Re: [PATCH 3/6] x86: clean up _TIF_SYSCALL_EMU handling using ptrace_syscall_enter hook

Sudeep Holla
2019-03-12T12:05:56Z

On Tue, Mar 12, 2019 at 01:34:44AM +0000, Haibo Xu (Arm Technology China) 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_SINGLESTE 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".
>

Ah right, sorry I missed that.

> > 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).
>

Understood

> 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.
>

I did run selftests after Andy Lutomirski pointed out. Nothing got flagged,
I haven't looked at the tests themselves yet, but it clearly misses this
case.

--
Regards,
Sudeep