linux-kernel - Re: Wanted: simple, safe x86 stack overflow detection

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Message-ID: <20070228204144.GA32316@one.firstfloor.org>
Date:	Wed, 28 Feb 2007 21:41:44 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: Wanted: simple, safe x86 stack overflow detection

On Wed, Feb 28, 2007 at 09:27:09AM -0500, Chuck Ebbert wrote:
> Can we just put a canary in the threadinfo and check it on every
> task switch? What are the drawbacks?

Likely already too late then -- if critical state is overwritten
you crashed before. Also a lot of stack intensive codes
relatively large unused holes so it might miss the canary completely

Anyways if you want a crash on context switch in the non
hole case you can probably get it by just rearranging thread_info a bit.
e.g. put preempt_count first. Any corruption of that will lead
to schedule complaining.

Don't think it is worth it though.

I suppose one could have a CONFIG_DEBUG_STACK_OVERFLOW that gets
the stacks from vmalloc which would catch any overflow with its
guard pages. This is you would need to change __pa() to handle
that too because there might be still some drivers that do
DMA on stack addresses.  Would be somewhat ugly but doable.

But I have my doubts it is worth it again -- in my experience static
analysis works well enough to trace them down and 
there are not that many anyways.

-Andi
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