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