linux-kernel - Re: fs: NULL deref in atime_needs

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Date:	Mon, 29 Feb 2016 08:45:37 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>, Ian Kent <raven@...maw.net>,
	Mickaël Salaün <mic@...ikod.net>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: fs: NULL deref in atime_needs_update

On Mon, Feb 29, 2016 at 5:09 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> The more I look at the situation with d_is_...() wrt barriers and ->d_seq,
> the less I understand it; outside of RCU mode we don't really need the
> barriers for that stuff and in RCU mode ->d_flags handling had been
> a serious headache all along...

Yeah, one of my least favorite "recent" vfs improvements.

> I'm tempted to do as below .. [ changing it to be unde the seqlock ]
>
> David, Linus, do you see any problems with that?  To me it looks saner
> that way and as cheap as the current code, but I might be missing something
> here...

I'd absolutely love to see this. The memory ordering for the flags
updates and reading was always really confusing, and I hated how it
was hidden inside the random access functions. And apparently it
wasn't just confusing, it was buggy too.

But I'd love it _more_ if this also means that we can get rid of the
rmb's, which your patch didn't. Can we? Or does the ordering still
remain for some other issue?

            Linus