[PATCH v3 6/8] x86: Split syscall_trace_enter into two phases
Andy Lutomirski
luto at amacapital.net
Mon Jul 28 13:23:13 PDT 2014
On Mon, Jul 28, 2014 at 10:37 AM, Oleg Nesterov <oleg at redhat.com> 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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