNEED might not lead to immediate freeing of the pages in the range. The kernel is free to delay freeing the pages until an appropriate moment. The resident set size (RSS) of the calling process will be immediately reduced however.

MADV_DONTNEED cannot be applied to locked pages, Huge TLB pages, or VM_PFNMAP pages. (Pages marked with the kernel-internal VM_PFNMAP flag are special memory areas that are not managed by the virtual memory subsystem. Such pages are typically created by device drivers that map the pages into user space.)

### Linux-specific advice values
The following Linux-specific advice values have no counterparts in the POSIX-specified posix_madvise(3), and may or may not have counterparts in the madvise() interface available on other implementations. Note that some of these operations change the semantics of memory accesses.

MADV_REMOVE (since Linux 2.6.16)

Free up a given range of pages and its associated backing store. This is equivalent to punching a hole in the corresponding byte range of the backing store (see fallocate(2)). Subsequent accesses in the specified address range will see bytes containing zero.

The specified address range must be mapped shared and writable. This flag cannot be applied to locked pages, Huge TLB pages, or VM_PFNMAP pages.

In the initial implementation, only tmpfs(5) was supported MADV_REMOVE; but since Linux 3.5, any filesystem which supports the fallocate(2) FALLOC_FL_PUNCH_HOLE mode also supports MADV_REMOVE. Hugetlbfs fails with the error EINVAL and other filesystems fail with the error EOPNOTSUPP.

MADV_DONTFORK (since Linux 2.6.16)

Do not make the pages in this range available to the child after a fork(2). This is useful to prevent copy-on-write semantics from changing the physical location of a page if the parent writes to it after a fork(2). (Such page relocations cause problems for hardware that DMAs into the page.)

MADV_DOFORK (since Linux 2.6.16)

Undo the effect of MADV_DONTFORK, restoring the default behavior, whereby a mapping is inherited across fork(2).

MADV_HWPOISON (since Linux 2.6.32)

Poison the pages in the range specified by addr and length and handle subsequent references to those pages like a hardware memory corruption. This operation is available only for privileged (CAP_SYS_ADMIN) processes. This operation may result in the calling process receiving a SIGBUS and the page being unmapped.

This feature is intended for testing of memory error-handling code; it is available only if the kernel was configured with CONFIG_MEMORY_FAILURE.

MADV_MERGEABLE (since Linux 2.6.32)

Enable Kernel Samepage Merging (KSM) for the pages in the range specified by addr and length. The kernel regularly scans those areas of user memory that have been marked as mergeable, looking for pages with identical content. These are replaced by a single write-protected page (which is automatically copied if a process later wants to update the content of the page). KSM merges only private anonymous pages (see mmap(2)).

The KSM feature is intended for applications that generate many instances of the same data (e.g., virtualization systems such as KVM). It can consume a lot of processing power; use with care. See the Linux kernel source file Documentation/admin-guide/mm/ksm.rst for more details.

The MADV_MERGEABLE and MADV_UNMERGEABLE operations are available only if the kernel was configured with CONFIG_KSM.

MADV_UNMERGEABLE (since Linux 2.6.32)

Undo the effect of an earlier MADV_MERGEABLE operation on the specified address range; KSM unmerges whatever pages it had merged in the address range specified by addr and length.

MADV_SOFT_OFFLINE (since Linux 2.6.33)

Soft offline the pages in the range specified by addr and length. The memory of each page in the specified range is preserved (i.e., when next accessed, the same content will be visible, but in a new physical page frame), and the original page is offlined (i.e., no longer used, and taken out of normal memory management). The effect of the MADV_SOFT_OFFLINE operation is invisible to (i.e., does not change the semantics of) the calling process.

This feature is intended for testing of memory error-handling code; it is available only if the kernel was configured with CONFIG_MEMORY_FAILURE.

MADV_HUGEPAGE (since Linux 2.6.38)

Enable Transparent Huge Pages (THP) for pages in the range specified by addr and length. Currently, Transparent Huge Pages work only with private anonymous pages (see mmap(2)). The kernel will regularly scan the areas marked as huge page candidates to replace them with huge pages. The kernel will also allocate huge pages directly when the region is naturally aligned to the huge page size (see posix_memalign(2)).

This feature is primarily aimed at applications that use large mappings of data and access large regions of that memory at a time (e.g., virtualization systems such as QEMU). It can very easily waste memory (e.g., a 2 MB mapping that only ever accesses 1 byte will result in 2 MB of wired memory instead of one 4 KB page). See the Linux kernel source file Documentation/admin-guide/mm/transhuge.rst for more details.

Most common kernels configurations provide MADV_HUGEPAGE-style behavior by default, and thus MADV_HUGEPAGE is normally not necessary. It is mostly intended for embedded systems, where MADV_HUGEPAGE-style behavior may not be enabled by default in the kernel. On such systems, this flag can be used in order to selectively enable THP. Whenever MADV_HUGEPAGE is used, it should always be in regions of memory with an access pattern that the developer knows in advance won't risk to increase the memory footprint of the application when transparent hugepages are enabled.

The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are available only if the kernel was configured with CONFIG_TRANSPARENT_HUGEPAGE.

MADV_NOHUGEPAGE (since Linux 2.6.38)

Ensures that memory in the address range specified by addr and length will not be backed by transparent hugepages.

MADV_DONTDUMP (since Linux 3.4)

Exclude from a core dump those pages in the range specified by addr and length. This is useful in applications that have large areas of memory that are known not to be useful in a core dump. The effect of MADV_DONTDUMP takes precedence over the bit mask that is set via the /proc/[pid]/coredump_filter file (see core(5)).

MADV_DODUMP (since Linux 3.4)

Undo the effect of an earlier MADV_DONTDUMP.

MADV_FREE (since Linux 4.5)

The application no longer requires the pages in the range specified by addr and len. The kernel can thus free these pages, but the freeing could be delayed until memory pressure occurs. For each of the pages that has been marked to be freed but has not yet been freed, the free operation will be canceled if the caller writes into the page. After a successful MADV_FREE operation, any stale data (i.e., dirty, unwritten pages) will be lost when the kernel frees the pages. However, subsequent writes to pages in the range will succeed and then kernel cannot free those dirtied pages, so that the caller can always see just written data. If there is no subsequent write, the kernel can free the pages at any time. Once pages in the range have been freed, the caller will see zero-fill-on-demand pages upon subsequent page references.

The MADV_FREE operation can be applied only to private anonymous pages (see mmap(2)). In Linux before version 4.12, when freeing pages on a swapless system, the pages in the given range are freed instantly, regardless of memory pressure.

MADV_WIPEONFORK (since Linux 4.14)

Present the child process with zero-filled memory in this range after a fork(2). This is useful in forking servers in order to ensure that sensitive per-process data (for example, PRNG seeds, cryptographic secrets, and so on) is not handed to child processes.

The MADV_WIPEONFORK operation can be applied only to private anonymous pages (see mmap(2)).

Within the child created by fork(2), the MADV_WIPEONFORK setting remains in place on the specified address range. This setting is cleared during execve(2).

MADV_KEEPONFORK (since Linux 4.14)

Undo the effect of an earlier MADV_WIPEONFORK.

MADV_COLD (since Linux 5.4)

Deactivate a given range of pages. This will make the pages a more probable reclaim target should there be a memory pressure. This is a nondestructive operation. The advice might be ignored for some pages in the range when it is not applicable.

MADV_PAGEOUT (since Linux 5.4)

Reclaim a given range of pages. This is done to free up memory occupied by these pages. If a page is anonymous, it will be swapped out. If a page is file-backed and dirty, it will be written back to the backing storage. The advice might be ignored for some pages in the range when it is not applicable.

## RETURN VALUE
On success, madvise() returns zero. On error, it returns -1 and errno is set to indicate the error.

## ERRORS
EACCES

advice is MADV_REMOVE, but the specified address range is not a shared writable mapping.

EAGAIN

A kernel resource was temporarily unavailable.

EBADF

The map exists, but the area maps something that isn't a file.

EINVAL

addr is not page-aligned or length is negative.

EINVAL

advice is not a valid.

EINVAL

advice is MADV_DONTNEED or MADV_REMOVE and the specified address range includes locked, Huge TLB pages, or VM_PFNMAP pages.

EINVAL

advice is MADV_MERGEABLE or MADV_UNMERGEABLE, but the kernel was not configured with CONFIG_KSM.

EINVAL

advice is MADV_FREE or MADV_WIPEONFORK but the specified address range includes file, Huge TLB, MAP_SHARED, or VM_PFNMAP ranges.

EIO

(for MADV_WILLNEED) Paging in this area would exceed the process's maximum resident set size.

ENOMEM

(for MADV_WILLNEED) Not enough memory: paging in failed.

ENOMEM

Addresses in the specified range are not currently mapped, or are outside the address space of the process.

EPERM

advice is MADV_HWPOISON, but the caller does not have the CAP_SYS_ADMIN capability.

## VERSIONS
Since Linux 3.18, support for this system call is optional, depending on the setting of the CONFIG_ADVISE_SYSCALLS configuration option.

## CONFORMING TO
madvise() is not specified by any standards. Versions of this system call, implementing a wide variety of advice values, exist on many other implementations. Other implementations typically implement at least the flags listed above under Conventional advice flags, albeit with some variation in semantics.

POSIX.1-2001 describes posix_madvise(3) with constants POSIX_MADV_NORMAL, POSIX_MADV_RANDOM, POSIX_MADV_SEQUENTIAL, POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED, and so on, with behavior close to the similarly named flags listed above.

## NOTES

### Linux notes
The Linux implementation requires that the address addr be page-aligned, and allows length to be zero. If there are some parts of the specified address range that are not mapped, the Linux version of madvise() ignores them and applies the call to the rest (but returns ENOMEM from the system call, as it should).

## SEE ALSO
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2), prctl(2), process_madvise(2), posix_madvise(3), core(5)