tcp: return a drop_reason from tcp_add_backlog()

Part of a stack canary removal from tcp_v{4,6}_rcv().

Return a drop_reason instead of a boolean, so that we no longer
have to pass the address of a local variable.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-37 (-37)
Function                                     old     new   delta
tcp_v6_rcv                                  3133    3129      -4
tcp_v4_rcv                                  3206    3202      -4
tcp_add_backlog                             1281    1252     -29
Total: Before=25567186, After=25567149, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260409101147.1642967-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 12 insertions, 15 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6156d1d068e1..fce4b653c23e 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1680,8 +1680,7 @@ static inline bool tcp_checksum_complete(struct sk_buff *skb)
 		__skb_checksum_complete(skb);
 }
 
-bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
-		     enum skb_drop_reason *reason);
+enum skb_drop_reason tcp_add_backlog(struct sock *sk, struct sk_buff *skb);
 
 static inline int tcp_filter(struct sock *sk, struct sk_buff *skb,
 			     enum skb_drop_reason *reason)
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 69ab236072e7..c9bbbf323648 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1900,8 +1900,7 @@ err_discard:
 }
 EXPORT_SYMBOL(tcp_v4_do_rcv);
 
-bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
-		     enum skb_drop_reason *reason)
+enum skb_drop_reason tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
 	u32 tail_gso_size, tail_gso_segs;
 	struct skb_shared_info *shinfo;
@@ -1929,10 +1928,9 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 	if (unlikely(tcp_checksum_complete(skb))) {
 		bh_unlock_sock(sk);
 		trace_tcp_bad_csum(skb);
-		*reason = SKB_DROP_REASON_TCP_CSUM;
 		__TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
 		__TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
-		return true;
+		return SKB_DROP_REASON_TCP_CSUM;
 	}
 
 	/* Attempt coalescing to last skb in backlog, even if we are
@@ -2006,7 +2004,7 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 		__NET_INC_STATS(sock_net(sk),
 				LINUX_MIB_TCPBACKLOGCOALESCE);
 		kfree_skb_partial(skb, fragstolen);
-		return false;
+		return SKB_NOT_DROPPED_YET;
 	}
 	__skb_push(skb, hdrlen);
 
@@ -2031,15 +2029,13 @@ no_coalesce:
 	if (unlikely(err)) {
 		bh_unlock_sock(sk);
 		if (err == -ENOMEM) {
-			*reason = SKB_DROP_REASON_PFMEMALLOC;
 			__NET_INC_STATS(sock_net(sk), LINUX_MIB_PFMEMALLOCDROP);
-		} else {
-			*reason = SKB_DROP_REASON_SOCKET_BACKLOG;
-			__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPBACKLOGDROP);
+			return SKB_DROP_REASON_PFMEMALLOC;
 		}
-		return true;
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPBACKLOGDROP);
+		return SKB_DROP_REASON_SOCKET_BACKLOG;
 	}
-	return false;
+	return SKB_NOT_DROPPED_YET;
 }
 
 static void tcp_v4_restore_cb(struct sk_buff *skb)
@@ -2247,7 +2243,8 @@ process:
 	if (!sock_owned_by_user(sk)) {
 		ret = tcp_v4_do_rcv(sk, skb);
 	} else {
-		if (tcp_add_backlog(sk, skb, &drop_reason))
+		drop_reason = tcp_add_backlog(sk, skb);
+		if (drop_reason)
 			goto discard_and_relse;
 	}
 	bh_unlock_sock(sk);
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 8dc3874e8b92..1d37826e8480 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1877,7 +1877,8 @@ process:
 	if (!sock_owned_by_user(sk)) {
 		ret = tcp_v6_do_rcv(sk, skb);
 	} else {
-		if (tcp_add_backlog(sk, skb, &drop_reason))
+		drop_reason = tcp_add_backlog(sk, skb);
+		if (drop_reason)
 			goto discard_and_relse;
 	}
 	bh_unlock_sock(sk);