linux-kernel - Re: tty-related oops in latest kernel(s)?

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Date:	Tue, 29 May 2007 21:57:23 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Tero Roponen" <teanropo@....fi>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Andy Whitcroft" <apw@...dowen.org>
Subject: Re: tty-related oops in latest kernel(s)?

Hi Tero,

On 5/29/07, Tero Roponen <teanropo@....fi> wrote:
> FYI, I just tested 2.6.21.3. I couldn't reproduce the problem with
> that kernel.

Well, I went through all tty related patches that went in after 2.6.21
and didn't really find anything interesting, except this:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=543691a6cd70b606dd9bed5e77b120c5d9c5c506

But it does seem correct. You could try reverting that from 2.6.22-rc3
and see if you can trigger the bug.

Looking at the original report again:

> release_dev: driver.table[3] not tty for (tty4)

So tty from filp->private_data does not match
tty->driver->ttys[tty->index] and release_dev bails out (thus messing
up reference count).

> Warning: dev (tty4) tty->count(3) != #fd's(2) in release_dev
> release_dev: driver.table[3] not tty for (tty4)

Presumably someone tries to close the file again which is why we get a
new complaint that reference counting has gone bad.

Unfortunately, I have no idea why drivers->tty does not match. It
could be a race with release_tty() or real use-after-free but I am
unable to find anything obvious in 2.6.21 -> 2.6.22-rc3 that would
break it. Doing the git bisect dance here would really help...
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