[PATCH 1/1] ptrace: __ptrace_detach: do __wake_up_parent() if we reap the tracee

[PATCH 1/1] ptrace: __ptrace_detach: do __wake_up_parent() if we reap the tracee

[Oleg Nesterov]

The bug is old, it wasn't cause by recent changes.

Test case:

	static void *tfunc(void *arg)
	{
		int pid = (long)arg;

		assert(ptrace(PTRACE_ATTACH, pid, NULL, NULL) == 0);
		kill(pid, SIGKILL);

		sleep(1);
		return NULL;
	}

	int main(void)
	{
		pthread_t th;
		long pid = fork();

		if (!pid)
			pause();

		signal(SIGCHLD, SIG_IGN);
		assert(pthread_create(&th, NULL, tfunc, (void*)pid) == 0);

		int r = waitpid(-1, NULL, __WNOTHREAD);
		printf("waitpid: %d %m\n", r);

		return 0;
	}

Before the patch this program hangs, after this patch waitpid() correctly
fails with errno == -ECHILD.

The problem is, __ptrace_detach() reaps the EXIT_ZOMBIE tracee if its
->real_parent is our sub-thread and we ignore SIGCHLD. But in this case
we should wake up other threads which can sleep in do_wait().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 include/linux/sched.h |    1 +
 kernel/signal.c       |    9 ---------
 kernel/exit.c         |    5 +++++
 kernel/ptrace.c       |   11 +++++++----
 4 files changed, 13 insertions(+), 13 deletions(-)

--- WAIT/include/linux/sched.h~PT_DETACH_WAKE_PARENT	2009-06-19 01:12:47.000000000 +0200
+++ WAIT/include/linux/sched.h	2009-07-01 20:20:57.000000000 +0200
@@ -1951,6 +1951,7 @@ extern int kill_pgrp(struct pid *pid, in
 extern int kill_pid(struct pid *pid, int sig, int priv);
 extern int kill_proc_info(int, struct siginfo *, pid_t);
 extern int do_notify_parent(struct task_struct *, int);
+extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
 extern void force_sig(int, struct task_struct *);
 extern void force_sig_specific(int, struct task_struct *);
 extern int send_sig(int, struct task_struct *, int);
--- WAIT/kernel/signal.c~PT_DETACH_WAKE_PARENT	2009-07-01 20:05:07.000000000 +0200
+++ WAIT/kernel/signal.c	2009-07-01 20:22:44.000000000 +0200
@@ -1383,15 +1383,6 @@ ret:
 }
 
 /*
- * Wake up any threads in the parent blocked in wait* syscalls.
- */
-static inline void __wake_up_parent(struct task_struct *p,
-				    struct task_struct *parent)
-{
-	wake_up_interruptible_sync(&parent->signal->wait_chldexit);
-}
-
-/*
  * Let a parent know about the death of a child.
  * For a stopped/continued status change, use do_notify_parent_cldstop instead.
  *
--- WAIT/kernel/exit.c~PT_DETACH_WAKE_PARENT	2009-07-01 20:05:11.000000000 +0200
+++ WAIT/kernel/exit.c	2009-07-01 20:23:42.000000000 +0200
@@ -1563,6 +1563,11 @@ static int ptrace_do_wait(struct wait_op
 	return 0;
 }
 
+void __wake_up_parent(struct task_struct *p, struct task_struct *parent)
+{
+	wake_up_interruptible_sync(&parent->signal->wait_chldexit);
+}
+
 static long do_wait(struct wait_opts *wo)
 {
 	DECLARE_WAITQUEUE(wait, current);
--- WAIT/kernel/ptrace.c~PT_DETACH_WAKE_PARENT	2009-07-01 19:59:01.000000000 +0200
+++ WAIT/kernel/ptrace.c	2009-07-01 20:52:58.000000000 +0200
@@ -266,9 +266,10 @@ static int ignoring_children(struct sigh
  * or self-reaping.  Do notification now if it would have happened earlier.
  * If it should reap itself, return true.
  *
- * If it's our own child, there is no notification to do.
- * But if our normal children self-reap, then this child
- * was prevented by ptrace and we must reap it now.
+ * If it's our own child, there is no notification to do. But if our normal
+ * children self-reap, then this child was prevented by ptrace and we must
+ * reap it now, in that case we must also wake up sub-threads sleeping in
+ * do_wait().
  */
 static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 {
@@ -278,8 +279,10 @@ static bool __ptrace_detach(struct task_
 		if (!task_detached(p) && thread_group_empty(p)) {
 			if (!same_thread_group(p->real_parent, tracer))
 				do_notify_parent(p, p->exit_signal);
-			else if (ignoring_children(tracer->sighand))
+			else if (ignoring_children(tracer->sighand)) {
+				__wake_up_parent(p, tracer);
 				p->exit_signal = -1;
+			}
 		}
 		if (task_detached(p)) {
 			/* Mark it as in the process of being reaped. */

Re: [PATCH 1/1] ptrace: __ptrace_detach: do __wake_up_parent() if we reap the tracee

[Oleg Nesterov]

On 07/01, Oleg Nesterov wrote:
>
> --- WAIT/kernel/ptrace.c~PT_DETACH_WAKE_PARENT	2009-07-01 19:59:01.000000000 +0200
> +++ WAIT/kernel/ptrace.c	2009-07-01 20:52:58.000000000 +0200
> @@ -266,9 +266,10 @@ static int ignoring_children(struct sigh
>   * or self-reaping.  Do notification now if it would have happened earlier.
>   * If it should reap itself, return true.
>   *
> - * If it's our own child, there is no notification to do.
> - * But if our normal children self-reap, then this child
> - * was prevented by ptrace and we must reap it now.
> + * If it's our own child, there is no notification to do. But if our normal
> + * children self-reap, then this child was prevented by ptrace and we must
> + * reap it now, in that case we must also wake up sub-threads sleeping in
> + * do_wait().
>   */
>  static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
>  {
> @@ -278,8 +279,10 @@ static bool __ptrace_detach(struct task_
>  		if (!task_detached(p) && thread_group_empty(p)) {
>  			if (!same_thread_group(p->real_parent, tracer))
>  				do_notify_parent(p, p->exit_signal);
> -			else if (ignoring_children(tracer->sighand))
> +			else if (ignoring_children(tracer->sighand)) {
> +				__wake_up_parent(p, tracer);
>  				p->exit_signal = -1;
> +			}

I wonder if we need more fixes here.

ignoring_children() is not exactly right afaics, we assume that
tracee->exit_signal == SIGCHLD.

But I guess this can be ignored, it falls into "ptracing with SIGCHLD
ignored asks for trouble" category.

But !same_thread_group() doesn't look 100% right too, for the same
reason. If ->exit_signal != SIGCHLD, we can't assume we already had
the correct notification. Hopefully this can be ignored too.

Oleg.