netdev - [PATCH v4 net-next 07/15] tcp: allow embedding leftover into option padding

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Message-Id: <20250417230029.21905-8-chia-yu.chang@nokia-bell-labs.com>
Date: Fri, 18 Apr 2025 01:00:21 +0200
From: chia-yu.chang@...ia-bell-labs.com
To: dsahern@...nel.org,
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v4 net-next 07/15] tcp: allow embedding leftover into option padding

From: Ilpo Järvinen <ij@...nel.org>

There is some waste space in the option usage due to padding
of 32-bit fields. AccECN option can take advantage of those
few bytes as its tail is often consuming just a few odd bytes.

Signed-off-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 net/ipv4/tcp_output.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b4eac0725682..d63f505a30e2 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -709,6 +709,8 @@ static __be32 *process_tcp_ao_options(struct tcp_sock *tp,
 	return ptr;
 }
 
+#define NOP_LEFTOVER	((TCPOPT_NOP << 8) | TCPOPT_NOP)
+
 /* Write previously computed TCP options to the packet.
  *
  * Beware: Something in the Internet is very sensitive to the ordering of
@@ -727,8 +729,10 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 			      struct tcp_out_options *opts,
 			      struct tcp_key *key)
 {
+	u16 leftover_bytes = NOP_LEFTOVER;      /* replace next NOPs if avail */
 	__be32 *ptr = (__be32 *)(th + 1);
 	u16 options = opts->options;	/* mungable copy */
+	int leftover_size = 2;
 
 	if (tcp_key_is_md5(key)) {
 		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
@@ -763,17 +767,22 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 	}
 
 	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
-		*ptr++ = htonl((TCPOPT_NOP << 24) |
-			       (TCPOPT_NOP << 16) |
+		*ptr++ = htonl((leftover_bytes << 16) |
 			       (TCPOPT_SACK_PERM << 8) |
 			       TCPOLEN_SACK_PERM);
+		leftover_bytes = NOP_LEFTOVER;
 	}
 
 	if (unlikely(OPTION_WSCALE & options)) {
-		*ptr++ = htonl((TCPOPT_NOP << 24) |
+		u8 highbyte = TCPOPT_NOP;
+
+		if (unlikely(leftover_size == 1))
+			highbyte = leftover_bytes >> 8;
+		*ptr++ = htonl((highbyte << 24) |
 			       (TCPOPT_WINDOW << 16) |
 			       (TCPOLEN_WINDOW << 8) |
 			       opts->ws);
+		leftover_bytes = NOP_LEFTOVER;
 	}
 
 	if (unlikely(opts->num_sack_blocks)) {
@@ -781,8 +790,7 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 			tp->duplicate_sack : tp->selective_acks;
 		int this_sack;
 
-		*ptr++ = htonl((TCPOPT_NOP  << 24) |
-			       (TCPOPT_NOP  << 16) |
+		*ptr++ = htonl((leftover_bytes << 16) |
 			       (TCPOPT_SACK <<  8) |
 			       (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
 						     TCPOLEN_SACK_PERBLOCK)));
@@ -794,6 +802,10 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 		}
 
 		tp->rx_opt.dsack = 0;
+	} else if (unlikely(leftover_bytes != NOP_LEFTOVER)) {
+		*ptr++ = htonl((leftover_bytes << 16) |
+			       (TCPOPT_NOP << 8) |
+			       TCPOPT_NOP);
 	}
 
 	if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
-- 
2.34.1