netdev - Re: [PATCH net-next] udp: expand SKB_DROP_REASON_UDP

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Message-ID: <CANn89iJi5GsaK6ZbuuiMDpHsWxj9fbAEG5Vj0CzoJeWFJvpj4Q@mail.gmail.com>
Date: Thu, 6 Mar 2025 21:14:55 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Willem de Bruijn <willemb@...gle.com>, David Ahern <dsahern@...nel.org>, 
	Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next] udp: expand SKB_DROP_REASON_UDP_CSUM use

On Thu, Mar 6, 2025 at 8:57 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Eric Dumazet wrote:
> > On Thu, Mar 6, 2025 at 8:01 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > Eric Dumazet wrote:
> > > > Use SKB_DROP_REASON_UDP_CSUM in __first_packet_length()
> > > > and udp_read_skb() when dropping a packet because of
> > > > a wrong UDP checksum.
> > > >
> > > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > > ---
> > > >  net/ipv4/udp.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > > > index 17c7736d8349433ad2d4cbcc9414b2f8112610af..39c3adf333b5f02ca53f768c918c75f2fc7f93ac 100644
> > > > --- a/net/ipv4/udp.c
> > > > +++ b/net/ipv4/udp.c
> > > > @@ -1848,7 +1848,7 @@ static struct sk_buff *__first_packet_length(struct sock *sk,
> > > >                       atomic_inc(&sk->sk_drops);
> > > >                       __skb_unlink(skb, rcvq);
> > > >                       *total += skb->truesize;
> > > > -                     kfree_skb(skb);
> > > > +                     kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);
> > > >               } else {
> > > >                       udp_skb_csum_unnecessary_set(skb);
> > > >                       break;
> > > > @@ -2002,7 +2002,7 @@ int udp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
> > > >               __UDP_INC_STATS(net, UDP_MIB_CSUMERRORS, is_udplite);
> > > >               __UDP_INC_STATS(net, UDP_MIB_INERRORS, is_udplite);
> > > >               atomic_inc(&sk->sk_drops);
> > > > -             kfree_skb(skb);
> > > > +             kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);
> > > >               goto try_again;
> > > >       }
> > >
> > > From a quick search for UDP_MIB_CSUMERRORS, one more case with regular
> > > kfree_skb:
> > >
> > > csum_copy_err:
> > >         if (!__sk_queue_drop_skb(sk, &udp_sk(sk)->reader_queue, skb, flags,
> > >                                  udp_skb_destructor)) {
> > >                 UDP_INC_STATS(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
> > >                 UDP_INC_STATS(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
> > >         }
> > >         kfree_skb(skb);
> >
> > Right, I was unsure because of the conditional SNMP updates.
>
> That seems to only suppress the update if peeking and the skb was
> already dequeued (ENOENT).
>
> Frankly, we probably never intended to increment this counter when
> peeking, as it is intended to be a per-datagram counter, not a
> per-recvmsg counter.
>
> I think ever erring on the side of extra increments in the unlikely
> case of ENOENT is fine.
>
> Cleaner is perhaps to have a kfree_skb_reason inside that branch and
> a consume_skb in the else.

I think it should be a  kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM),
because only one real 'free' will happen, if two or more threads were
using MSG_PEEK for this packet.

Using a consume_skb() would be racy, because if the skb refcount
reaches 0, a wrong event would be generated.

I will squash in v2 :

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 39c3adf333b5f02ca53f768c918c75f2fc7f93ac..d0bffcfa56d8deb14f38cc48a4a2b1d899ad6af4
100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2117,7 +2117,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr
*msg, size_t len, int flags,
                UDP_INC_STATS(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
                UDP_INC_STATS(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
        }
-       kfree_skb(skb);
+       kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);

        /* starting over for a new packet, but check if we need to yield */
        cond_resched();
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 3a0d6c5a8286b1685e8a1dec50365fe392ab9a87..024458ef163c9e24dfb37aea2690b2030f6a0fbc
100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -586,7 +586,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr
*msg, size_t len,
                SNMP_INC_STATS(mib, UDP_MIB_CSUMERRORS);
                SNMP_INC_STATS(mib, UDP_MIB_INERRORS);
        }
-       kfree_skb(skb);
+       kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);

        /* starting over for a new packet, but check if we need to yield */
        cond_resched();