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


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