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    SubjectRe: [PATCH 3/6] x86: clean up _TIF_SYSCALL_EMU handling using ptrace_syscall_enter hook
    On Mon, Mar 11, 2019 at 6:35 PM Haibo Xu (Arm Technology China)
    <Haibo.Xu@arm.com> wrote:
    >
    > On 2019/3/12 2:34, Sudeep Holla wrote:
    > > (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;
    > >
    >
    > I don't think so! As I mentioned in the last email loop, when PTRACE_SYSEMU_SINGLESTEP
    > is enabled, both the _TIF_SYSCALL_EMU and _TIF_SINGLESTEP flags are set, in which case
    > the step should be "false" for the old logic. But with the new logic, the step is "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
    > >
    >
    > For the PTRACE_SYSEMU_SINGLESTEP request, 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.That's
    > why the old logic-{step = ((flags & (_TIF_SINGLESTEP | _TIF_SYSCALL_EMU)) == _TIF_SINGLESTEP)}
    > here try to filter out the special case(PTRACE_SYSEMU_SINGLESTEP).
    >
    > Another way to make sure the logic is fine, you can run some tests with respect to both logic,
    > and to check whether they have the same behavior.


    tools/testing/selftests/x86/ptrace_syscall.c has a test intended to
    exercise this. Can one of you either confirm that it does exercise it
    and that it still passes or can you improve the test?

    Thanks,
    Andy

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