Re: [PATCH net-next 1/5] tcp: fix SO_RCVLOWAT and RCVBUF autotuning

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Mon, Apr 16, 2018 at 10:33:35AM -0700, Eric Dumazet wrote:
> Applications might use SO_RCVLOWAT on TCP socket hoping to receive
> one [E]POLLIN event only when a given amount of bytes are ready in socket
> receive queue.
>
> Problem is that receive autotuning is not aware of this constraint,
> meaning sk_rcvbuf might be too small to allow all bytes to be stored.
>
> Add a new (struct proto_ops)->set_rcvlowat method so that a protocol
> can override the default setsockopt(SO_RCVLOWAT) behavior.
>

...

> +/* Make sure sk_rcvbuf is big enough to satisfy SO_RCVLOWAT hint */
> +int tcp_set_rcvlowat(struct sock *sk, int val)
> +{
> +	sk->sk_rcvlowat = val ? : 1;
> +	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
> +		return 0;
> +
> +	/* val comes from user space and might be close to INT_MAX */
> +	val <<= 1;
> +	if (val < 0)
> +		val = INT_MAX;
> +
> +	val = min(val, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);

Hi Eric,

As val may be changed to a smaller value by the line above, shouldn't
it assign sk->sk_rcvlowat again?  Otherwise it may still be bigger
than sk_rcvbuf.

Say val = 512k, sysctl_tcp_rmem[2] = 256k
val <<= 1 ,  val = 1M
val = min() ,  val = 256k
val > sk_rcvbuf
   sk_rcvbuf = 256k , at most, which is smaller than sk_rcvlowat

Without reassigning the application has to check how big is
tcp_rmem[2] and be sure to not go above /2 of it to not trip on this
again.

Or, as you have added a return value here, it could return -EINVAL in
such cases. Probably better, as then the application will not get a
smaller buffer than wanted later.

> +	if (val > sk->sk_rcvbuf) {
> +		sk->sk_rcvbuf = val;
> +		tcp_sk(sk)->window_clamp = tcp_win_from_space(sk, val);
> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL(tcp_set_rcvlowat);
> +
...