linux-kernel - Re: [PATCH 3/6] x86: clean up _TIF_SYSCALL_EMU handling using ptrace_syscall

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Message-ID: <20190311183403.GA31062@e107155-lin>
Date:   Mon, 11 Mar 2019 18:34:03 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     "Haibo Xu (Arm Technology China)" <Haibo.Xu@....com>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Richard Weinberger <richard@....at>,
        "jdike@...toit.com" <jdike@...toit.com>,
        Steve Capper <Steve.Capper@....com>,
        "Bin Lu (Arm Technology China)" <Bin.Lu@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 3/6] x86: clean up _TIF_SYSCALL_EMU handling using
 ptrace_syscall_enter hook

(I thought I had sent this email, last Tuesday itself, but saw this in my
draft today, something went wrong, sorry for the delay)

On Tue, Mar 05, 2019 at 02:14:47AM +0000, Haibo Xu (Arm Technology China) wrote:
> On 2019/3/4 18:12, Sudeep Holla wrote:
> > On Mon, Mar 04, 2019 at 08:25:28AM +0000, Haibo Xu (Arm Technology China) wrote:
> >> On 2019/3/1 2:32, Sudeep Holla wrote:
> >>> Now that we have a new hook ptrace_syscall_enter that can be called from
> >>> syscall entry code and it handles PTRACE_SYSEMU in generic code, we
> >>> can do some cleanup using the same in syscall_trace_enter.
> >>>
> >>> Further the extra logic to find single stepping PTRACE_SYSEMU_SINGLESTEP
> >>> in syscall_slow_exit_work seems unnecessary. Let's remove the same.
> >>
> >> I think we should not change the logic here. Is so, it will double the report of syscall
> >> when PTRACE_SYSEMU_SINGLESTEP is enabled.
> >>
> >
> > I don't think that should happen, but I may be missing something.
> > Can you explain how ?
> >
>
> When PTRACE_SYSEMU_SINGLESTEP is enabled, both the _TIF_SYSCALL_EMU and
> _TIF_SINGLESTEP flags are set, but ptrace only need to report(send SIGTRAP)
> at the entry of a system call, no need to report at the exit of a system
> call.
>
Sorry, but I still not get it, we have:

	step = ((flags & (_TIF_SINGLESTEP | _TIF_SYSCALL_EMU)) == _TIF_SINGLESTEP);

For me, this is same as:
	step = ((flags & _TIF_SINGLESTEP) == _TIF_SINGLESTEP)
	or
	if (flags & _TIF_SINGLESTEP)
		step = true;

So when PTRACE_SYSEMU_SINGLESTEP, _TIF_SYSCALL_EMU and _TIF_SINGLESTEP
are set and step evaluates to true.

So dropping _TIF_SYSCALL_EMU here should be fine. Am I still missing
something ?

--
Regards,
Sudeep