Subject: Re: [Patch bpf-next v3 02/10] af_unix: implement ->read_sock() for sockmap

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 05 May 2021 19:14:06 +0200

On Mon, Apr 26, 2021 at 04:49 AM CEST, Cong Wang wrote:
> From: Cong Wang <[email-masked]>
>
> Implement ->read_sock() for AF_UNIX datagram socket, it is
> pretty much similar to udp_read_sock().
>
> Cc: John Fastabend <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: Jakub Sitnicki <[email-masked]>
> Cc: Lorenz Bauer <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>
> ---
>  net/unix/af_unix.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
>
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 5a31307ceb76..f4dc22db371d 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -661,6 +661,8 @@ static ssize_t unix_stream_splice_read(struct socket *,  loff_t *ppos,
>  				       unsigned int flags);
>  static int unix_dgram_sendmsg(struct socket *, struct msghdr *, size_t);
>  static int unix_dgram_recvmsg(struct socket *, struct msghdr *, size_t, int);
> +static int unix_read_sock(struct sock *sk, read_descriptor_t *desc,
> +			  sk_read_actor_t recv_actor);
>  static int unix_dgram_connect(struct socket *, struct sockaddr *,
>  			      int, int);
>  static int unix_seqpacket_sendmsg(struct socket *, struct msghdr *, size_t);
> @@ -738,6 +740,7 @@ static const struct proto_ops unix_dgram_ops = {
>  	.listen =	sock_no_listen,
>  	.shutdown =	unix_shutdown,
>  	.sendmsg =	unix_dgram_sendmsg,
> +	.read_sock =	unix_read_sock,
>  	.recvmsg =	unix_dgram_recvmsg,
>  	.mmap =		sock_no_mmap,
>  	.sendpage =	sock_no_sendpage,
> @@ -2183,6 +2186,41 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
>  	return err;
>  }
>
> +static int unix_read_sock(struct sock *sk, read_descriptor_t *desc,
> +			  sk_read_actor_t recv_actor)
> +{
> +	int copied = 0;
> +
> +	while (1) {
> +		struct unix_sock *u = unix_sk(sk);
> +		struct sk_buff *skb;
> +		int used, err;
> +
> +		mutex_lock(&u->iolock);
> +		skb = skb_recv_datagram(sk, 0, 1, &err);
> +		if (!skb) {
> +			mutex_unlock(&u->iolock);
> +			return err;
> +		}
> +
> +		used = recv_actor(desc, skb, 0, skb->len);
> +		if (used <= 0) {
> +			if (!copied)
> +				copied = used;
> +			mutex_unlock(&u->iolock);
> +			break;
> +		} else if (used <= skb->len) {
> +			copied += used;
> +		}
> +		mutex_unlock(&u->iolock);

Do we need hold the mutex for recv_actor to process the skb?

> +
> +		if (!desc->count)
> +			break;
> +	}
> +
> +	return copied;
> +}
> +
>  /*
>   *	Sleep until more data has arrived. But check for races..
>   */