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PERSISTENT-KEYRING(7)      Linux Programmer's ManualPERSISTENT-KEYRING(7)NAME       persistent-keyring - per-user persistent keyringDESCRIPTION       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 has a name (description) of the  form  _persistent.<UID>  where       <UID> is the user ID of the corresponding user.       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       thekeyctl_get_persistent(3) function.       If  a  persistent  keyring  does  not  exist when it is accessed by thekeyctl_get_persistent(3) operation, it will be automatically created.       Each time thekeyctl_get_persistent(3) operation is performed, the per-       sistent key's expiration timer is reset to the value in:           /proc/sys/kernel/keys/persistent_keyring_expiry       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 tokeyctl_get_persistent(3).       The  persistent  keyring is not directly searched byrequest_key(2); it       is searched only if it is linked into  one  of  the  keyrings  that  is       searched byrequest_key(2).       The  persistent  keyring is independent ofclone(2),fork(2),vfork(2),execve(2), and_exit(2).  It persists until its expiration timer  trig-       gers,  at which point it is garbage collected.  This allows the persis-       tent keyring to carry keys beyond the life of the  kernel's  record  of       the corresponding UID (the destruction of which results in the destruc-       tion of theuser-keyring(7) and theuser-session-keyring(7)).  The per-       sistent keyring can thus be used to hold authentication tokens for pro-       cesses that run without user interaction, such as programs  started  bycron(8).       The persistent keyring is used to store UID-specific objects that them-       selves 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.   Special operations       The keyutils library provides thekeyctl_get_persistent(3) function for       manipulating persistent keyrings.  (This function is  an  interface  to       thekeyctl(2) KEYCTL_GET_PERSISTENT operation.)  This operation allows       the calling thread to get the persistent keyring corresponding  to  its       own UID or, if the thread has the CAP_SETUID capability, the persistent       keyring corresponding to some other UID in the same user namespace.NOTES       Each user namespace owns a  keyring  called  .persistent_register  that       contains  links  to all of the persistent keys in that namespace.  (The       .persistent_register keyring can be seen when reading the  contents  of       the   /proc/keys   file   for   the  UID  0  in  the  namespace.)   Thekeyctl_get_persistent(3) operation looks for a key with a name  of  the       form  _persistent.<UID> in that keyring, creates the key if it does not       exist, and links it into the keyring.SEE ALSOkeyctl(1),keyctl(3),keyctl_get_persistent(3),keyrings(7),process-keyring(7),session-keyring(7),thread-keyring(7),user-keyring(7),user-session-keyring(7)COLOPHON       This page is part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs, and the       latest version of this page, can be found at       https://www.kernel.org/doc/man-pages/.Linux                             2017-03-13PERSISTENT-KEYRING(7)