[PATCH net-next] tcp: replace per-packet memset in __tcp_transmit_skb()

[Keita Morisaki <kmta1236@gmail.com>]

Replace memset(&opts, 0, sizeof(opts)) with targeted initialization of
only the three fields read unconditionally by tcp_options_write() and
bpf_skops_write_hdr_opt(): mss, num_sack_blocks, and bpf_opt_len.

struct tcp_out_options is 40 bytes without MPTCP and 96 bytes with
CONFIG_MPTCP=y (typical distro config). Every remaining field is either
assigned before first use by tcp_established_options()/tcp_syn_options(),
or gated behind its OPTION_* flag in tcp_options_write(). This memset
runs on every transmitted TCP packet, so removing it reduces per-packet
overhead on the hot path.

Assembly comparison (x86-64, GCC 13, CONFIG_MPTCP=n):

  Before: 5 store instructions zeroing 40 bytes
  After:  3 store instructions zeroing 4 bytes

With CONFIG_MPTCP=y the savings are larger: 12 stores (96 bytes) become
3 stores (4 bytes).

A BUILD_BUG_ON guards against future field additions: if the struct size
changes, the assertion fires and forces the developer to audit whether
the new field needs explicit zeroing.

Also add opts->options = 0 at the top of tcp_syn_options(), which
already used |= without a prior clear. tcp_established_options() already
clears opts->options at its top.

Signed-off-by: Keita Morisaki <kmta1236@gmail.com>
---
 net/ipv4/tcp_output.c | 16 ++++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 326b58ff1118d..ae04c697dbacc 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -965,6 +965,8 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 	struct tcp_fastopen_request *fastopen = tp->fastopen_req;
 	bool timestamps;

+	opts->options = 0;
+
 	/* Better than switch (key.type) as it has static branches */
 	if (tcp_key_is_md5(key)) {
 		timestamps = false;
@@ -1549,7 +1551,20 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,

 	inet = inet_sk(sk);
 	tcb = TCP_SKB_CB(skb);
-	memset(&opts, 0, sizeof(opts));
+	/* Only zero fields read unconditionally by tcp_options_write()
+	 * or bpf_skops_write_hdr_opt(). Other fields are set by
+	 * tcp_established_options()/tcp_syn_options() before use, or
+	 * gated behind their OPTION_* flag.
+	 *
+	 * If you add a field to tcp_out_options, this will fire —
+	 * audit whether the new field needs zeroing here.
+	 */
+	BUILD_BUG_ON(sizeof(opts) !=
+		     sizeof(struct mptcp_out_options) + 40);
+	opts.mss = 0;
+	opts.num_sack_blocks = 0;
+	opts.bpf_opt_len = 0;

 	tcp_get_current_key(sk, &key);
 	if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {


base-commit: 7dff99b354601e04378b4490e1b1b5765a3c3a88
--
2.43.0