The user keyring is a keyring used to anchor keys on behalf of a user. Each UID the kernel deals with has its own user keyring. This keyring is associated with the record that the kernel maintains for the UID and, once created, is retained as long as that record persists. It is shared amongst all processes of that UID.
The user keyring is created on demand when a thread requests it. Normally, this happens when pam_keyinit is invoked when a user logs in.
The user keyring is not searched by default by request_key(). When the pam_keyinit module creates a session keyring, it adds to it a link to the user keyring so that the user keyring will be searched when the session keyring is.
A special serial number value, KEY_SPEC_USER_KEYRING, is defined that can be used in lieu of the calling process's user keyring's actual serial number.
From the keyctl utility, '@u' can be used instead of a numeric key ID in much the same way.
User keyrings are independent of clone(), fork(), vfork(), execve() and exit() excepting that the keyring is destroyed when the UID record is destroyed when the last process pinning it exits.
If it necessary to for a key associated with a user to exist beyond the UID record being garbage collected - for example for use by a cron script - then the persistent keyring should be used instead.
If a user keyring does not exist when it is accessed, it will be created.