On Mon, Mar 29, 2021 at 1:54 PM John Fastabend [off-list ref] wrote:

Cong Wang wrote:

quoted

From: Cong Wang <redacted>

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 <redacted>
Signed-off-by: Cong Wang <redacted>
---
 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.

quoted

+             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.