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authorOleg Nesterov <oleg@redhat.com>2012-08-11 18:06:42 +0200
committerAnton Arapov <anton@redhat.com>2012-10-29 11:42:40 +0100
commitde8cda52ece1384da801a0b581a4754e0408b502 (patch)
tree13fe20f979248b3dfef32034224883f2b28484fa
parentadb233ddeb579a9711c94b54ac0ce05778462fef (diff)
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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>
-rw-r--r--arch/x86/kernel/step.c14
1 files changed, 13 insertions, 1 deletions
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 7a51498..f89cdc6 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();
}
/*