linux-kernel - Re: arm64: Register modification during syscall entry/exit stop

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Date:   Mon, 1 Jun 2020 05:23:01 -0400
From:   Keno Fischer <keno@...iacomputing.com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     Will Deacon <will@...nel.org>, Kyle Huey <khuey@...nos.co>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: arm64: Register modification during syscall entry/exit stop

On Mon, Jun 1, 2020 at 5:14 AM Dave Martin <Dave.Martin@....com> wrote:
> Can you explain why userspace would write a changed value for x7
> but at the same time need that new to be thrown away?

The discarding behavior is the primary reason things aren't completely
broken at the moment. If it read the wrong x7 value and didn't know about
the Aarch64 quirk, it's often just trying to write that same wrong
value back during the next stop, so if that's just ignored,
that's probably fine in 99% of cases, since the value in the
tracee will be undisturbed.

I don't think there's a sane way to change the aarch64 NT_PRSTATUS
semantics without just completely removing the x7 behavior, but of course
people may be relying on that (I think somebody said upthread that strace does?)

Keno