[PATCH net-next v1] tcp: remove redundant memset in hot paths with CONFIG_INIT_STACK_ALL_ZERO

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Modern compilers with CONFIG_INIT_STACK_ALL_ZERO enabled
(-ftrivial-auto-var-init=zero) automatically zero-initialize all stack
variables via `rep stos` in the function prologue. However, the compiler
cannot eliminate explicit memset() calls on those same variables,
resulting in redundant zeroing of the same memory region.

Replace explicit memset() with `= {0}` initializers for stack variables
in TCP hot path functions. With `= {0}`, the compiler recognizes a single
initialization and generates only one zeroing sequence, whereas a
separate memset() after declaration always produces a second `rep stos`.

scripts/bloat-o-meter -t ../vmlinux.old vmlinux

  add/remove: 0/0 grow/shrink: 0/5 up/down: 0/-148 (-148)
  Function                                     old     new   delta
  __tcp_transmit_skb                          5256    5247      -9
  tcp_v6_send_response                        2629    2612     -17
  tcp_make_synack                             3106    3089     -17
  tcp_v4_send_ack                             1465    1428     -37
  tcp_v4_send_reset                           3269    3201     -68
  Total: Before=30075907, After=30075759, chg -0.00%

No functional change intended.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/ipv4/tcp_ipv4.c   | 13 ++++---------
 net/ipv4/tcp_output.c |  6 ++----
 net/ipv6/tcp_ipv6.c   |  3 +--
 3 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 2ea8253b737a..1a413ca0e3bf 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -726,10 +726,10 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
 	struct {
 		struct tcphdr th;
 		__be32 opt[REPLY_OPTIONS_LEN];
-	} rep;
+	} rep = {0};
 	const __u8 *md5_hash_location = NULL;
 	const struct tcp_ao_hdr *aoh;
-	struct ip_reply_arg arg;
+	struct ip_reply_arg arg = {0};
 #ifdef CONFIG_TCP_MD5SIG
 	struct tcp_md5sig_key *key = NULL;
 	unsigned char newhash[16];
@@ -751,7 +751,6 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
 		return;
 
 	/* Swap the send and the receive. */
-	memset(&rep, 0, sizeof(rep));
 	rep.th.dest   = th->source;
 	rep.th.source = th->dest;
 	rep.th.doff   = sizeof(struct tcphdr) / 4;
@@ -765,7 +764,6 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
 				       skb->len - (th->doff << 2));
 	}
 
-	memset(&arg, 0, sizeof(arg));
 	arg.iov[0].iov_base = (unsigned char *)&rep;
 	arg.iov[0].iov_len  = sizeof(rep.th);
 
@@ -921,15 +919,12 @@ static void tcp_v4_send_ack(const struct sock *sk,
 	struct {
 		struct tcphdr th;
 		__be32 opt[(MAX_TCP_OPTION_SPACE  >> 2)];
-	} rep;
+	} rep = {0};
 	struct net *net = sock_net(sk);
-	struct ip_reply_arg arg;
+	struct ip_reply_arg arg = {0};
 	struct sock *ctl_sk;
 	u64 transmit_time;
 
-	memset(&rep.th, 0, sizeof(struct tcphdr));
-	memset(&arg, 0, sizeof(arg));
-
 	arg.iov[0].iov_base = (unsigned char *)&rep;
 	arg.iov[0].iov_len  = sizeof(rep.th);
 	if (tsecr) {
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 34a25ef61006..a0d5bed59f44 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1533,7 +1533,7 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 	struct inet_sock *inet;
 	struct tcp_sock *tp;
 	struct tcp_skb_cb *tcb;
-	struct tcp_out_options opts;
+	struct tcp_out_options opts = {0};
 	unsigned int tcp_options_size, tcp_header_size;
 	struct sk_buff *oskb = NULL;
 	struct tcp_key key;
@@ -1566,7 +1566,6 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 
 	inet = inet_sk(sk);
 	tcb = TCP_SKB_CB(skb);
-	memset(&opts.cleared, 0, sizeof(opts.cleared));
 
 	tcp_get_current_key(sk, &key);
 	if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
@@ -3934,7 +3933,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 {
 	struct inet_request_sock *ireq = inet_rsk(req);
 	const struct tcp_sock *tp = tcp_sk(sk);
-	struct tcp_out_options opts;
+	struct tcp_out_options opts = {0};
 	struct tcp_key key = {};
 	struct sk_buff *skb;
 	int tcp_header_size;
@@ -3972,7 +3971,6 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 
 	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
-	memset(&opts, 0, sizeof(opts));
 	now = tcp_clock_ns();
 #ifdef CONFIG_SYN_COOKIES
 	if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 8dc3874e8b92..e801927a8ce3 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -847,7 +847,7 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 	struct dst_entry *dst;
 	struct sk_buff *buff;
 	struct tcphdr *t1;
-	struct flowi6 fl6;
+	struct flowi6 fl6 = {0};
 	u32 mark = 0;
 
 	if (tsecr)
@@ -922,7 +922,6 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 	}
 #endif
 
-	memset(&fl6, 0, sizeof(fl6));
 	fl6.daddr = ipv6_hdr(skb)->saddr;
 	fl6.saddr = ipv6_hdr(skb)->daddr;
 	fl6.flowlabel = label;
-- 
2.43.0