linux-kernel - Re: [PATCH] use canary at end of stack to indicate overruns at oops time

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Message-ID: <20080422084404.GA2388@elte.hu>
Date:	Tue, 22 Apr 2008 10:44:04 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] use canary at end of stack to indicate overruns at
	oops time


* Eric Sandeen <sandeen@...hat.com> wrote:

> Use a canary at the end of the stack to clearly indicate at oops time 
> whether the stack has ever overflowed.
> 
> This is a very simple implementation with a couple of drawbacks:
> 
> 1) a thread may legitimately use exactly up to the last
>    word on the stack
> 
>  -- but the chances of doing this and then oopsing later seem slim

even that narrow case is a bug - an NMI might arrive at exactly that 
point and overflow the stack for real.

> 2) it's possible that the stack usage isn't dense enough
>    that the canary location could get skipped over
> 
>  -- but the worst that happens is that we don't flag the overrun

yeah.

> With the code in place, an intentionally-bloated stack oops does:
> 
> BUG: unable to handle kernel paging request at ffff8103f84cc680
> IP: [<ffffffff810253df>] update_curr+0x9a/0xa8
> PGD 8063 PUD 0 
> Thread overran stack or stack corrupted
> Oops: 0000 [1] SMP 
> CPU 0 
> ...

excellent. I've queued this up, it's definitely an improvement in 
debuggability.

we used to have something comparable in ancient kernels (it was called 
the stack red zone IIRC) - but it was not printed in oopses and we lost 
the feature somewhere anyway.

	Ingo
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