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

[Haibo Xu (Arm Technology China)]

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.

Regards,

Haibo
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