Re: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases

[Andy Lutomirski]

On Mon, Jul 28, 2014 at 10:37 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> Hi Andy,
>
> I am really sorry for delay.
>
> This is on top of the recent change from Kees, right? Could me remind me
> where can I found the tree this series based on? So that I could actually
> apply these changes...

https://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/log/?h=seccomp/fastpath

The first four patches are already applied there.

>
> On 07/21, Andy Lutomirski wrote:
>>
>> +long syscall_trace_enter_phase2(struct pt_regs *regs, u32 arch,
>> +                             unsigned long phase1_result)
>>  {
>>       long ret = 0;
>> +     u32 work = ACCESS_ONCE(current_thread_info()->flags) &
>> +             _TIF_WORK_SYSCALL_ENTRY;
>> +
>> +     BUG_ON(regs != task_pt_regs(current));
>>
>>       user_exit();
>>
>> @@ -1458,17 +1562,20 @@ long syscall_trace_enter(struct pt_regs *regs)
>>        * do_debug() and we need to set it again to restore the user
>>        * state.  If we entered on the slow path, TF was already set.
>>        */
>> -     if (test_thread_flag(TIF_SINGLESTEP))
>> +     if (work & _TIF_SINGLESTEP)
>>               regs->flags |= X86_EFLAGS_TF;
>
> This looks suspicious, but perhaps I misread this change.
>
> If I understand correctly, syscall_trace_enter() can avoid _phase2() above.
> But we should always call user_exit() unconditionally?

Damnit.  I read that every function called by user_exit, and none of
them give any indication of why they're needed for traced syscalls but
not for untraced syscalls.  On a second look, it seems that TIF_NOHZ
controls it.  I'll update the code to call user_exit iff TIF_NOHZ is
set.  If that's still wrong, then I don't see how the current code is
correct either.

>
> And we should always set X86_EFLAGS_TF if TIF_SINGLESTEP? IIRC, TF can be
> actually cleared on a 32bit kernel if we step over sysenter insn?

I don't follow.  If TIF_SINGLESTEP, then phase1 will return a nonzero
value, and phase2 will set TF.

I admit I don't really understand all the TF machinations.

--Andy
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