KEYCTL(2)

NAME
keyctl - manipulate the kernel's key management facility

SYNOPSIS
#include <sys/types.h>
#include <keyutils.h>

long keyctl(int operation, ...)

/* For direct call via syscall(2): */
#include <asm/unistd.h>
#include <linux/keyctl.h>
#include <unistd.h>

long syscall(__NR_keyctl, int operation, __kernel_ulong_t arg2,
             __kernel_ulong_t arg3, __kernel_ulong_t arg4,
             __kernel_ulong_t arg5);
No glibc wrapper is provided for this system call; see NOTES.

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 non-zero 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. 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. The arg5 argument is reserved and must be 0.

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 non-zero 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 appropriately to indicate the error.

ERRORS

EACCES
    The requested operation wasn't permitted.

EDEADLK
    operation is KEYCTL_LINK and the requested link 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.

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. 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
    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.

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

ENFILE (Linux kernels before 3.13)
    operation is 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 is KEYCTL_UNLINK and the key to be unlinked isn't linked to the keyring.

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).

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

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

EOPNOTSUPP
    operation is KEYCTL_UPDATE and the key type does not support updating.

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.

VERSIONS
This system call first appeared in Linux 2.6.10.

CONFORMING TO
This system call is a nonstandard Linux extension.

NOTES
No wrapper for this system call is provided in glibc. 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.

EXAMPLE
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 <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 */
    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: %lx\n",
            (long) key_to_instantiate);
    fprintf(fp, "  UID:                %ld\n", (long) uid);
    fprintf(fp, "  GID:                %ld\n", (long) gid);
    fprintf(fp, "  thread_keyring:     %lx\n", (long) thread_keyring);
    fprintf(fp, "  process_keyring:    %lx\n", (long) process_keyring);
    fprintf(fp, "  session_keyring:    %lx\n", (long) 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:          %lx\n", (long) 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:  %lx\n", (long) 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_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 Documentation/security/keys.txt and Documentation/security/keys-request-key.txt.

COLOPHON
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