Re: [Patch bpf-next v7 09/13] udp: implement ->read_sock() for sockmap

[Cong Wang <xiyou.wangcong@gmail.com>]

On Mon, Mar 29, 2021 at 1:54 PM John Fastabend <john.fastabend@gmail.com> wrote:
>
> Cong Wang wrote:
> > From: Cong Wang <cong.wang@bytedance.com>
> >
> > This is similar to tcp_read_sock(), except we do not need
> > to worry about connections, we just need to retrieve skb
> > from UDP receive queue.
> >
> > Note, the return value of ->read_sock() is unused in
> > sk_psock_verdict_data_ready().
> >
> > Cc: John Fastabend <john.fastabend@gmail.com>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > Cc: Jakub Sitnicki <jakub@cloudflare.com>
> > Cc: Lorenz Bauer <lmb@cloudflare.com>
> > Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> > ---
> >  include/net/udp.h   |  2 ++
> >  net/ipv4/af_inet.c  |  1 +
> >  net/ipv4/udp.c      | 35 +++++++++++++++++++++++++++++++++++
> >  net/ipv6/af_inet6.c |  1 +
> >  4 files changed, 39 insertions(+)
> >
> > diff --git a/include/net/udp.h b/include/net/udp.h
> > index df7cc1edc200..347b62a753c3 100644
> > --- a/include/net/udp.h
> > +++ b/include/net/udp.h
> > @@ -329,6 +329,8 @@ struct sock *__udp6_lib_lookup(struct net *net,
> >                              struct sk_buff *skb);
> >  struct sock *udp6_lib_lookup_skb(const struct sk_buff *skb,
> >                                __be16 sport, __be16 dport);
> > +int udp_read_sock(struct sock *sk, read_descriptor_t *desc,
> > +               sk_read_actor_t recv_actor);
> >
> >  /* UDP uses skb->dev_scratch to cache as much information as possible and avoid
> >   * possibly multiple cache miss on dequeue()
> > diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> > index 1355e6c0d567..f17870ee558b 100644
> > --- a/net/ipv4/af_inet.c
> > +++ b/net/ipv4/af_inet.c
> > @@ -1070,6 +1070,7 @@ const struct proto_ops inet_dgram_ops = {
> >       .setsockopt        = sock_common_setsockopt,
> >       .getsockopt        = sock_common_getsockopt,
> >       .sendmsg           = inet_sendmsg,
> > +     .read_sock         = udp_read_sock,
> >       .recvmsg           = inet_recvmsg,
> >       .mmap              = sock_no_mmap,
> >       .sendpage          = inet_sendpage,
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 38952aaee3a1..04620e4d64ab 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -1782,6 +1782,41 @@ struct sk_buff *__skb_recv_udp(struct sock *sk, unsigned int flags,
> >  }
> >  EXPORT_SYMBOL(__skb_recv_udp);
> >
> > +int udp_read_sock(struct sock *sk, read_descriptor_t *desc,
> > +               sk_read_actor_t recv_actor)
> > +{
> > +     int copied = 0;
> > +
> > +     while (1) {
> > +             int offset = 0, err;
>
> Should this be
>
>  int offset = sk_peek_offset()?

What are you really suggesting? sk_peek_offset() is just 0 unless
we have MSG_PEEK here and we don't, because we really want to
dequeue the skb rather than peeking it.

Are you suggesting we should do peeking? I am afraid we can't.
Please be specific, guessing your mind is not an effective way to
address your reviews.

>
> MSG_PEEK should work from recv side, at least it does on TCP side. If
> its handled in some following patch a comment would be nice. I was
> just reading udp_recvmsg() so maybe its not needed.

Please explain why do we need peeking in sockmap? At very least
it has nothing to do with my patchset.

I do not know why you want to use TCP as a "standard" here, TCP
also supports splice(), UDP still doesn't even with ->read_sock().
Of course they are very different.

>
> > +             struct sk_buff *skb;
> > +
> > +             skb = __skb_recv_udp(sk, 0, 1, &offset, &err);
> > +             if (!skb)
> > +                     return err;
> > +             if (offset < skb->len) {
> > +                     size_t len;
> > +                     int used;
> > +
> > +                     len = skb->len - offset;
> > +                     used = recv_actor(desc, skb, offset, len);
> > +                     if (used <= 0) {
> > +                             if (!copied)
> > +                                     copied = used;
> > +                             break;
> > +                     } else if (used <= len) {
> > +                             copied += used;
> > +                             offset += used;
>
> The while loop is going to zero this? What are we trying to do
> here with offset?

offset only matters for MSG_PEEK and we do not support peeking
in sockmap case, hence it is unnecessary here. I "use" it here just
to make the code as complete as possible.

To further answer your question, it is set to 0 when we return a
valid skb on line 201 inside __skb_try_recv_from_queue(), as
"_off" is set to 0 and won't change unless we have MSG_PEEK.

173         bool peek_at_off = false;
174         struct sk_buff *skb;
175         int _off = 0;
176
177         if (unlikely(flags & MSG_PEEK && *off >= 0)) {
178                 peek_at_off = true;
179                 _off = *off;
180         }
181
182         *last = queue->prev;
183         skb_queue_walk(queue, skb) {
184                 if (flags & MSG_PEEK) {
185                         if (peek_at_off && _off >= skb->len &&
186                             (_off || skb->peeked)) {
187                                 _off -= skb->len;
188                                 continue;
189                         }
190                         if (!skb->len) {
191                                 skb = skb_set_peeked(skb);
192                                 if (IS_ERR(skb)) {
193                                         *err = PTR_ERR(skb);
194                                         return NULL;
195                                 }
196                         }
197                         refcount_inc(&skb->users);
198                 } else {
199                         __skb_unlink(skb, queue);
200                 }
201                 *off = _off;
202                 return skb;

Of course, when we return NULL, we return immediately without
using offset:

1794                 skb = __skb_recv_udp(sk, 0, 1, &offset, &err);
1795                 if (!skb)
1796                         return err;

This should not be hard to figure out. Hope it is clear now.

Thanks.