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From:   Keno Fischer <keno@juliacomputing.com>
Date:   Sun, 31 May 2020 12:20:51 -0400
Message-ID: <CABV8kRwhsPhhqUXS46Rwh-xDEDY2q=KSd-xz1W-pu4Gy4KVp8Q@mail.gmail.com>
Subject: Re: arm64: Register modification during syscall entry/exit stop
To:     Dave Martin <Dave.Martin@arm.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Kyle Huey <khuey@pernos.co>, Oleg Nesterov <oleg@redhat.com>,
        linux-arm-kernel@lists.infradead.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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> 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.


Keno