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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

Date: Mon, 13 May 2024 12:15:57 +0200
From: Michal Luczaj <mhal@rbox.co>
> On 5/13/24 11:24, Kuniyuki Iwashima wrote:
> > From: Michal Luczaj <mhal@rbox.co>
> > Date: Mon, 13 May 2024 11:14:39 +0200
> >> On 5/13/24 09:44, Kuniyuki Iwashima wrote:
> >>> From: Michal Luczaj <mhal@rbox.co>
> >>> Date: Mon, 13 May 2024 08:40:34 +0200
> >>>> What I'm talking about is the quoted above (unchanged) part in manage_oob():
> >>>>
> >>>> 	if (!WARN_ON_ONCE(skb_unref(skb)))
> >>>>   		kfree_skb(skb);
> >>>
> >>> Ah, I got your point, good catch!
> >>>
> >>> Somehow I was thinking of new GC where alive recvq is not touched
> >>> and lockdep would end up with false-positive.
> >>>
> >>> We need to delay freeing oob_skb in that case like below.
> >>> ...
> >>
> >> So this not a lockdep false positive after all?
> >>
> >> Here's my understanding: the only way manage_oob() can lead to an inverted locking
> >> order is when the receiver socket is _not_ in gc_candidates. And when it's not
> >> there, no risk of deadlock. What do you think?
> > 
> > For the new GC, it's false positive, but for the old GC, it's not.
> >
> > The old GC locks unix_gc_lock and could iterate alive sockets if
> > they are linked to gc_inflight_list, and then recvq is locked.
> > ...
> 
> The recvq is locked not for all sockets in gc_inflight_list, but only its
> subset, gc_candidates, i.e. sockets that fulfil the 'total_refs == u->inflight'
> condition, right? So doesn't this imply that our receiver is not user-reachable
> and manage_oob() cannot be called/raced?

Ah, yes, the splat was false-positive for the old GC too.

Instead of using a differenct class for the recvq in GC, it would
be better to unlock it earlier in manage_oob(), so I'll post v3.

Thanks!