[musl] Re: [PATCH v2] powerpc/64/signal: balance return predictor stack in signal trampoline

[Rich Felker]

On Fri, Jan 22, 2021 at 03:19:22PM -0300, Raoni Fassina Firmino wrote:
> On Fri, Jan 22, 2021 at 09:44:05AM -0500, Rich Felker wrote:
> > Maybe I'm missing something but I don't see how this would break musl;
> > we just inspect the PC in the mcontext, which I don't see any changes
> > to and which should still point to the next instruction of the
> > interrupted context. I don't have a test environment though so I'll
> > have to wait for feedback from ppc users to be sure. Are there any
> > further details on how it's breaking glibc?
> 
> For glibc, backtrace() compares the return-address from each stack frame
> to the value of `__kernel_sigtramp_rt64` to identify the frame with the
> mcontext information, but now the return-address is not the start of the
> routine, but the middle of it, so it fails to catch this special frame.

Is there a reason it's backtracing rather than just looking at the
interrupted context (pointed to by the third argument to the signal
handler)?

Rich