netdev - Re: [Patch bpf-next v3 1/4] tcp: introduce tcp_read

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Date:   Thu, 9 Jun 2022 11:50:34 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Cong Wang <cong.wang@...edance.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [Patch bpf-next v3 1/4] tcp: introduce tcp_read_skb()

On Thu, Jun 9, 2022 at 8:08 AM John Fastabend <john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > This patch inroduces tcp_read_skb() based on tcp_read_sock(),
> > a preparation for the next patch which actually introduces
> > a new sock ops.
> >
> > TCP is special here, because it has tcp_read_sock() which is
> > mainly used by splice(). tcp_read_sock() supports partial read
> > and arbitrary offset, neither of them is needed for sockmap.
> >
> > Cc: Eric Dumazet <edumazet@...gle.com>
> > Cc: John Fastabend <john.fastabend@...il.com>
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: Jakub Sitnicki <jakub@...udflare.com>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > ---
> >  include/net/tcp.h |  2 ++
> >  net/ipv4/tcp.c    | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 49 insertions(+)
> >
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index 1e99f5c61f84..878544d0f8f9 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -669,6 +669,8 @@ void tcp_get_info(struct sock *, struct tcp_info *);
> >  /* Read 'sendfile()'-style from a TCP socket */
> >  int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
> >                 sk_read_actor_t recv_actor);
> > +int tcp_read_skb(struct sock *sk, read_descriptor_t *desc,
> > +              sk_read_actor_t recv_actor);
> >
> >  void tcp_initialize_rcv_mss(struct sock *sk);
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 9984d23a7f3e..a18e9ababf54 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -1709,6 +1709,53 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
> >  }
> >  EXPORT_SYMBOL(tcp_read_sock);
> >
> > +int tcp_read_skb(struct sock *sk, read_descriptor_t *desc,
> > +              sk_read_actor_t recv_actor)
> > +{
> > +     struct tcp_sock *tp = tcp_sk(sk);
> > +     u32 seq = tp->copied_seq;
> > +     struct sk_buff *skb;
> > +     int copied = 0;
> > +     u32 offset;
> > +
> > +     if (sk->sk_state == TCP_LISTEN)
> > +             return -ENOTCONN;
> > +
> > +     while ((skb = tcp_recv_skb(sk, seq, &offset)) != NULL) {
> > +             int used;
> > +
> > +             __skb_unlink(skb, &sk->sk_receive_queue);
> > +             used = recv_actor(desc, skb, 0, skb->len);
> > +             if (used <= 0) {
> > +                     if (!copied)
> > +                             copied = used;
> > +                     break;
> > +             }
> > +             seq += used;
> > +             copied += used;
> > +
> > +             if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
> > +                     kfree_skb(skb);
>
> Hi Cong, can you elaborate here from v2 comment.
>
> "Hm, it is tricky here, we use the skb refcount after this patchset, so
> it could be a real drop from another kfree_skb() in net/core/skmsg.c
> which initiates the drop."

Sure.

This is the source code of consume_skb():

 911 void consume_skb(struct sk_buff *skb)
 912 {
 913         if (!skb_unref(skb))
 914                 return;
 915
 916         trace_consume_skb(skb);
 917         __kfree_skb(skb);
 918 }

and this is kfree_skb (or kfree_skb_reason()):

 770 void kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
 771 {
 772         if (!skb_unref(skb))
 773                 return;
 774
 775         DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >=
SKB_DROP_REASON_MAX);
 776
 777         trace_kfree_skb(skb, __builtin_return_address(0), reason);
 778         __kfree_skb(skb);
 779 }

So, both do refcnt before tracing, very clearly.

Now, let's do a simple case:

tcp_read_skb():
 -> tcp_recv_skb() // Let's assume skb refcnt == 1 here
  -> recv_actor()
   -> skb_get() // refcnt == 2
   -> kfree_skb() // Let's assume users drop it intentionally
 ->kfree_skb() // refcnt == 0 here, if we had consume_skb() it would
not be counted as a drop

Of course you can give another example where consume_skb() is
correct, but the point here is it is very tricky when refcnt, I even doubt
we can do anything here, maybe moving trace before refcnt.

>
> The tcp_read_sock() hook is using tcp_eat_recv_skb(). Are we going
> to kick tracing infra even on good cases with kfree_skb()? In
> sk_psock_verdict_recv() we do an skb_clone() there.

I don't get your point here, are you suggesting we should sacrifice
performance just to make the drop tracing more accurate??

Thanks.