linux-kernel - Re: [BUG 2.6.36-rc6] list corruption in module_bug

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Message-Id: <201010051448.34984.rusty@rustcorp.com.au>
Date:	Tue, 5 Oct 2010 14:48:34 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Adrian Bunk <bunk@...sta.de>
Subject: Re: [BUG 2.6.36-rc6] list corruption in module_bug_finalize

On Mon, 4 Oct 2010 06:21:08 am Thomas Gleixner wrote:
> Current mainline triggers a list corruption bug in
> module_bug_finalize(). dmesg excerpt below.
> 
> The corresponding code says:
> 
>         /*
>          * Strictly speaking this should have a spinlock to protect against
>          * traversals, but since we only traverse on BUG()s, a spinlock
>          * could potentially lead to deadlock and thus be counter-productive.
>          */
>         list_add(&mod->bug_list, &module_bug_list);
> 
> I can see the traversal problem vs. BUG(), but what's protecting the
> list_add() ? BKL probably did, but is that true anymore ?

I've never even *seen* this code before :(

Looks like it went through Adrian Bunk to Andrew, but despite the fact that
it (foolishly) doesn't touch kernel/module.c, it's generic code and I should
have seen it.  It did change the linux/module.h header.

So, it used to be protected by module_mutex, but Linus and I cleaned that up.

So, we need a lock around this list for adding and removal.  I'd use
list_add_rcu to try to help the lockless traversal too...

And moving it from all the archs into kernel/module.c would be a nice bonus.

Nice catch!
Rusty.
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