tcp: allow MPTCP to update the announced window

The MPTCP RFC requires that the MPTCP-level receive window's
right edge never moves backward. Currently the MPTCP code
enforces such constraint while tracking the right edge, but it
does not reflects it on the wire, as MPTCP lacks a suitable hook
to update accordingly the TCP header.

This change modifies the existing mptcp_write_options() hook,
providing the current packet's TCP header to the MPTCP protocol,
so that the next patch could implement the above mentioned
constraint.

No functional changes intended.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

include/net/mptcp.h

@@ -125,7 +125,7 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
 			       struct mptcp_out_options *opts);
 bool mptcp_incoming_options(struct sock *sk, struct sk_buff *skb);
 
-void mptcp_write_options(__be32 *ptr, const struct tcp_sock *tp,
+void mptcp_write_options(struct tcphdr *th, __be32 *ptr, struct tcp_sock *tp,
 			 struct mptcp_out_options *opts);
 
 void mptcp_diag_fill_info(struct mptcp_sock *msk, struct mptcp_info *info);

net/ipv4/tcp_output.c

@@ -445,12 +445,13 @@ struct tcp_out_options {
 	struct mptcp_out_options mptcp;
 };
 
-static void mptcp_options_write(__be32 *ptr, const struct tcp_sock *tp,
+static void mptcp_options_write(struct tcphdr *th, __be32 *ptr,
+				struct tcp_sock *tp,
 				struct tcp_out_options *opts)
 {
 #if IS_ENABLED(CONFIG_MPTCP)
 	if (unlikely(OPTION_MPTCP & opts->options))
-		mptcp_write_options(ptr, tp, &opts->mptcp);
+		mptcp_write_options(th, ptr, tp, &opts->mptcp);
 #endif
 }
 
@@ -606,9 +607,10 @@ static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
  * At least SACK_PERM as the first option is known to lead to a disaster
  * (but it may well be that other scenarios fail similarly).
  */
-static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
+static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 			      struct tcp_out_options *opts)
 {
+	__be32 *ptr = (__be32 *)(th + 1);
 	u16 options = opts->options;	/* mungable copy */
 
 	if (unlikely(OPTION_MD5 & options)) {
@@ -702,7 +704,7 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
 
 	smc_options_write(ptr, &options);
 
-	mptcp_options_write(ptr, tp, opts);
+	mptcp_options_write(th, ptr, tp, opts);
 }
 
 static void smc_set_option(const struct tcp_sock *tp,
@@ -1355,7 +1357,7 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 		th->window	= htons(min(tp->rcv_wnd, 65535U));
 	}
 
-	tcp_options_write((__be32 *)(th + 1), tp, &opts);
+	tcp_options_write(th, tp, &opts);
 
 #ifdef CONFIG_TCP_MD5SIG
 	/* Calculate the MD5 hash, as we have all we need now */
@@ -3591,7 +3593,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 
 	/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
 	th->window = htons(min(req->rsk_rcv_wnd, 65535U));
-	tcp_options_write((__be32 *)(th + 1), NULL, &opts);
+	tcp_options_write(th, NULL, &opts);
 	th->doff = (tcp_header_size >> 2);
 	__TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
 

net/mptcp/options.c

@@ -1265,7 +1265,7 @@ static u16 mptcp_make_csum(const struct mptcp_ext *mpext)
 				 ~csum_unfold(mpext->csum));
 }
 
-void mptcp_write_options(__be32 *ptr, const struct tcp_sock *tp,
+void mptcp_write_options(struct tcphdr *th, __be32 *ptr, struct tcp_sock *tp,
 			 struct mptcp_out_options *opts)
 {
 	const struct sock *ssk = (const struct sock *)tp;