Re: [PATCH net-next] udp: no longer touch sk->sk_refcnt in early demux

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2024-03-08 at 09:37 +0100, Paolo Abeni wrote:
> On Thu, 2024-03-07 at 22:00 +0000, Eric Dumazet wrote:
> > After commits ca065d0cf80f ("udp: no longer use SLAB_DESTROY_BY_RCU")
> > and 7ae215d23c12 ("bpf: Don't refcount LISTEN sockets in sk_assign()")
> > UDP early demux no longer need to grab a refcount on the UDP socket.
> > 
> > This save two atomic operations per incoming packet for connected
> > sockets.
> 
> This reminds me of a old series:
> 
> https://lore.kernel.org/netdev/cover.1506114055.git.pabeni@redhat.com/
> 
> and I'm wondering if we could reconsider such option.
> 
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Cc: Martin KaFai Lau <kafai@fb.com>
> > Cc: Joe Stringer <joe@wand.net.nz>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> > Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> > ---
> >  net/ipv4/udp.c | 5 +++--
> >  net/ipv6/udp.c | 5 +++--
> >  2 files changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index a8acea17b4e5344d022ae8f8eb674d1a36f8035a..e43ad1d846bdc2ddf5767606b78bbd055f692aa8 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -2570,11 +2570,12 @@ int udp_v4_early_demux(struct sk_buff *skb)
> >  					     uh->source, iph->saddr, dif, sdif);
> >  	}
> >  
> > -	if (!sk || !refcount_inc_not_zero(&sk->sk_refcnt))
> > +	if (!sk)
> >  		return 0;
> >  
> >  	skb->sk = sk;
> > -	skb->destructor = sock_efree;
> > +	DEBUG_NET_WARN_ON_ONCE(sk_is_refcounted(sk));
> > +	skb->destructor = sock_pfree;
> 
> I *think* that the skb may escape the current rcu section if e.g. if
> matches a nf dup target in the input tables.
> 
> Back then I tried to implement some debug infra to track such accesses:
> 
> https://lore.kernel.org/lkml/cover.1507294365.git.pabeni@redhat.com/
> 
> which was buggy (prone to false negative). I think it can be improved
> to something more reliable, perhaps I should revamp it?
> 
> I'm also wondering if the DEBUG_NET_WARN_ON_ONCE is worthy?!? the sk is
> an hashed UDP socket so is a full sock and has the bit SOCK_RCU_FREE
> set.
> 
> Perhaps we could use a simple 'noop' destructor as in:
> 
> https://lore.kernel.org/netdev/b16163e3a4fa4d772edeabd8743acb4a07206bb9.1506114055.git.pabeni@redhat.com/

Please ignore this last part, too late I noticed we need 'sock_pfree'
to let inet_steal_sock() work as expected.

Paolo