linux-kernel - Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry

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Date:   Mon, 12 Mar 2018 18:28:30 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())

Al Viro <viro@...IV.linux.org.uk> writes:

> 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.

If I am parsing the code correctly path_connected is broken for nfsv2
and nfsv3 when NFS_MOUNT_UNSHARED is not set.   nfsv4 appears to make
a kernel mount of the real root of the filesystem properly setting
s_root and then finds the child it is mounting.

> 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().

Agreed.

Which means that if you mount server:/foo/bar/baz first and then
mount server:/foo with an appropriate rename you might be able
to see all of server:/foo or possibly server:/

Hmm..

Given that nfs_kill_super calls generic_shutdown_super and
generic_shutdown_super calls shrink_dcache_for_umount
I would argue that nfsv2 and nfsv3 are buggy in the same
case, as shrink_dcache_for_umount is called on something
that is not the root of the filesystem's dentry tree.

Eric