mount-util: don't trigger automounts when cloning submounts

get_sub_mounts() clones each submount of the given prefix with
OPEN_TREE_CLONE. The kernel resolves the path of an OPEN_TREE_CLONE
with LOOKUP_AUTOMOUNT, i.e. if the submount is an autofs automount
point that has not been triggered yet, cloning it forces the automount
to trigger, and open_tree() blocks until the automount request has
been served.

This is particularly problematic during boot: setting up a private
/proc for the first sandboxed service (e.g. systemd-userdbd.service,
which uses ProtectProc=invisible) clones the submounts of /proc, which
include PID 1's own /proc/sys/fs/binfmt_misc automount point. The
executor then blocks until PID 1 gets around to dispatching the
resulting proc-sys-fs-binfmt_misc.mount job, which competes with the
ongoing boot transaction. On a Fedora 44 VM this delayed
systemd-userdbd.service by ~0.9s, and with it every early-boot NSS
user/group lookup that ends up in nss-systemd's varlink queries — most
importantly systemd-tmpfiles-setup-dev-early.service, which
systemd-udevd.service is ordered after, stalling the whole boot
critical path:

  [2.131846] proc-sys-fs-binfmt_misc.automount: Got automount request
             for /proc/sys/fs/binfmt_misc, triggered by 323 ((systemd-userd))
  [2.943574] Mounting proc-sys-fs-binfmt_misc.mount...
  [2.968507] Mounted proc-sys-fs-binfmt_misc.mount.

Triggering the automount here also defeats its purpose, since
binfmt_misc ends up mounted on every boot even if nothing ever
accesses it.

Pass AT_NO_AUTOMOUNT so that untriggered automount points are cloned
as they are instead.

Before (Fedora 44 VM, 4 vCPUs):
  Startup finished in ... + 2.559s (userspace)
    1.058s systemd-tmpfiles-setup-dev-early.service
     938ms systemd-userdbd.service

After:
  Startup finished in ... + 1.582s (userspace)
     137ms systemd-tmpfiles-setup-dev-early.service
      22ms systemd-userdbd.service

diff --git a/src/shared/mount-util.c b/src/shared/mount-util.c
index 20eda5b0ef3a978e41ab80efdfc5a6f1a8b42eac..543682f11179bf1c3ced06e1e1671c0f154c4160 100644 (file)
--- a/src/shared/mount-util.c
+++ b/src/shared/mount-util.c
@@ -1841,13 +1841,21 @@ int get_sub_mounts(const char *prefix, SubMount **ret_mounts, size_t *ret_n_moun
                 /* If possible on a newer kernel, use MS_PRIVATE to decouple it from the original mount.
                  * Otherwise MNT_DETACH of the source path could propagate through and unmount the
                  * just-moved nested children at the destination (relevant for preserving nested mounts
-                 * under sysext hierarchies). */
+                 * under sysext hierarchies).
+                 *
+                 * Also, pass AT_NO_AUTOMOUNT so that we clone automount points (i.e. autofs mounts) as
+                 * they are, instead of triggering them. OPEN_TREE_CLONE would otherwise force them to be
+                 * mounted, and — worse — block until the automount request has been served. If the
+                 * automount point is managed by PID 1 itself (as is the case for systemd's own
+                 * /proc/sys/fs/binfmt_misc automount point, which is cloned whenever a private /proc is
+                 * set up for a service) this can take a long time during boot, since the resulting mount
+                 * job competes with the ongoing boot transaction. */
                 static bool mount_attr_unsupported = false;
 
                 if (!mount_attr_unsupported) {
                         mount_fd = open_tree_attr_with_fallback(
                                         AT_FDCWD, path,
-                                        OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC|AT_RECURSIVE,
+                                        OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC|AT_RECURSIVE|AT_NO_AUTOMOUNT,
                                         &(struct mount_attr) { .propagation = MS_PRIVATE });
                         if (mount_fd == -ENOENT) /* The path may be hidden by another over-mount or already unmounted. */
                                 continue;
@@ -1861,7 +1869,7 @@ int get_sub_mounts(const char *prefix, SubMount **ret_mounts, size_t *ret_n_moun
                 }
 
                 if (mount_attr_unsupported) {
-                        mount_fd = RET_NERRNO(open_tree(AT_FDCWD, path, OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC|AT_RECURSIVE));
+                        mount_fd = RET_NERRNO(open_tree(AT_FDCWD, path, OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC|AT_RECURSIVE|AT_NO_AUTOMOUNT));
                         if (mount_fd == -ENOENT)
                                 continue;
                         if (mount_fd < 0)