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Re: [libvirt] Potential problem with /proc/mounts items that don't exist inside Kubernetes

On 07/06/2017 12:11 PM, Daniel P. Berrange wrote:
On Thu, Jul 06, 2017 at 11:48:43AM +0200, Juan Hernández wrote:
On 07/06/2017 11:33 AM, Daniel P. Berrange wrote:
On Thu, Jul 06, 2017 at 11:26:58AM +0200, Juan Hernández wrote:
On 07/06/2017 11:18 AM, Daniel P. Berrange wrote:
On Thu, Jul 06, 2017 at 11:11:03AM +0200, Juan Hernández wrote:
Hello all,

This is my first mail to this list, so let me introduce myself. My name is
Juan Hernandez, and I work in the oVirt team. Currently I am experimenting
with the integration between ManageIQ and KubeVirt.

I recently detected a potential issue when running libvirt inside
Kubernetes, as part of KubeVirt. There are entries in /proc/mounts that
don't exist, and libvirt can't start virtual machines because of that. This
is specific to this enviroment, but I think it may be worth addressing it in
libvirt itself. See the following issue for details:

     Libvirt fails when there are hidden cgroup mount points in `/proc/mounts`

I suggested a possible fix there, which seems simple, but it makes all tests
fail. I'd be happy to fix the tests as well, but I would need some guidance
on how to do so. Any suggestion is welcome.

The root cause problem will be the code that parse /proc/mounts. It needs
to pick the last entry in the mounts file, since the earlier ones can be
hidden. For some reason virCgroupDetectMountsFromFile instead picks the
first entry, so that function needs updating todo the reverse.

Is the order of /proc/mounts guaranteed? It may be, but I'd suggest to not
rely on that. Instead of that libvirt could check if the mount point does
actually exist, and skip if it it doesn't. That is the fix I proposed:

The order of /proc/mounts reflects the order in which the mounts were
performed. IOW, later entries will override earlier entries if there
is path overlap.

diff --git a/src/util/vircgroup.c b/src/util/vircgroup.c
index 5aa1db5..021a3f2 100644
--- a/src/util/vircgroup.c
+++ b/src/util/vircgroup.c
@@ -393,6 +393,14 @@ virCgroupDetectMountsFromFile(virCgroupPtr group,
           if (STRNEQ(entry.mnt_type, "cgroup"))

+        /* Some mount points in the /proc/mounts file may be
+         * hidden by others, and may not actually exist from
+         * the point of the view of the process, so we need
+         * to skip them.
+         */
+       if (!virFileExists(entry.mnt_dir))
+            continue;

This is fragile because it is possible for the mount point to
still exist, but for its contents to have been replaced or
hidden. So we really do want to explicitly take only the last
entry, instead of doing this check.

That makes sense to me. Should I open a BZ to request that change?

Yes, its worth tracking this in BZ.

If you wanted to try to write a patch too, that'd be awesome :-)

I created the following BZ:

  Don't use cgroup mount points from /proc/mounts that are hidden

I will assign it to myself, and I will try to create a fix. Should I fail, I will ask for help.

           for (i = 0; i < VIR_CGROUP_CONTROLLER_LAST; i++) {
               const char *typestr = virCgroupControllerTypeToString(i);
               int typelen = strlen(typestr);

That fix makes things work, for it doesn't pass the tests.


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