path: root/arch
diff options
authorOleg Nesterov <>2012-08-11 18:06:42 +0200
committerAnton Arapov <>2012-10-29 11:23:44 +0100
commit06650b7b504327d3ffd53bc0867e8eff3f79b86c (patch)
tree44cbbb74d8bbe55d77058999f273461393325d61 /arch
parentc6eb3a04ee21277be2ac4ee43c309eab39d2c00f (diff)
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 <>
Diffstat (limited to 'arch')
1 files changed, 13 insertions, 1 deletions
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 7a514986ca0..f89cdc6ccd5 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -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);
- update_debugctlmsr(debugctl);
+ if (task == current)
+ update_debugctlmsr(debugctl);
+ local_irq_enable();