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SubjectRe: Q: user_enable_single_step() && update_debugctlmsr()
On 08/01/2012 04:31 PM, Oleg Nesterov wrote:
> And I think you missed my point. I'll try again.
Okay.

> We have the GDB process and the (stopped) tracee T. And we have
> another task X which have TIF_SINGLESTEP but not TIF_BLOCKSTEP.
> To simplify, suppose that X is already TASK_RUNNING but not on rq.
>
> GDB does ptrace(PTRACE_SINGLEBLOCK, T). This sets X->TIF_BLOCKSTEP.
> Now suppose that GDB is preempted right after it does
> update_debugctlmsr(), and the scheduler choses X as the next task.
>
> Both GDB and X do not have TIF_BLOCKSTEP, so __switch_to_extra()
> does not update DEBUGCTLMSR_BTF.
>
> X returns to the user-mode with TIF_SINGLESTEP and TIF_BLOCKSTEP,
> the latter is wrong.
>
> No?

Yup, correct, you are right sir. Thank you for trying so hard to make
me see this :)

So a patch like
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -173,8 +173,8 @@ static void enable_step(struct task_struct *child,
bool block)
unsigned long debugctl = get_debugctlmsr();

debugctl |= DEBUGCTLMSR_BTF;
- update_debugctlmsr(debugctl);
set_tsk_thread_flag(child, TIF_BLOCKSTEP);
+ update_debugctlmsr(debugctl);
} else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
unsigned long debugctl = get_debugctlmsr();

should fix the race and _yes_ I also would follow your suggestion to
remove this update_debugctlmsr() here since switch_to() should do this.
>
> Oleg.
>

Sebastian


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