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

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> 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>
> > > ---

[...]

> > >  }
> > >  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.

I was only asking for further details because the offset addition
below struck me as odd.

> 
> >
> > 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.

We need MSG_PEEK to work from application side. From sockmap
side I agree its not needed.

> 
> 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.

Not claiming any "standard" here only that user application needs
to work correctly if it passes MSG_PEEK.

> 
> >
> > > +             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.

huh? If its not used the addition is just confusing. Can we drop it?

> 
> 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.
> 

Yes, but tracking offset only to clear it a couple lines later
is confusing.

> Thanks.