ACK: [SRU][FOCAL][PATCH 2/2] UBUNTU: SAUCE: shiftfs: prevent ESTALE for LOOKUP_JUMP lookups

[Seth Forshee]

On Tue, Jun 23, 2020 at 07:46:16PM +0200, Christian Brauner wrote:
> BugLink: https://bugs.launchpad.net/bugs/1872757
> 
> Users reported that creating temporary files shiftfs reports ESTALE.
> This can be reproduced via:
> 
> import tempfile
> import os
> 
> def test():
>     with tempfile.TemporaryFile() as fd:
>         fd.write("data".encode('utf-8'))
>         # re-open the file to get a read-only file descriptor
>         return open(f"/proc/self/fd/{fd.fileno()}", "r")
> 
> def main():
>    fd = test()
>    fd.close()
> 
> if __name__ == "__main__":
>     main()
> 
> a similar issue was reported here:
> https://github.com/systemd/systemd/issues/14861
> 
> Our revalidate methods were very opinionated about whether or not a
> lower dentry was valid especially when it became unlinked we simply
> invalidated the lower dentry which caused above bug to surface. This has
> led to bugs where a ESTALE was returned for e.g.  temporary files that
> were created and directly re-opened afterwards through
> /proc/<pid>/fd/<nr-of-deleted-file>. When a file is re-opened through
> /proc/<pid>/fd/<nr> LOOKUP_JUMP is set and the vfs will revalidate via
> d_weak_revalidate(). Since the file has been unhashed or even already
> gone negative we'd fail the open when we should've succeeded.
> 
> Reported-by: Christian Kellner <ckellner at redhat.com>
> Reported-by: Evgeny Vereshchagin <evvers at ya.ru>
> Cc: Seth Forshee <seth.forshee at canonical.com>
> Link: https://github.com/systemd/systemd/issues/14861
> Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>

This looks okay to me, and no other stacked filesystem checks i_nlink of
the lower filesystem inode.

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