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Re: [PATCH] dump_stack() based on prologue code analysis

To: David Daney <ddaney@avtrex.com>
Subject: Re: [PATCH] dump_stack() based on prologue code analysis
From: Thiemo Seufer <ths@networkno.de>
Date: Fri, 28 Jul 2006 20:31:27 +0100
Cc: Nigel Stephens <nigel@mips.com>, Atsushi Nemoto <anemo@mba.ocn.ne.jp>, vagabon.xyz@gmail.com, linux-mips@linux-mips.org, ralf@linux-mips.org
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David Daney wrote:
> Nigel Stephens wrote:
> >
> >
> >David Daney wrote:
> >
> >>Atsushi Nemoto wrote:
> >>
> >>>On Thu, 27 Jul 2006 20:12:45 +0100, Thiemo Seufer <ths@networkno.de> 
> >>>wrote:
> >>>
> >>>>IOW, binary analysis can't be expected to provide full accuracy, but
> >>>>we can live with a reasonable approximation, I think.
> >>>
> >>>
> >>>
> >>>Yes, this is a starting point.
> >>>
> >>>The patch (and current mips get_wchan() implementation) tries to do is
> >>>what I used to do to analyze stack dump by hand.
> >>>
> >>>1. Determine PC and SP.
> >>>2. Disassemble a function containing the PC address.
> >>>3. If the function is leaf, make use RA for new PC.
> >>
> >>
> >>This was always the tricky part for me.  How do you know if the 
> >>function is a leaf?
> >>
> >
> >I think that if you cannot find a store instruction which saves RA to 
> >the stack -- either because it's a real leaf and there is no such store, 
> >or because the PC hasn't yet reached the store instruction -- then in 
> >both cases it can be treated as a leaf.
> 
> Presumably you are walking the code back from the PC until you find the 
> prolog.  How would you tell if you had gone past the beginning of a leaf 
> function?  If you find a j $31 you might assume that it was the end of 
> the previous function.
> 
> But that does not work if you are in a function that has multiple return 
> points.  On encountering a j $31 you have no way of telling if you are 
> in a leaf function or a non-leaf function with multiple return points.

... or in a fragment of a single return function which was moved out
of the hot path, after the return point.


Thiemo

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