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Date:   Wed, 27 May 2020 10:55:29 +0100
From:   Will Deacon <will@kernel.org>
To:     Keno Fischer <keno@juliacomputing.com>
Cc:     linux-arm-kernel@lists.infradead.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kyle Huey <khuey@pernos.co>
Subject: Re: arm64: Register modification during syscall entry/exit stop
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On Sun, May 24, 2020 at 02:56:35AM -0400, Keno Fischer wrote:
> Just ran into this issue again, with what I think may be most compelling
> example yet why this is problematic:
> 
> The tracee incurred a signal, we PTRACE_SYSEMU'd to the rt_sigreturn,
> which the tracer tried to emulate by applying the state from the signal frame.
> However, the PTRACE_SYSEMU stop is a syscall-stop, so the tracer's write
> to x7 was ignored and x7 retained the value it had in the signal handler,
> which broke the tracee.

Yeah, that sounds like a good justification to add a way to stop this. Could
you send a patch, please?

Interestingly, I *thought* the current behaviour was needed by strace, but I
can't find anything there that seems to require it. Oh well, we're stuck
with it anyway.

Will