netdev - Re: [PATCH net 1/2] af_unix: Fix garbage collector racing against connect()

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Date: Mon, 8 Apr 2024 17:22:31 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <mhal@...x.co>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH net 1/2] af_unix: Fix garbage collector racing against connect()

From: Michal Luczaj <mhal@...x.co>
Date: Tue, 9 Apr 2024 01:25:23 +0200
> On 4/8/24 23:18, Kuniyuki Iwashima wrote:
> > From: Michal Luczaj <mhal@...x.co>
> > Date: Mon,  8 Apr 2024 17:58:45 +0200
> ...
> >>  	list_for_each_entry_safe(u, next, &gc_inflight_list, link) {

Please move sk declaration here and


> >> -		long total_refs;
> >> -
> >> -		total_refs = file_count(u->sk.sk_socket->file);

keep these 3 lines for reverse xmax tree order.


> >> +		struct sock *sk = &u->sk;
> >> +		long total_refs = file_count(sk->sk_socket->file);
> >>  
> >>  		WARN_ON_ONCE(!u->inflight);
> >>  		WARN_ON_ONCE(total_refs < u->inflight);
> >> @@ -286,6 +295,11 @@ static void __unix_gc(struct work_struct *work)
> >>  			list_move_tail(&u->link, &gc_candidates);
> >>  			__set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
> >>  			__set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
> >> +
> >> +			if (sk->sk_state == TCP_LISTEN) {
> >> +				unix_state_lock(sk);
> >> +				unix_state_unlock(sk);
> > 
> > Less likely though, what if the same connect() happens after this ?
> > 
> > connect(S, addr)	sendmsg(S, [V]); close(V)	__unix_gc()
> > ----------------	-------------------------	-----------
> > NS = unix_create1()
> > skb1 = sock_wmalloc(NS)
> > L = unix_find_other(addr)
> > 						for u in gc_inflight_list:
> > 						  if (total_refs == inflight_refs)
> > 						    add u to gc_candidates
> > 						    // L was already traversed
> > 						    // in a previous iteration.
> > unix_state_lock(L)
> > unix_peer(S) = NS
> > 
> > 						// gc_candidates={L, V}
> > 
> > 						for u in gc_candidates:
> > 						  scan_children(u, dec_inflight)
> > 
> > 						// embryo (skb1) was not
> > 						// reachable from L yet, so V's
> > 						// inflight remains unchanged
> > __skb_queue_tail(L, skb1)
> > unix_state_unlock(L)
> > 						for u in gc_candidates:
> > 						  if (u.inflight)
> > 						    scan_children(u, inc_inflight_move_tail)
> > 
> > 						// V count=1 inflight=2 (!)
> 
> If I understand your question, in this case L's queue technically does change
> between scan_children()s: embryo appears, but that's meaningless. __unix_gc()
> already holds unix_gc_lock, so the enqueued embryo can not carry any SCM_RIGHTS
> (i.e. it doesn't affect the inflight graph). Note that unix_inflight() takes the
> same unix_gc_lock.
> 
> Is there something I'm missing?

Ah exactly, you are right.

Could you repost this patch only with my comment above addressed ?

Thanks!