ACK: [SRU][F][G][H][PATCH v2] UBUNTU: SAUCE: overlayfs: fix incorrect mnt_id of files opened from map_files

[Seth Forshee]

On Mon, Apr 26, 2021 at 11:11:21AM +0300, alexander at mihalicyn.com wrote:
> From: Alexander Mikhalitsyn <alexander at mihalicyn.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1857257
> 
> The hack was introduced in ("UBUNTU: SAUCE: overlayfs: allow with
> shiftfs as underlay") and it broke checkpoint/restore of docker
> contains:
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1857257
> 
> The following script can be used to trigger the issue:
>   #!/bin/bash
> 
>   cat > test.py << EOF
>   import sys
> 
>   f = open("/proc/self/maps")
> 
>   for l in f.readlines():
>     if "python" not in l:
>       continue
>     print(l)
>     s = l.split()
>     start, end = s[0].split("-")
>     fname = s[-1]
>     print(start, end, fname)
>     break
>   else:
>     sys.exit(1)
> 
>   test_file1 = open(fname)
>   test_file2 = open("/proc/self/map_files/%s-%s" % (start, end))
> 
>   fdinfo1 = open("/proc/self/fdinfo/%d" % test_file1.fileno()).read()
>   fdinfo2 = open("/proc/self/fdinfo/%d" % test_file2.fileno()).read()
> 
>   if fdinfo1 != fdinfo2:
>     print("FAIL")
>     print(test_file1)
>     print(fdinfo1)
>     print(test_file2)
>     print(fdinfo2)
>     sys.exit(1)
>   print("PASS")
>   EOF
>   sudo docker run -it --privileged --rm -v `pwd`:/mnt python python /mnt/test.py
> 
> Thanks to Andrei Vagin for the reproducer and investigation of this problem.
> 
> Cc: Andrei Vagin <avagin at gmail.com>
> Cc: Adrian Reber <areber at redhat.com>
> Cc: Christian Brauner <christian.brauner at ubuntu.com>
> Cc: Stefan Bader <stefan.bader at canonical.com>
> Cc: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski at canonical.com>
> 
> Fixes: d24b8a5 ("UBUNTU: SAUCE: overlayfs: allow with shiftfs as underlay")
> Signed-off-by: Alexander Mikhalitsyn <alexander at mihalicyn.com>

I tested with and without this patch using the xfstests overlay tests
and saw no regressions. The use of vm_prfile is limited to very few use
cases, all within proc, so I think this change should be safe.

Acked-by: Seth Forshee <seth.forshee at canonical.com>