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

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Date:   Mon, 1 Jun 2020 10:52:23 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Keno Fischer <keno@...iacomputing.com>
Cc:     Kyle Huey <khuey@...nos.co>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: arm64: Register modification during syscall entry/exit stop

On Mon, Jun 01, 2020 at 05:23:01AM -0400, Keno Fischer wrote:
> 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 guess that's my question: when is x7 "disturbed".

Other than sigreturn, I can't think of a case.

I'm likely missing some aspect of what you're trying to do.

> 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?)

Since rt_sigreturn emulation was always broken, can we just say
that the effect of updating any reg other than x0 is unspecified in this
case?

Even fixing the x7 issue won't magically teach your tracer how to
deal with unrecognised data in the signal frame, so new hardware or
a new kernel could cause your tracer to become subtly broken.  Would you
be better off tweaking the real signal frame as desired and doing a real
rt_sigreturn for example, instead of attempting to emulate it?

I'm somewhat playing devil's advocate here...

Cheers
---Dave