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

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Message-ID: <20200601091441.GW5031@arm.com>
Date:   Mon, 1 Jun 2020 10:14:41 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Keno Fischer <keno@...iacomputing.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 Sun, May 31, 2020 at 12:13:18PM -0400, Keno Fischer wrote:
> > Keno -- are you planning to send out a patch? You previously spoke about
> > implementing this using PTRACE_SETOPTIONS.
> 
> Yes, I'll have a patch for you. Though I've come to the conclusion
> that introducing a new regset is probably a better way to solve it.
> We can then also expose orig_x0 at the same time and give it sane semantics
> (there's some problems with the way it works currently - I'll write it up
> together with the patch).

I'd worry that having a new ptrace option would be useless bug-
compatibility that is just going to bitrot.

Can you explain why userspace would write a changed value for x7
but at the same time need that new to be thrown away?

That sounds like a nonsensical thing for userspace to be doing.

Cheers
---Dave