Re: [PATCH nft] src: mnl: fix --echo buffer size -- again

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, Sep 10, 2019 at 12:19:18AM +0200, Florian Westphal wrote:
[...]
> diff --git a/src/mnl.c b/src/mnl.c
> index 9c1f5356c9b9..d664564e16af 100644
> --- a/src/mnl.c
> +++ b/src/mnl.c
> @@ -311,8 +311,6 @@ int mnl_batch_talk(struct netlink_ctx *ctx, struct list_head *err_list,
>  	int ret, fd = mnl_socket_get_fd(nl), portid = mnl_socket_get_portid(nl);
>  	uint32_t iov_len = nftnl_batch_iovec_len(ctx->batch);
>  	char rcv_buf[MNL_SOCKET_BUFFER_SIZE];
> -	unsigned int enobuf_restarts = 0;
> -	size_t avg_msg_size, batch_size;
>  	const struct sockaddr_nl snl = {
>  		.nl_family = AF_NETLINK
>  	};
> @@ -321,17 +319,22 @@ int mnl_batch_talk(struct netlink_ctx *ctx, struct list_head *err_list,
>  		.tv_usec	= 0
>  	};
>  	struct iovec iov[iov_len];
> -	unsigned int scale = 4;
>  	struct msghdr msg = {};
>  	fd_set readfds;
>  
>  	mnl_set_sndbuffer(ctx->nft->nf_sock, ctx->batch);
>  
> -	batch_size = mnl_nft_batch_to_msg(ctx, &msg, &snl, iov, iov_len);
> -	avg_msg_size = div_round_up(batch_size, num_cmds);
> +	mnl_nft_batch_to_msg(ctx, &msg, &snl, iov, iov_len);
>  
> -restart:
> -	mnl_set_rcvbuffer(ctx->nft->nf_sock, num_cmds * avg_msg_size * scale);
> +	if (nft_output_echo(&ctx->nft->output)) {
> +		size_t buffer_size = MNL_SOCKET_BUFFER_SIZE * 1024;
> +		size_t new_buffer_size = num_cmds * 1024;

Probably all simplify this to?

		mnl_set_rcvbuffer(ctx->nft->nf_sock, (1 << 10) * num_cmds);

Upper limit for us is NLMSG_GOODSIZE, which is not exposed to
userspace, although we have control over that upper limit from
nf_tables. In practise, I think we go over the 1 Kbytes per message
boundary.

> +
> +		if (new_buffer_size > buffer_size)
> +			buffer_size = new_buffer_size;
> +
> +		mnl_set_rcvbuffer(ctx->nft->nf_sock, buffer_size);
> +	}
>  
>  	ret = mnl_nft_socket_sendmsg(ctx, &msg);
>  	if (ret == -1)