Re: [PATCH v2 net] af_unix: Update unix_sk(sk)->oob_skb under sk_receive_queue lock.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Michal Luczaj <mhal@rbox.co>
Date: Sun, 12 May 2024 16:47:11 +0200
> On 5/10/24 11:39, Kuniyuki Iwashima wrote:
> > @@ -2655,6 +2661,8 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
> >  		consume_skb(skb);
> >  		skb = NULL;
> >  	} else {
> > +		spin_lock(&sk->sk_receive_queue.lock);
> > +
> >  		if (skb == u->oob_skb) {
> >  			if (copied) {
> >  				skb = NULL;
> > @@ -2666,13 +2674,15 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
> >  			} else if (flags & MSG_PEEK) {
> >  				skb = NULL;
> >  			} else {
> > -				skb_unlink(skb, &sk->sk_receive_queue);
> > +				__skb_unlink(skb, &sk->sk_receive_queue);
> >  				WRITE_ONCE(u->oob_skb, NULL);
> >  				if (!WARN_ON_ONCE(skb_unref(skb)))
> >  					kfree_skb(skb);
> >  				skb = skb_peek(&sk->sk_receive_queue);
> >  			}
> >  		}
> > +
> > +		spin_unlock(&sk->sk_receive_queue.lock);
> >  	}
> >  	return skb;
> >  }
> 
> Now it is
>   
>   spin_lock(&sk->sk_receive_queue.lock)
>   kfree_skb

This does not free skb actually and just drops a refcount by skb_get()
in queue_oob().


>     unix_destruct_scm

So, here we don't reach unix_destruct_scm().

That's why I changed kfree_skb() to skb_unref() in __unix_gc().

Thanks!


>       unix_notinflight
>         spin_lock(&unix_gc_lock)
> 
> I.e. sk_receive_queue.lock -> unix_gc_lock, inversion of what unix_gc() does.
> But that's benign, right?