USER-SESSION-KEYRING(7) Linux Programmer's Manual USER-SESSION-KEYRING(7)
user-session-keyring - per-user default session keyring
The user session keyring is a keyring used to anchor keys on behalf of
a user. Each UID the kernel deals with has its own user session
keyring that is shared by all processes with that UID. The user ses‐
sion keyring has a name (description) of the form _uid_ses.<UID> where
<UID> is the user ID of the corresponding user.
The user session keyring is associated with the record that the kernel
maintains for the UID. It comes into existence upon the first attempt
to access either the user session keyring, the user-keyring(7), or the
session-keyring(7). The keyring remains pinned in existence so long as
there are processes running with that real UID or files opened by those
processes remain open. (The keyring can also be pinned indefinitely by
linking it into another keyring.)
The user session keyring is created on demand when a thread requests it
or when a thread asks for its session-keyring(7) and that keyring
doesn't exist. In the latter case, a user session keyring will be cre‐
ated and, if the session keyring wasn't to be created, the user session
keyring will be set as the process's actual session keyring.
The user session keyring is searched by request_key(2) if the actual
session keyring does not exist and is ignored otherwise.
A special serial number value, KEY_SPEC_USER_SESSION_KEYRING, is
defined that can be used in lieu of the actual serial number of the
calling process's user session keyring.
From the keyctl(1) utility, '@us' can be used instead of a numeric key
ID in much the same way.
User session keyrings are independent of clone(2), fork(2), vfork(2),
execve(2), and _exit(2) excepting that the keyring is destroyed when
the UID record is destroyed when the last process pinning it exits.
If a user session keyring does not exist when it is accessed, it will
Rather than relying on the user session keyring, it is strongly recom‐
mended—especially if the process is running as root—that a session-
keyring(7) be set explicitly, for example by pam_keyinit(8).
The user session keyring was added to support situations where a
process doesn't have a session keyring, perhaps because it was created
via a pathway that didn't involve PAM (e.g., perhaps it was a daemon
started by inetd(8)). In such a scenario, the user session keyring
acts as a substitute for the session-keyring(7).
keyctl(1), keyctl(3), keyrings(7), persistent-keyring(7),
process-keyring(7), session-keyring(7), thread-keyring(7),
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Linux 2017-03-13 USER-SESSION-KEYRING(7)