Section (2) add_key

Linux manual pages Section 2  


add_key — add a key to the kernel_zsingle_quotesz_s key management facility


#include <sys/types.h>
#include <keyutils.h>
key_serial_t add_key( const char *type,
  const char *description,
  const void *payload,
  size_t plen,
  key_serial_t keyring);
No glibc wrapper is provided 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_zsingle_quotesz_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_zsingle_quotesz_s thread-specific keyring (thread-keyring(7)).


This specifies the caller_zsingle_quotesz_s process-specific keyring (process-keyring(7)).


This specifies the caller_zsingle_quotesz_s session-specific keyring (session-keyring(7)).


This specifies the caller_zsingle_quotesz_s UID-specific keyring (user-keyring(7)).


This specifies the caller_zsingle_quotesz_s UID-session keyring (user-session-keyring(7)).

Key types

The key type is a string that specifies the key_zsingle_quotesz_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.


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 _zsingle_quotesz_:_zsingle_quotesz_ 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 cause of the error.



The keyring wasn_zsingle_quotesz_t available for modification by the user.


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


One or more of type, description, and payload points outside process_zsingle_quotesz_s accessible address space.


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


The payload data was invalid.


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.


The keyring doesn_zsingle_quotesz_t exist.


Insufficient memory to create a key.


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


type was keyring and the description started with a period (_zsingle_quotesz_._zsingle_quotesz_). 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.


No wrapper for this system call is provided in glibc. 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 program:

$ ./a.out user mykey Some payload
Key ID is 64a4dca
$ grep _zsingle_quotesz_64a4dca_zsingle_quotesz_ /proc/keys
064a4dca I--Q---    1 perm 3f010000  1000  1000 user    mykey: 12

Program source

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

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

    printf(Key ID is %lx
, (long) 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), user-session-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 Documentation/security/keys−request−key.txt).


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    Written by David Howells (
and Copyright (C) 2016 Michael Kerrisk <>

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