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Date: Mon, 12 Mar 2018 20:39:17 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: John Ogness <john.ogness@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent()
 breakage when used from shrink_dentry_list())
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On Mon, Mar 12, 2018 at 03:23:44PM -0500, Eric W. Biederman wrote:

> Of the two code paths you are concert about:
> 
> For path path_connected looking at s_root is a heuristic to avoid
> calling is_subdir every time we need to do that check.  If the heuristic
> fails we still have is_subdir which should remain accurate.  If
> is_subdir fails the path is genuinely not connected at that moment
> and failing is the correct thing to do.
 
Umm...  That might be not good enough - the logics is "everything's
reachable from ->s_root anyway, so we might as well not bother checking".
For NFS it's simply not true.

We can mount server:/foo/bar/baz on /tmp/a, then server:/foo on /tmp/b
and we'll have ->s_root pointing to a subtree of what's reachable at
/tmp/b.  Play with renames under /tmp/b and you just might end up with
a problem.  And mount on /tmp/a will be (mistakenly) considered to
be safe, since it satisfies the heuristics in path_connected().