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mount: rescan /proc/self/mountinfo before processing waitid() results

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(The interesting bits about the what and why are in a comment in the
patch, please have a look there instead of looking here in the commit
msg).

Fixes: #10872
This commit is contained in:
Lennart Poettering 2019-07-17 18:57:13 +02:00
parent fcd8e119c2
commit 350804867d
1 changed files with 27 additions and 3 deletions
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30
src/core/mount.c
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@ -50,6 +50,7 @@ static const UnitActiveState state_translation_table[_MOUNT_STATE_MAX] = {
static int mount_dispatch_timer(sd_event_source *source, usec_t usec, void *userdata);
static int mount_dispatch_io(sd_event_source *source, int fd, uint32_t revents, void *userdata);
static int mount_process_proc_self_mountinfo(Manager *m);
static bool MOUNT_STATE_WITH_PROCESS(MountState state) {
return IN_SET(state,
@ -1280,6 +1281,22 @@ static void mount_sigchld_event(Unit *u, pid_t pid, int code, int status) {
if (pid != m->control_pid)
return;
/* So here's the thing, we really want to know before /usr/bin/mount or /usr/bin/umount exit whether
* they established/remove a mount. This is important when mounting, but even more so when unmounting
* since we need to deal with nested mounts and otherwise cannot safely determine whether to repeat
* the unmounts. In theory, the kernel fires /proc/self/mountinfo changes off before returning from
* the mount() or umount() syscalls, and thus we should see the changes to the proc file before we
* process the waitid() for the /usr/bin/(u)mount processes. However, this is unfortunately racy: we
* have to waitid() for processes using P_ALL (since we need to reap unexpected children that got
* reparented to PID 1), but when using P_ALL we might end up reaping processes that terminated just
* instants ago, i.e. already after our last event loop iteration (i.e. after the last point we might
* have noticed /proc/self/mountinfo events via epoll). This means event loop priorities for
* processing SIGCHLD vs. /proc/self/mountinfo IO events are not as relevant as we want. To fix that
* race, let's explicitly scan /proc/self/mountinfo before we start processing /usr/bin/(u)mount
* dying. It's ugly, but it makes our ordering systematic again, and makes sure we always see
* /proc/self/mountinfo changes before our mount/umount exits. */
(void) mount_process_proc_self_mountinfo(u->manager);
m->control_pid = 0;
if (is_clean_exit(code, status, EXIT_CLEAN_COMMAND, NULL))
@ -1774,16 +1791,14 @@ static int drain_libmount(Manager *m) {
return rescan;
}
static int mount_dispatch_io(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
static int mount_process_proc_self_mountinfo(Manager *m) {
_cleanup_set_free_free_ Set *around = NULL, *gone = NULL;
Manager *m = userdata;
const char *what;
Iterator i;
Unit *u;
int r;
assert(m);
assert(revents & EPOLLIN);
r = drain_libmount(m);
if (r <= 0)
@ -1888,6 +1903,15 @@ static int mount_dispatch_io(sd_event_source *source, int fd, uint32_t revents,
return 0;
}
static int mount_dispatch_io(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
Manager *m = userdata;
assert(m);
assert(revents & EPOLLIN);
return mount_process_proc_self_mountinfo(m);
}
static void mount_reset_failed(Unit *u) {
Mount *m = MOUNT(u);