Commit 95cf00fa authored Aug 11, 2012 by Oleg Nesterov

ptrace/x86: Partly fix set_task_blockstep()->update_debugctlmsr() logic



Afaics the usage of update_debugctlmsr() and TIF_BLOCKSTEP in
step.c was always very wrong.

1. update_debugctlmsr() was simply unneeded. The child sleeps
   TASK_TRACED, __switch_to_xtra(next_p => child) should notice
   TIF_BLOCKSTEP and set/clear DEBUGCTLMSR_BTF after resume if
   needed.

2. It is wrong. The state of DEBUGCTLMSR_BTF bit in CPU register
   should always match the state of current's TIF_BLOCKSTEP bit.

3. Even get_debugctlmsr() + update_debugctlmsr() itself does not
   look right. Irq can change other bits in MSR_IA32_DEBUGCTLMSR
   register or the caller can be preempted in between.

4. It is not safe to play with TIF_BLOCKSTEP if task != current.
   DEBUGCTLMSR_BTF and TIF_BLOCKSTEP should always match each
   other if the task is running. The tracee is stopped but it
   can be SIGKILL'ed right before set/clear_tsk_thread_flag().

However, now that uprobes uses user_enable_single_step(current)
we can't simply remove update_debugctlmsr(). So this patch adds
the additional "task == current" check and disables irqs to avoid
the race with interrupts/preemption.

Unfortunately this patch doesn't solve the last problem, we need
another fix. Probably we should teach ptrace_stop() to set/clear
single/block stepping after resume.

And afaics there is yet another problem: perf can play with
MSR_IA32_DEBUGCTLMSR from nmi, this obviously means that even
__switch_to_xtra() has problems.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

parent 848e8f5f

arch/x86/kernel/step.c

+13 −1

Original line number	Diff line number	Diff line
		@@ -161,6 +161,16 @@ static void set_task_blockstep(struct task_struct *task, bool on)
		{
		unsigned long debugctl;

		/*
		* Ensure irq/preemption can't change debugctl in between.
		* Note also that both TIF_BLOCKSTEP and debugctl should
		* be changed atomically wrt preemption.
		* FIXME: this means that set/clear TIF_BLOCKSTEP is simply
		* wrong if task != current, SIGKILL can wakeup the stopped
		* tracee and set/clear can play with the running task, this
		* can confuse the next __switch_to_xtra().
		*/
		local_irq_disable();
		debugctl = get_debugctlmsr();
		if (on) {
		debugctl \|= DEBUGCTLMSR_BTF;
		@@ -169,7 +179,9 @@ static void set_task_blockstep(struct task_struct *task, bool on)
		debugctl &= ~DEBUGCTLMSR_BTF;
		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
		}
		if (task == current)
		update_debugctlmsr(debugctl);
		local_irq_enable();
		}

		/*

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