[PATCH -mm 4/2] ptrace_stop: fix racy nonstop_code setting

[Oleg Nesterov <oleg@tv-sign.ru>]

If the tracer is gone and we are not going to stop, ptrace_stop() sets
->exit_code = nostop_code. However, the tracer could actually clear the
exit code before detaching. In that case get_signal_to_deliver() "resends"
the signal which was cancelled by the debugger. For example, it is possible
that a quick PTRACE_ATTACH + PTRACE_DETACH can leave the tracee in STOPPED
state.

Change the behaviour of ptrace_stop(). If the caller is ptrace notify(), we
should always clear ->exit_code. If the caller is get_signal_to_deliver(), we
should not touch it at all. To do so, change the nonstop_code parameter to
"bool clear_code" and change the callers accordingly.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- PT/kernel/signal.c~4_non_stop	2007-11-22 16:59:35.000000000 +0300
+++ PT/kernel/signal.c	2007-11-22 20:29:11.000000000 +0300
@@ -1601,10 +1601,10 @@ static inline int may_ptrace_stop(void)
  * That makes it a way to test a stopped process for
  * being ptrace-stopped vs being job-control-stopped.
  *
- * If we actually decide not to stop at all because the tracer is gone,
- * we leave nostop_code in current->exit_code.
+ * If we actually decide not to stop at all because the tracer
+ * is gone, we keep current->exit_code unless clear_code.
  */
-static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info)
+static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
 {
 	/*
 	 * If there is a group stop in progress,
@@ -1631,7 +1631,8 @@ static void ptrace_stop(int exit_code, i
 		 * Don't drop the lock yet, another tracer may come.
 		 */
 		__set_current_state(TASK_RUNNING);
-		current->exit_code = nostop_code;
+		if (clear_code)
+			current->exit_code = 0;
 		read_unlock(&tasklist_lock);
 	}
 
@@ -1665,7 +1666,7 @@ void ptrace_notify(int exit_code)
 
 	/* Let the debugger run.  */
 	spin_lock_irq(&current->sighand->siglock);
-	ptrace_stop(exit_code, 0, &info);
+	ptrace_stop(exit_code, 1, &info);
 	spin_unlock_irq(&current->sighand->siglock);
 }
 
@@ -1813,7 +1814,7 @@ relock:
 			ptrace_signal_deliver(regs, cookie);
 
 			/* Let the debugger run.  */
-			ptrace_stop(signr, signr, info);
+			ptrace_stop(signr, 0, info);
 
 			/* We're back.  Did the debugger cancel the sig?  */
 			signr = current->exit_code;

-
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