Re: Stack unwind across signal frame

[David Daney <david.s.daney@gmail.com>]

On 02/17/2012 01:13 PM, Alan Cooper wrote:
> I'm seeing a problem on both 2.6.37 and 3.3 MIPS kernels where I can't
> unwind through a MIPS signal frame.
You don't tell us the version of the unwinder (likely from libgcc) you 
are using.  There was a lot of work in this area four or five years ago, 
I didn't take the time to do the required archaeology to determine the 
exact patch, but likely you are missing this.

>   It looks like this is caused by
> the VDSO code that was added 2/2010.
Some CPUs have errata necessitating a different signal frame layout, on 
these CPUs, you wouldn't be able to unwind either, even pre mips-vdso.

>   When the unwinder tries to find
> the frame info for the caller of the signal handler (the trampoline in
> VDSO), it can't find the eh_frame info because the address is in the
> VDSO area and stops unwinding. It looks like other platforms solve
> this by adding the eh_frame info for the VDSO area so the lookup
> works.

That's right.  However all 'modern' GCCs and GDBs can unwind through 
signal frames on all 2.4.x and later kernels.  I would recommend 
upgrading your GCC to 4.6.2, and see if you obtain better results.

> This problem ends up breaking pthread cleanup for C++ programs because
> the cleanup is done using a class with the expectation that the
> destructor will be called when the thread gets canceled by a cancel
> signal. This seems like a big problem for all current MIPS kernels so
> I was wondering if I'm missing something?

A modern libgcc I think.

>
> If this is correct, then it seems like the best solution would be to
> add the VDSO eh_frame info to MIPS.

Having a correct eh_frame in the vdso, would be nice, but is not the 
highest priority for me.


David Daney