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

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Date:   Thu, 7 Oct 2021 22:00:24 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     bpf@...r.kernel.org, Cong Wang <cong.wang@...edance.com>,
        Yucong Sun <sunyucong@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf v3 3/4] net: implement ->sock_is_readable() for UDP
 and AF_UNIX

On 10/2/21 2:37 AM, Cong Wang wrote:
> From: Cong Wang <cong.wang@...edance.com>
> 
> 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 <sunyucong@...il.com>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Jakub Sitnicki <jakub@...udflare.com>
> Cc: Lorenz Bauer <lmb@...udflare.com>
> Signed-off-by: Cong Wang <cong.wang@...edance.com>
> ---
>   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.

Thanks,
Daniel

>   	return mask;
>   
>   }