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Date: Fri, 22 Jan 2021 13:31:27 -0500
From: Rich Felker <dalias@...c.org>
To: Florian Weimer <fweimer@...hat.com>, musl@...ts.openwall.com,
libc-alpha@...rceware.org, linuxppc-dev@...ts.ozlabs.org,
Nicholas Piggin <npiggin@...il.com>, Alan Modra <amodra@...il.com>
Subject: Re: Re: [PATCH v2] powerpc/64/signal: balance return
predictor stack in signal trampoline
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
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