linux-kernel - Re: [RFC PATCH ghak100 V1 1/2] audit: avoid fcaps on MNT

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Date:   Mon, 19 Nov 2018 13:47:53 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-audit@...hat.com,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...hat.com>, sgrubb@...hat.com
Subject: Re: [RFC PATCH ghak100 V1 1/2] audit: avoid fcaps on MNT_FORCE

On Fri, Nov 16, 2018 at 6:34 PM Richard Guy Briggs <rgb@...hat.com> wrote:
>
> Don't fetch fcaps when umount2 is called with MNT_FORCE to avoid a
> process hang while it waits for the missing resource to (possibly never)
> re-appear.

The patch would be pretty good if the dependence on MNT_FORCE wasn't
added.   As it is, it's buggy in more ways than one:

 - It does the opposite of the above (i.e. skips fcaps *unless*
MNT_FORCE is set)
 - sets LOOKUP_NO_REVAL from caller of path lookup, which is invalid
(LOOKUP_NO_REVAL is used only internally by path lookup)
 - the fact that *_path_mountpoint_at() shouldn't touch the mount root
is independent of MNT_FORCE

I still don't quite understand what audit is trying to do here, but
apparently it's okay to skip getxattr in the MNT_FORCE case.  So why
is it not okay to skip it in the non-MNT_FORCE case?

Thanks,
Miklos