From 4bc21bbc61acd1ce114da381a9742f6bcd4ffde8 Mon Sep 17 00:00:00 2001

From: Lennart Poettering <lennart@poettering.net>

Date: Wed, 17 Jul 2019 18:57:13 +0200

Subject: [PATCH] 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

(cherry picked from commit 350804867dbcc9b7ccabae1187d730d37e2d8a21)

Conflicts:

	src/core/mount.c

Resolves: #1696178

---

 src/core/mount.c | 30 +++++++++++++++++++++++++++---

 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/src/core/mount.c b/src/core/mount.c

index 85b07375e2..2ac04e3874 100644

--- a/src/core/mount.c

+++ b/src/core/mount.c

@@ -53,6 +53,7 @@ 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,

@@ -1241,6 +1242,22 @@ 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))

@@ -1781,16 +1798,14 @@ 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_ 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)

@@ -1898,6 +1913,15 @@ 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);