From 6b3a0480cf2de402abce168aa0b093a8dc4f7a57 Mon Sep 17 00:00:00 2001

Message-Id: <6b3a0480cf2de402abce168aa0b093a8dc4f7a57@dist-git>

From: Michal Privoznik <mprivozn@redhat.com>

Date: Tue, 6 Sep 2022 13:43:22 +0200

Subject: [PATCH] qemu_namespace: Fix a corner case in

 qemuDomainGetPreservedMounts()

When setting up namespace for QEMU we look at mount points under

/dev (like /dev/pts, /dev/mqueue/, etc.) because we want to

preserve those (which is done by moving them to a temp location,

unshare(), and then moving them back). We have a convenience

helper - qemuDomainGetPreservedMounts() - that processes the

mount table and (optionally) moves the other filesystems too.

This helper is also used when attempting to create a path in NS,

because the path, while starting with "/dev/" prefix, may

actually lead to one of those filesystems that we preserved.

And here comes the corner case: while we require the parent mount

table to be in shared mode (equivalent of `mount --make-rshared /'),

these mount events propagate iff the target path exist inside the

slave mount table (= QEMU's private namespace). And since we

create only a subset of /dev nodes, well, that assumption is not

always the case.

For instance, assume that a domain is already running, no

hugepages were configured for it nor any hugetlbfs is mounted.

Now, when a hugetlbfs is mounted into '/dev/hugepages', this is

propagated into the QEMU's namespace, but since the target dir

does not exist in the private /dev, the FS is not mounted in the

namespace.

Fortunately, this difference between namespaces is visible when

comparing /proc/mounts and /proc/$PID/mounts (where PID is the

QEMU's PID). Therefore, if possible we should look at the latter.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

(cherry picked from commit 46b03819ae8d833b11c2aaccb2c2a0361727f51b)

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2152083

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

---

 src/qemu/qemu_namespace.c | 10 +++++++++-

 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/qemu/qemu_namespace.c b/src/qemu/qemu_namespace.c

index 71e3366ca5..807ec37c91 100644

--- a/src/qemu/qemu_namespace.c

+++ b/src/qemu/qemu_namespace.c

@@ -109,6 +109,8 @@ qemuDomainGetPreservedMountPath(virQEMUDriverConfig *cfg,

  * b) generate backup path for all the entries in a)

  *

  * Any of the return pointers can be NULL. Both arrays are NULL-terminated.

+ * Get the mount table either from @vm's PID (if running), or from the

+ * namespace we're in (if @vm's not running).

  *

  * Returns 0 on success, -1 otherwise (with error reported)

  */

@@ -123,12 +125,18 @@ qemuDomainGetPreservedMounts(virQEMUDriverConfig *cfg,

     size_t nmounts = 0;

     g_auto(GStrv) paths = NULL;

     g_auto(GStrv) savePaths = NULL;

+    g_autofree char *mountsPath = NULL;

     size_t i;

     if (ndevPath)

         *ndevPath = 0;

-    if (virFileGetMountSubtree(QEMU_PROC_MOUNTS, "/dev", &mounts, &nmounts) < 0)

+    if (vm->pid > 0)

+        mountsPath = g_strdup_printf("/proc/%lld/mounts", (long long) vm->pid);

+    else

+        mountsPath = g_strdup(QEMU_PROC_MOUNTS);

+

+    if (virFileGetMountSubtree(mountsPath, "/dev", &mounts, &nmounts) < 0)

         return -1;

     if (nmounts == 0)

-- 

2.39.0