From: Eric Dumazet <edumazet@google.com>
Date: Tue, 16 Sep 2025 16:09:46 +0000 (+0000)
Subject: udp: refine __udp_enqueue_schedule_skb() test
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=9aaec660b5be29f23aaa7d1b0ae426b895dc0ca5;p=users%2Fhch%2Fmisc.git

udp: refine __udp_enqueue_schedule_skb() test

Commit 5a465a0da13e ("udp: Fix multiple wraparounds
of sk->sk_rmem_alloc.") allowed to slightly overshoot
sk->sk_rmem_alloc, when many cpus are trying
to feed packets to a common UDP socket.

This patch, combined with the following one reduces
false sharing on the victim socket under DDOS.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250916160951.541279-6-edumazet@google.com
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index cca41c569f37..edd846fee90f 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1739,8 +1739,8 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 		if (rcvbuf > INT_MAX >> 1)
 			goto drop;
 
-		/* Always allow at least one packet for small buffer. */
-		if (rmem > rcvbuf)
+		/* Accept the packet if queue is empty. */
+		if (rmem)
 			goto drop;
 	}