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Re: [Libunwind-devel] [RFC][patch] Fix crash in _ULx86_64_tdep_trace whe

From: Lassi Tuura
Subject: Re: [Libunwind-devel] [RFC][patch] Fix crash in _ULx86_64_tdep_trace when sigaltstack is too far away
Date: Fri, 11 Nov 2011 20:59:24 +0100


>> Hum... In principle I agree. I do have the concern about 3 that the layout
>> (and dwarf info) change depending on registers in use, specifically things
>> like whether process is saving expensive (AVX etc.) registers or not.
> I am not sure I understand.
> The sigcontext_addr points to 'ucontext_t', and in the patch we fetch
> the register state from there. Surely you are not suggesting that ucontext_t
> layout changes depending on AVX use?

No, I was just being paranoid... I submitted the bits to use kernel-provided
dwarf info for signal frames on x86_64, instead of the previous heuristic
signal frame detector/unwinder. The previous one caused problems, but I did
not remember off the top of my head what it was about the dwarf-based signal
unwinder that was better.

Now that I thought it over, it's clear the current fast trace wouldn't work
either if what I wrote is true: it freezes the stack deltas on first hit to
the signal frame. I reviewed a few versions of linux kernel signal code, and
it seems it always allocates a fixed amount of space, so indeed it seems safe
assumption for now.

For reference some of the past x86_64 discussions related to this:


You might be interested in some of those related to the x86 version of the
fast trace in the other thread.


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