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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 (thread-keyring(7)).
This specifies the caller's process-specific keyring (process-keyring(7)).
This specifies the caller's session-specific keyring (session-keyring(7)).
This specifies the caller's UID-specific keyring (user-keyring(7)).
This specifies the caller's UID-session keyring (user-session-keyring(7)).

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.
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 error.


The keyring wasn'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'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't exist.
Insufficient memory to create a key.
The type started with a period ('.'). Key types that begin with a period are reserved to the implementation.
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 library. (The accompanying package provides the <keyutils.h> header file.) 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 '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), 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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2021-08-27 Linux