[PATCH 2/4] qemu_namespace: Fix a corner case in qemuDomainGetPreservedMounts()

[Michal Privoznik]

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 at 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)
-- 
2.35.1