Re: [patch V2 1/8] signal: Prevent exec() race

[Oleg Nesterov]

On 09/08, Frederic Weisbecker wrote:
>
> Old leader                           Exec'ing New leader     CPU 2
> -----                                -----                   -----
>
> WRITE q->next = q
> WRITE q->prev = q

Damn ;) I am shy to suggest this again, but if we have _any_
concerns about the races with list_del_init()...

The lockless flush_sigqueue_list() doesn't need to it, right?
So perhaps something like below (on top of this series) to avoid
the (potential) race explicitly?

Most probably this change is wrong. And in any case we can do better.
Just to explain what I mean.

In short, I mean that exit_signals() -> flush path can do
__sigqueue_free(q) without list_del_init(&q->list).

Yes! I agree in advance that if this change can fix something,
then it fixes the symptom.

But at the same time, why does flush_sigqueue_list(head) need
list_del_init() before __sigqueue_free() ? AFAICS only to make
list_empty(head) == true. This looks confusing to me.

Oh, I am sure I missed something again ;)

Oleg.
---

diff --git a/kernel/signal.c b/kernel/signal.c
index f93d8f77ec1a..32c344ee4769 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -457,14 +457,19 @@ static void __sigqueue_free(struct sigqueue *q)
 	kmem_cache_free(sigqueue_cachep, q);
 }
 
-static void flush_sigqueue_list(struct list_head *head)
+static void __flush_sigqueue_list(struct list_head *head)
 {
 	struct sigqueue *q, *tmp;
 
-	list_for_each_entry_safe(q, tmp, head, list) {
-		list_del_init(&q->list);
+	list_for_each_entry_safe(q, tmp, head, list)
 		__sigqueue_free(q);
-	}
+}
+
+static void flush_sigqueue_list(struct list_head *head)
+{
+	LIST_HEAD(flush);
+	list_splice_init(head, &flush);
+	__flush_sigqueue_list(&flush);
 }
 
 void flush_sigqueue(struct sigpending *queue)
@@ -3196,7 +3201,7 @@ void exit_signals(struct task_struct *tsk)
 
 	cgroup_threadgroup_change_end(tsk);
 
-	flush_sigqueue_list(&sigq_list);
+	__flush_sigqueue_list(&sigq_list);
 
 	/*
 	 * If group stop has completed, deliver the notification.  This