Re: Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKAROUND_WAR

[David Daney <ddaney@avtrex.com>]

David Daney wrote:
> With the current kernel (2.6.23.1) in my R5000 based O2 it seems 
> impossible for GCC's exception unwinding machinery to unwind through 
> signal frames.  The cause of the problems is the 
> ICACHE_REFILLS_WORKAROUND_WAR which puts the sigcontext at an almost 
> impossible to determine offset from the signal return trampoline.  The 
> unwinder depends on being able to find the sigcontext given a known 
> location of the trampoline.
>
> It seems there are a couple of possible solutions:
>
> 1) The comments in war.h indicate the problem only exists in R7000 and 
> E9000 processors.  We could turn off the workaround if the kernel is 
> configured for R5000.  That would help me, but not those with the 
> effected systems.
>
> 2) In the non-workaround case, the siginfo immediately follows the 
> trampoline and the first member is the signal number.  For the 
> workaround case the first word following the trampoline is zero.  We 
> could replace this with the offset to the sigcontext which is always a 
> small negative value.  The unwinder could then distinguish the two 
> cases (signal numbers are positive and the offset negative).  If we 
> did this, the change would have to be coordinated with GCC's unwinder 
> (in libgcc_s.so.1).
>
I think I have found a solution that doesn't require kernel changes.

The CFA (i.e. the stack pointer of the signal handler) of the the 
context when calling mips_fallback_frame_state is at a constant offset 
from the sigcontext.  I can just use the CFA instead of the trampoline's 
address.

Does this seem plausible?

Thanks,
David Daney