David Daney wrote:
With the current kernel (18.104.22.168) 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
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
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
Does this seem plausible?