linux-kernel - Re: [PATCH] arm64/ptrace: add PTRACE_SYSEMU and PTRACE_SYSEMU

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Date:   Mon, 03 Sep 2018 18:40:51 +0200
From:   Richard Weinberger <richard@....at>
To:     Will Deacon <will.deacon@....com>
Cc:     "Haibo.Xu" <haibo.xu@....com>, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        nd@....com, jdike@...toit.com
Subject: Re: [PATCH] arm64/ptrace: add PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP support

Am Montag, 3. September 2018, 18:31:03 CEST schrieb Will Deacon:
> On Mon, Sep 03, 2018 at 02:23:17PM +0800, Haibo.Xu wrote:
> > Add PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP support on ARM64.
> > This copies the x86 semantics for invoking ptrace hooks, and have
> > been verified on ARM64 machine.
> > 
> > Signed-off-by: Haibo.Xu <haibo.xu@....com>
> > Signed-off-by: Bin.Lu <bin.lu@....com>
> > ---
> >  arch/arm64/include/asm/thread_info.h |  5 ++++-
> >  arch/arm64/include/uapi/asm/ptrace.h |  2 ++
> >  arch/arm64/kernel/ptrace.c           | 17 +++++++++++++++++
> >  3 files changed, 23 insertions(+), 1 deletion(-)
> 
> What is PTRACE_SYSEMU and what is its semantics? Why isn't it done in the
> core ptrace code?

It is an optimization added for UserModeLinux many years ago.
PTRACE_SYSEMU basically allows you to handle system calls in user space
without the kernel seeing them.
Before that UML had to render every system call into a no-op, e.h. getpid().
This was complicated and slow.

The ptrace() manpage has a section on PTRACE_SYSEMU, more documentation on the
semantics is not available.

And yes, I think this should also done in the core. Like many other ptrace() areas
this needs a cleanup. ;-)

I wonder what Haibo Xu want to do with PTRACE_SYSEMU on arm64.
Are you porting UML or gvisor to arm64?

Thanks,
//richard