[PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[Oleg Nesterov]

No functional changes, preparation for the next patch.

Move the "should we release this child" logic into the separate handler,
__ptrace_detach().

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

--- 6.29-rc3/kernel/exit.c~3_REINTRODUCE_DETACH	2009-01-29 02:46:42.000000000 +0100
+++ 6.29-rc3/kernel/exit.c	2009-01-29 04:09:40.000000000 +0100
@@ -737,6 +737,39 @@ static int ignoring_children(struct sigh
 		sigh->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT;
 }
 
+/* Returns nonzero if the tracee should be released. */
+int __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
+{
+	__ptrace_unlink(p);
+
+	if (p->exit_state != EXIT_ZOMBIE)
+		return 0;
+	/*
+	 * If it's a zombie, our attachedness prevented normal
+	 * parent notification or self-reaping.  Do notification
+	 * now if it would have happened earlier.  If it should
+	 * reap itself, add it to the @dead list.  We can't call
+	 * release_task() here because we already hold tasklist_lock.
+	 *
+	 * 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 (!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))
+			p->exit_signal = -1;
+	}
+
+	if (!task_detached(p))
+		return 0;
+
+	/* Mark it as in the process of being reaped. */
+	p->exit_state = EXIT_DEAD;
+	return 1;
+}
+
 /*
  * Detach all tasks we were using ptrace on.
  * Any that need to be release_task'd are put on the @dead list.
@@ -748,36 +781,8 @@ static void ptrace_exit(struct task_stru
 	struct task_struct *p, *n;
 
 	list_for_each_entry_safe(p, n, &parent->ptraced, ptrace_entry) {
-		__ptrace_unlink(p);
-
-		if (p->exit_state != EXIT_ZOMBIE)
-			continue;
-
-		/*
-		 * If it's a zombie, our attachedness prevented normal
-		 * parent notification or self-reaping.  Do notification
-		 * now if it would have happened earlier.  If it should
-		 * reap itself, add it to the @dead list.  We can't call
-		 * release_task() here because we already hold tasklist_lock.
-		 *
-		 * 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 (!task_detached(p) && thread_group_empty(p)) {
-			if (!same_thread_group(p->real_parent, parent))
-				do_notify_parent(p, p->exit_signal);
-			else if (ignoring_children(parent->sighand))
-				p->exit_signal = -1;
-		}
-
-		if (task_detached(p)) {
-			/*
-			 * Mark it as in the process of being reaped.
-			 */
-			p->exit_state = EXIT_DEAD;
+		if (__ptrace_detach(parent, p))
 			list_add(&p->ptrace_entry, dead);
-		}
 	}
 }
 

Re: [PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[Roland McGrath]

> No functional changes, preparation for the next patch.
> 
> Move the "should we release this child" logic into the separate handler,
> __ptrace_detach().

My inclination is to use bool in new code for true/false return values,
but I don't really care.

Please canonicalize the comment formatting for your new comments.

The preserved comment no longer makes sense, there is no "dead list" in
that function.  Make it a coherent comment at the top that explains the
return value.  

Given its content, this function now better belongs in ptrace.c, I think.


Thanks,
Roland

=====

/*
 * Called with tasklist_lock held for writing.
 * Unlink a traced task, and clean it up if it was a traced zombie.
 * Return true if it needs to be reaped with release_task().
 * (We can't call release_task() here because we already hold tasklist_lock.)
 *
 * If it's a zombie, our attachedness prevented normal parent notification
 * 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.
 */
bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
{
	__ptrace_unlink(p);

	if (p->exit_state == EXIT_ZOMBIE) {
		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))
				p->exit_signal = -1;
		}
		if (task_detached(p)) {
			/*
			 * Mark it as in the process of being reaped.
			 */
			p->exit_state = EXIT_DEAD;
			return true;
		}
	}

	return false;
}

Re: [PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[Oleg Nesterov]

On 02/04, Roland McGrath wrote:
>
> > No functional changes, preparation for the next patch.
> > 
> > Move the "should we release this child" logic into the separate handler,
> > __ptrace_detach().
> 
> My inclination is to use bool in new code for true/false return values,
> but I don't really care.
> 
> Please canonicalize the comment formatting for your new comments.
> 
> The preserved comment no longer makes sense, there is no "dead list" in
> that function.  Make it a coherent comment at the top that explains the
> return value.

OK, I'll send the cleanup patch.

> Given its content, this function now better belongs in ptrace.c, I think.

I don't completely agree... This helper imho has nothing to do with
ptracing, except it does __ptrace_unlink(). But OK, I will move it
if you prefer. In that case we should export task_detached().

Oleg.

Re: [PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[Roland McGrath]

> > Given its content, this function now better belongs in ptrace.c, I think.
> 
> I don't completely agree... This helper imho has nothing to do with
> ptracing, except it does __ptrace_unlink(). But OK, I will move it
> if you prefer. 

Obviously where it goes is not a big deal.  But I think it's clear that it
has everything to do with ptrace and nothing to do with anything else.
It resolves a situation that can only arise because of ptrace magic.

> In that case we should export task_detached().

Or just recognize that this trivial wrapper around == -1 has little
value two lines away from a place where = -1 is done explicitly.  
Really, the "abstraction" is more confusing than not in this function, IMHO.


Thanks,
Roland

Re: [PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[Oleg Nesterov]

On 02/05, Roland McGrath wrote:
>
> > > Given its content, this function now better belongs in ptrace.c, I think.
> >
> > I don't completely agree... This helper imho has nothing to do with
> > ptracing, except it does __ptrace_unlink(). But OK, I will move it
> > if you prefer.
>
> Obviously where it goes is not a big deal.  But I think it's clear that it
> has everything to do with ptrace and nothing to do with anything else.
> It resolves a situation that can only arise because of ptrace magic.

OK, OK, I will move it.

> > In that case we should export task_detached().
>
> Or just recognize that this trivial wrapper around == -1 has little
> value two lines away from a place where = -1 is done explicitly.
> Really, the "abstraction" is more confusing than not in this function, IMHO.

Well, yes. The only problem it is not easy to grep for this check
without a helper.


(And I still hope we can change the rules sometimes, I think there
 is no good reason to have task_detached() or EXIT_DEAD tasks on
 ->children list. But this is offtopic.)

Oleg.

Re: [PATCH 3/4] ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[Roland McGrath]

> (And I still hope we can change the rules sometimes, I think there
>  is no good reason to have task_detached() or EXIT_DEAD tasks on
>  ->children list. But this is offtopic.)

Agreed on both counts. :-)


Thanks,
Roland