Per-user persistent keyring
The persistent keyring is a keyring used to anchor keys on behalf of a user. Each UID the kernel deals with has its own persistent keyring that is shared between all threads owned by that UID.
The persistent keyring is created on demand when a thread requests it. The keyring's expiration timer is reset every time it is accessed to the value in:
/proc/sys/kernel/keys/persistent_keyring_expiry
The persistent keyring is not searched by request_key() unless it is referred to by a keyring that is.
The persistent keyring may not be accessed directly, even by processes with the appropriate UID. Instead it must be linked to one of a process's keyrings first before that keyring can access it by virtue of its possessor permits. This is done with keyctl_get_persistent().
Persistent keyrings are independent of clone(), fork(), vfork(), execve() and exit(). They persist until their expiration timers trigger - at which point they are garbage collected. This allows them 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 and user session keyrings).
If a persistent keyring does not exist when it is accessed, it will be created.
The keyutils library provides a special operation for manipulating persistent keyrings:
This operation allows the caller to get the persistent keyring corresponding to their own UID or, if they have CAP_SETUID, the persistent keyring corresponding to some other UID in the same user namespace.