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

[Ingo Molnar <mingo@elte.hu>]

* Eric Sandeen <sandeen@redhat.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