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 <[email protected]> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
