Re: [PATCH] net: pass back data left in socket after receive

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 27 Apr 2022 13:49:34 -0600 Jens Axboe wrote:
> This is currently done for CMSG_INQ, add an ability to do so via struct
> msghdr as well and have CMSG_INQ use that too. If the caller sets
> msghdr->msg_get_inq, then we'll pass back the hint in msghdr->msg_inq.
> 
> Rearrange struct msghdr a bit so we can add this member while shrinking
> it at the same time. On a 64-bit build, it was 96 bytes before this
> change and 88 bytes afterwards.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

LGTM, but I said that once before..  Eric?

FWIW the io_uring patch that uses it is here:
https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-flags2

> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 6f85f5d957ef..12085c9a8544 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -50,6 +50,9 @@ struct linger {
>  struct msghdr {
>  	void		*msg_name;	/* ptr to socket address structure */
>  	int		msg_namelen;	/* size of socket address structure */
> +
> +	int		msg_inq;	/* output, data left in socket */
> +
>  	struct iov_iter	msg_iter;	/* data */
>  
>  	/*
> @@ -62,8 +65,9 @@ struct msghdr {
>  		void __user	*msg_control_user;
>  	};
>  	bool		msg_control_is_user : 1;
> -	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
> +	bool		msg_get_inq : 1;/* return INQ after receive */
>  	unsigned int	msg_flags;	/* flags on received message */
> +	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
>  	struct kiocb	*msg_iocb;	/* ptr to iocb for async requests */
>  };
>  
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index cf18fbcbf123..78d79e26fb4d 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2335,8 +2335,10 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  	if (sk->sk_state == TCP_LISTEN)
>  		goto out;
>  
> -	if (tp->recvmsg_inq)
> +	if (tp->recvmsg_inq) {
>  		*cmsg_flags = TCP_CMSG_INQ;
> +		msg->msg_get_inq = 1;
> +	}
>  	timeo = sock_rcvtimeo(sk, nonblock);
>  
>  	/* Urgent data needs to be handled specially. */
> @@ -2559,7 +2561,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int nonblock,
>  		int flags, int *addr_len)
>  {
> -	int cmsg_flags = 0, ret, inq;
> +	int cmsg_flags = 0, ret;
>  	struct scm_timestamping_internal tss;
>  
>  	if (unlikely(flags & MSG_ERRQUEUE))
> @@ -2576,12 +2578,14 @@ int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int nonblock,
>  	release_sock(sk);
>  	sk_defer_free_flush(sk);
>  
> -	if (cmsg_flags && ret >= 0) {
> +	if ((cmsg_flags || msg->msg_get_inq) && ret >= 0) {
>  		if (cmsg_flags & TCP_CMSG_TS)
>  			tcp_recv_timestamp(msg, sk, &tss);
> +		if (msg->msg_get_inq)
> +			msg->msg_inq = tcp_inq_hint(sk);
>  		if (cmsg_flags & TCP_CMSG_INQ) {
> -			inq = tcp_inq_hint(sk);
> -			put_cmsg(msg, SOL_TCP, TCP_CM_INQ, sizeof(inq), &inq);
> +			put_cmsg(msg, SOL_TCP, TCP_CM_INQ, sizeof(msg->msg_inq),
> +				 &msg->msg_inq);
>  		}
>  	}
>  	return ret;
>