linux-kernel - Re: [patch 09/35 error-handling] reiserfs: add locking around error buffer

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Message-ID: <49D11E57.7060502@suse.com>
Date:	Mon, 30 Mar 2009 15:32:39 -0400
From:	Jeff Mahoney <jeffm@...e.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	ReiserFS Development List <reiserfs-devel@...r.kernel.org>
Subject: Re: [patch 09/35 error-handling] reiserfs: add locking around error
 buffer

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Andi Kleen wrote:
> Jeff Mahoney <jeffm@...e.com> writes:
> 
>>  The formatting of the error buffer is race prone. It uses static buffers
>>  for both formatting and output. While overwriting the error buffer
>>  can product garbled output, overwriting the format buffer with incompatible
>>  % directives can cause crashes.
> 
> Really? Just curious but could you describe the steps how a garbled printk can
> leads to a crash?

Yeah, I was surprised by this one as well, but I was convinced by seeing
too many crashes that ended in prepare_error_buf with no other explanation.

Reiserfs has this really hacky way of formatting extended pointer types
where it copies the format buffer into a static buffer. If another
thread copies its format string into the format buffer while the first
thread is formatting its output, it can hit a pointer format that will
consume whatever happens to be on the stack whether it's valid or not.

I posted patches implementing extensible pointer types for printk to
avoid problems like this, but nobody commented on them.
prepare_error_buf() depends on undefined varargs behavior that just
happens to work. Something changed recently where I had to change it
stop consuming args manually, which is in another patch.

- -Jeff

- --
Jeff Mahoney
SUSE Labs
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