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Date:   Mon, 20 Jan 2020 19:35:58 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc:     Pali =?iso-8859-1?Q?Roh=E1r?= <pali.rohar@gmail.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        Namjae Jeon <linkinjeon@gmail.com>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: oopsably broken case-insensitive support in ext4 and f2fs (Re:
 vfat: Broken case-insensitive support for UTF-8)
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On Mon, Jan 20, 2020 at 08:07:21AM +0000, Al Viro wrote:

> > > I hadn't checked ->d_compare() instances for a while; somebody needs to
> > > do that again, by the look of it.  The above definitely is broken;
> > > no idea how many other instaces had grown such bugs...
> > 
> > f2fs one also has the same bug.  Anyway, I'm going down right now, will
> > check the rest tomorrow morning...
> 
> We _probably_ can get away with just checking that inode for NULL and
> buggering off if it is (->d_seq mismatch is guaranteed in that case),
> but I suspect that we might need READ_ONCE() on both dereferences.
> I hate memory barriers...

FWIW, other instances seem to be OK; HFS+ one might or might not be
OK in the face of concurrent rename (wrong result in that case is
no problem; oops would be), but it doesn't play silly buggers with
pointer-chasing.

ext4 and f2fs do, and ->d_compare() is broken in both of them.