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logind: save /run/systemd/users/UID before starting user@.service

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Previously, this had a race condition during a user's first login.
Some component calls CreateSession (most likely by a PAM service
other than 'systemd-user' running pam_systemd), with the following
results:

- logind:
  * create the user's XDG_RUNTIME_DIR
  * tell pid 1 to create user-UID.slice
  * tell pid 1 to start user@UID.service

Then these two processes race:

- logind:
  * save information including XDG_RUNTIME_DIR to /run/systemd/users/UID

- the subprocess of pid 1 responsible for user@service:
  * start a 'systemd-user' PAM session, which reads XDG_RUNTIME_DIR
    and puts it in the environment
  * run systemd --user, which requires XDG_RUNTIME_DIR in the
    environment

If logind wins the race, which usually happens, everything is fine;
but if the subprocesses of pid 1 win the race, which can happen
under load, then systemd --user exits unsuccessfully.

To avoid this race, we have to write out /run/systemd/users/UID
even though the service has not "officially" started yet;
previously this did an early-return without saving anything.
Record its state as OPENING in this case.

Bug: https://github.com/systemd/systemd/issues/232
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
This commit is contained in:
Simon McVittie 2015-06-17 16:45:49 +01:00
parent eef8c1f6fd
commit 71161305f1
1 changed files with 17 additions and 5 deletions
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22
src/login/logind-user.c
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@ -105,7 +105,7 @@ void user_free(User *u) {
free(u);
}
int user_save(User *u) {
static int user_save_internal(User *u) {
_cleanup_free_ char *temp_path = NULL;
_cleanup_fclose_ FILE *f = NULL;
int r;
@ -113,9 +113,6 @@ int user_save(User *u) {
assert(u);
assert(u->state_file);
if (!u->started)
return 0;
r = mkdir_safe_label("/run/systemd/users", 0755, 0, 0);
if (r < 0)
goto finish;
@ -258,6 +255,15 @@ finish:
return r;
}
int user_save(User *u) {
assert(u);
if (!u->started)
return 0;
return user_save_internal (u);
}
int user_load(User *u) {
_cleanup_free_ char *display = NULL, *realtime = NULL, *monotonic = NULL;
Session *s = NULL;
@ -453,6 +459,12 @@ int user_start(User *u) {
if (r < 0)
return r;
/* Save the user data so far, because pam_systemd will read the
* XDG_RUNTIME_DIR out of it while starting up systemd --user.
* We need to do user_save_internal() because we have not
* "officially" started yet. */
user_save_internal(u);
/* Spawn user systemd */
r = user_start_service(u);
if (r < 0)
@ -704,7 +716,7 @@ UserState user_get_state(User *u) {
if (u->stopping)
return USER_CLOSING;
if (u->slice_job || u->service_job)
if (!u->started || u->slice_job || u->service_job)
return USER_OPENING;
if (u->sessions) {