[libvirt] [PATCH 1/2] vircgroupv2: Fix VM startup when legacy cgroups are defined

[Pavel Hrdina]

On Thu, Sep 26, 2019 at 05:18:40PM -0400, Cole Robinson wrote:
> On Fedora 31, starting a 'mock' build alters /proc/$pid/cgroup,
> probably due to usage of systemd-nspawn.
> 
> Before:
> $ cat /proc/self/cgroup
> 0::/user.slice/user-1000.slice/...
> 
> After:
> $ cat /proc/self/cgroup
> 1:name=systemd:/
> 0::/user.slice/user-1000.slice/...
> 
> The cgroupv2 code mishandles that first line in the second case, which
> causes VM startup to fail with: Unable to read from
> '/sys/fs/cgroup/machine/cgroup.controllers': No such file or directory
> 
> The kernel docs[1] say that the cgroupv2 path will always start with
> '0::', which in the code here controllers="". Only set the v2 placement
> path when we see that cgroup file entry.
> 
> [1] https://www.kernel.org/doc/html/v5.3/admin-guide/cgroup-v2.html#processes
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1751120
> 
> Signed-off-by: Cole Robinson <crobinso at redhat.com>
> ---
>  src/util/vircgroupv2.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
> index 0663c67190..0cb20e4896 100644
> --- a/src/util/vircgroupv2.c
> +++ b/src/util/vircgroupv2.c
> @@ -194,12 +194,16 @@ virCgroupV2DetectMounts(virCgroupPtr group,
>  static int
>  virCgroupV2DetectPlacement(virCgroupPtr group,
>                             const char *path,
> -                           const char *controllers ATTRIBUTE_UNUSED,
> +                           const char *controllers,
>                             const char *selfpath)
>  {
>      if (group->unified.placement)
>          return 0;
>  
> +    /* controllers="" indicates the cgroupv2 controller path */
> +    if (STRNEQ_NULLABLE(controllers, ""))
> +        return 0;

Fixes the issue reported by several users and looks reasonable.

I would use /* controllers == "" ...*/ to mach the following comment
and there is no need to use STRNEQ_NULLABLE as the caller already
checks if !controllers.

Reviewed-by: Pavel Hrdina <phrdina at redhat.com>
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