Re: [Patch bpf v3 3/4] net: implement ->sock_is_readable() for UDP and AF_UNIX

Cong Wang wrote:

On Thu, Oct 7, 2021 at 1:00 PM Daniel Borkmann [off-list ref] wrote:

quoted

On 10/2/21 2:37 AM, Cong Wang wrote:

quoted

From: Cong Wang <redacted>

Yucong noticed we can't poll() sockets in sockmap even
when they are the destination sockets of redirections.
This is because we never poll any psock queues in ->poll(),
except for TCP. With ->sock_is_readable() now we can
overwrite >sock_is_readable(), invoke and implement it for
both UDP and AF_UNIX sockets.

Reported-by: Yucong Sun <redacted>
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>
---
  net/ipv4/udp.c      | 2 ++
  net/ipv4/udp_bpf.c  | 1 +
  net/unix/af_unix.c  | 4 ++++
  net/unix/unix_bpf.c | 2 ++
  4 files changed, 9 insertions(+)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 2a7825a5b842..4a7e15a43a68 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c

@@ -2866,6 +2866,8 @@ __poll_t udp_poll(struct file *file, struct socket *sock, poll_table *wait)
          !(sk->sk_shutdown & RCV_SHUTDOWN) && first_packet_length(sk) == -1)
              mask &= ~(EPOLLIN | EPOLLRDNORM);

+     if (sk_is_readable(sk))
+             mask |= EPOLLIN | EPOLLRDNORM;

udp_poll() has this extra logic around first_packet_length() which drops all bad csum'ed
skbs. How does this stand in relation to sk_msg_is_readable()? Is this a concern as well
there? Maybe makes sense to elaborate a bit more in the commit message for context / future
reference.

We don't validate UDP checksums on sockmap RX path, so
it is okay to leave it as it is, but it is worth a comment like
you suggest. I will add a comment in this code.

If we really need to validate the checksum, it should be addressed
in a separate patch(set), not in this one.

Thanks.

We should validate the checksum before creating the sk_msg so that any parsers running
on top of UDP don't parse a bad checksum payload or pass a bad checksum up to user
space. I thought there would be a check in the read_sock side, but I didn't see it.