|PERSISTENT-KEYRING(7)||Linux Programmer's Manual||PERSISTENT-KEYRING(7)|
The persistent keyring may not be accessed directly, even by processes with the appropriate UID. Instead, it must first be linked to one of a process's keyrings, before that keyring can access the persistent keyring by virtue of its possessor permits. This linking is done with the keyctl_get_persistent(3) function.
If a persistent keyring does not exist when it is accessed by the keyctl_get_persistent(3) operation, it will be automatically created.
Each time the keyctl_get_persistent(3) operation is performed, the persistent key's expiration timer is reset to the value in:
Should the timeout be reached, the persistent keyring will be removed and everything it pins can then be garbage collected. The key will then be re-created on a subsequent call to keyctl_get_persistent(3).
The persistent keyring is not directly searched by request_key(2); it is searched only if it is linked into one of the keyrings that is searched by request_key(2).
The persistent keyring is independent of clone(2), fork(2), vfork(2), execve(2), and _exit(2). It persists until its expiration timer triggers, at which point it is garbage collected. This allows the persistent keyring to carry keys beyond the life of the kernel's record of the corresponding UID (the destruction of which results in the destruction of the user-keyring(7) and the user-session-keyring(7)). The persistent keyring can thus be used to hold authentication tokens for processes that run without user interaction, such as programs started by cron(8).
The persistent keyring is used to store UID-specific objects that themselves have limited lifetimes (e.g., kerberos tokens). If those tokens cease to be used (i.e., the persistent keyring is not accessed), then the timeout of the persistent keyring ensures that the corresponding objects are automatically discarded.