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

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Message-ID: <20200601092329.GX5031@arm.com>
Date:   Mon, 1 Jun 2020 10:23:29 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Keno Fischer <keno@...iacomputing.com>
Cc:     Kyle Huey <khuey@...nos.co>,
        Catalin Marinas <catalin.marinas@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: arm64: Register modification during syscall entry/exit stop

On Sun, May 31, 2020 at 12:20:51PM -0400, Keno Fischer wrote:
> > Can't PTRACE_SYSEMU be emulated by using PTRACE_SYSCALL, cancelling the
> > syscall at the syscall enter stop, then modifying the regs at the
> > syscall exit stop?
> 
> Yes, it can. The idea behind SYSEMU is to be able to save half the
> ptrace traps that would require, in theory making the ptracer
> a decent amount faster. That said, the x7 issue is orthogonal to
> SYSEMU, you'd have the same issues if you used PTRACE_SYSCALL.

Right, I just wondered whether there was some deeper difference between
the two approaches.

Cheers
---Dave