ADD_KEY(2)                 Linux Key Management Calls                 ADD_KEY(2)

       add_key - add a key to the kernel's key management facility

       #include <keyutils.h>

       key_serial_t add_key(const char *type, const char *description,
                            const void *payload, size_t plen,
                            key_serial_t keyring);

       Note: There is no glibc wrapper for this system call; see NOTES.

       add_key() creates or updates a key of the given type and description,
       instantiates it with the payload of length plen, attaches it to the
       nominated keyring, and returns the key's serial number.

       The key may be rejected if the provided data is in the wrong format or it
       is invalid in some other way.

       If the destination keyring already contains a key that matches the
       specified type and description, then, if the key type supports it, that
       key will be updated rather than a new key being created; if not, a new
       key (with a different ID) will be created and it will displace the link
       to the extant key from the keyring.

       The destination keyring serial number may be that of a valid keyring for
       which the caller has write permission.  Alternatively, it may be one of
       the following special keyring IDs:

              This specifies the caller's thread-specific keyring

              This specifies the caller's process-specific keyring

              This specifies the caller's session-specific keyring

              This specifies the caller's UID-specific keyring

              This specifies the caller's UID-session keyring

   Key types
       The key type is a string that specifies the key's type.  Internally, the
       kernel defines a number of key types that are available in the core key
       management code.  Among the types that are available for user-space use
       and can be specified as the type argument to add_key() are the following:

              Keyrings are special key types that may contain links to sequences
              of other keys of any type.  If this interface is used to create a
              keyring, then payload should be NULL and plen should be zero.

       "user" This is a general purpose key type whose payload may be read and
              updated by user-space applications.  The key is kept entirely
              within kernel memory.  The payload for keys of this type is a blob
              of arbitrary data of up to 32,767 bytes.

       "logon" (since Linux 3.3)
              This key type is essentially the same as "user", but it does not
              permit the key to read.  This is suitable for storing payloads
              that you do not want to be readable from user space.

       This key type vets the description to ensure that it is qualified by a
       "service" prefix, by checking to ensure that the description contains a
       ':' that is preceded by other characters.

       "big_key" (since Linux 3.13)
              This key type is similar to "user", but may hold a payload of up
              to 1 MiB.  If the key payload is large enough, then it may be
              stored encrypted in tmpfs (which can be swapped out) rather than
              kernel memory.

       For further details on these key types, see keyrings(7).

       On success, add_key() returns the serial number of the key it created or
       updated.  On error, -1 is returned and errno is set to indicate the

       EACCES The keyring wasn't available for modification by the user.

       EDQUOT The key quota for this user would be exceeded by creating this key
              or linking it to the keyring.

       EFAULT One or more of type, description, and payload points outside
              process's accessible address space.

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

       EINVAL The payload data was invalid.

       EINVAL type was "logon" and the description was not qualified with a
              prefix string of the form "service:".

              The keyring has expired.

              The keyring has been revoked.

       ENOKEY The keyring doesn't exist.

       ENOMEM Insufficient memory to create a key.

       EPERM  The type started with a period ('.').  Key types that begin with a
              period are reserved to the implementation.

       EPERM  type was "keyring" and the description started with a period
              ('.').  Keyrings with descriptions (names) that begin with a
              period are reserved to the implementation.

       This system call first appeared in Linux 2.6.10.

       This system call is a nonstandard Linux extension.

       Glibc does not provide a wrapper for this system call.  A wrapper is
       provided in the libkeyutils package.  When employing the wrapper in that
       library, link with -lkeyutils.

       The program below creates a key with the type, description, and payload
       specified in its command-line arguments, and links that key into the
       session keyring.  The following shell session demonstrates the use of the

           $ ./a.out user mykey "Some payload"
           Key ID is 64a4dca
           $ grep '64a4dca' /proc/keys
           064a4dca I--Q---    1 perm 3f010000  1000  1000 user    mykey: 12

   Program source

       #include <sys/types.h>
       #include <keyutils.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>

       main(int argc, char *argv[])
           key_serial_t key;

           if (argc != 4) {
               fprintf(stderr, "Usage: %s type description payload\n",

           key = add_key(argv[1], argv[2], argv[3], strlen(argv[3]),
           if (key == -1) {

           printf("Key ID is %jx\n", (uintmax_t) key);


       keyctl(1), keyctl(2), request_key(2), keyctl(3), keyrings(7),
       keyutils(7), persistent-keyring(7), process-keyring(7),
       session-keyring(7), thread-keyring(7), user-keyring(7),

       The kernel source files Documentation/security/keys/core.rst and
       Documentation/keys/request-key.rst (or, before Linux 4.13, in the files
       Documentation/security/keys.txt and

       This page is part of release 5.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22                         ADD_KEY(2)