Re: [RFC Patch 0/2] Improvements to stack traces

[Keir Fraser <keir.xen@gmail.com>]

On 08/08/2013 17:19, "Andrew Cooper" <andrew.cooper3@citrix.com> wrote:

> This series is RFC for two reasons; firstly because I have not dev-tested it
> yet, but mainly because of a specific question.
> 
> In the algorithm using frame pointers, the lower bound is adjusted by two
> words from the provided stack pointer.
> 
> This appears to be the behaiour right from its introduction in:
> 
>     commit aa24d38a469b59abf1b95b732b6ea9ed86e511cf
>     Author: kaf24@firebug.cl.cam.ac.uk <kaf24@firebug.cl.cam.ac.uk>
>     Date:   Thu Sep 1 15:31:12 2005 +0000
> 
> What is the reason for the adjustment?  Tim and I couldn't think of a case
> where a valid frame pointer could be outside the stack. Any well formed use of
> frame pointers should require the callee to push the old frame pointer at
> entry, and pop it on right before exit.
> 
> Am I missing something obvious?
> 
> The potential problem comes in the stack overflow case, where rsp points to
> the boundary of the primary stack, and rbp points just below it, at which
> point the bounday condition will pass but referencing rbp will cause a triple
> fault.
> 
> This can be detected and worked around, but if the adjustment is erronious
> then by far the easiest solution is to just discard the adjustment.

I think it was just an attempt at paranoia when I implemented this. I'm
happy for it to be ripped out.

 -- Keir

> ~Andrew
> 
> CC: Keir Fraser <keir@xen.org>
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Tim Deegan <tim@xen.org>
> 
> --
> 1.7.10.4
> 
> 
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