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systemd/SOURCES/0213-mount-rescan-proc-self...

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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);