[PATCH 2/2] ptrace: fix set_task_blockstep()->update_debugctlmsr() logic

[Oleg Nesterov <oleg@redhat.com>]

Afaics the usage of update_debugctlmsr() 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.

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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/step.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index afa60db..636402e 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -166,12 +166,18 @@ static void set_task_blockstep(struct task_struct *task, bool on)
 	else
 		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
 
+	if (task != current)
+		return;
+
+	/* ensure irq/preemption can't change debugctl in between */
+	local_irq_disable();
 	debugctl = get_debugctlmsr();
 	if (on)
 		debugctl |= DEBUGCTLMSR_BTF;
 	else
 		debugctl &= ~DEBUGCTLMSR_BTF;
 	update_debugctlmsr(debugctl);
+	local_irq_enable();
 }
 
 /*
-- 
1.5.5.1