[PATCH net-next] net: fix off-by-one in udp_flow_src_port() / psp_write_headers()

[PATCH net-next] net: fix off-by-one in udp_flow_src_port() / psp_write_headers()

[Eric Dumazet]

udp_flow_src_port() and psp_write_headers() use ip_local_port_range.

ip_local_port_range is inclusive : all ports between min and max
can be used.

Before this patch, if ip_local_port_range was set to 40000-40001
40001 would not be used as a source port.

Use reciprocal_scale() to help code readability.

Not tagged for stable trees, as this change could break user
expectations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/net/udp.h  | 3 ++-
 net/psp/psp_main.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/udp.h b/include/net/udp.h
index da68702ddf6edc6a28440490be07be7eef2d1a4e..b648003e5792a167822f28aeb728c3d64a2141f4 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -29,6 +29,7 @@
 #include <linux/seq_file.h>
 #include <linux/poll.h>
 #include <linux/indirect_call_wrapper.h>
+#include <linux/math.h>
 
 /**
  *	struct udp_skb_cb  -  UDP(-Lite) private variables
@@ -376,7 +377,7 @@ static inline __be16 udp_flow_src_port(struct net *net, struct sk_buff *skb,
 	 */
 	hash ^= hash << 16;
 
-	return htons((((u64) hash * (max - min)) >> 32) + min);
+	return htons(reciprocal_scale(hash, max - min + 1) + min);
 }
 
 static inline int udp_rqueue_get(struct sock *sk)
diff --git a/net/psp/psp_main.c b/net/psp/psp_main.c
index d4c04c923c5ac6aead8b5bac41c54ae1143f4110..9508b6c380038b311a9b778933789b5c4cb61007 100644
--- a/net/psp/psp_main.c
+++ b/net/psp/psp_main.c
@@ -202,7 +202,7 @@ static void psp_write_headers(struct net *net, struct sk_buff *skb, __be32 spi,
 		 * reciprocal divide.
 		 */
 		hash ^= hash << 16;
-		uh->source = htons((((u64)hash * (max - min)) >> 32) + min);
+		uh->source = htons(reciprocal_scale(hash, max - min + 1) + min);
 	} else {
 		uh->source = udp_flow_src_port(net, skb, 0, 0, false);
 	}
-- 
2.53.0.473.g4a7958ca14-goog

Re: [PATCH net-next] net: fix off-by-one in udp_flow_src_port() / psp_write_headers()

[Willem de Bruijn]

Eric Dumazet wrote:
> udp_flow_src_port() and psp_write_headers() use ip_local_port_range.
> 
> ip_local_port_range is inclusive : all ports between min and max
> can be used.
> 
> Before this patch, if ip_local_port_range was set to 40000-40001
> 40001 would not be used as a source port.
> 
> Use reciprocal_scale() to help code readability.
> 
> Not tagged for stable trees, as this change could break user
> expectations.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

Re: [PATCH net-next] net: fix off-by-one in udp_flow_src_port() / psp_write_headers()

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon,  2 Mar 2026 16:39:33 +0000 you wrote:
> udp_flow_src_port() and psp_write_headers() use ip_local_port_range.
> 
> ip_local_port_range is inclusive : all ports between min and max
> can be used.
> 
> Before this patch, if ip_local_port_range was set to 40000-40001
> 40001 would not be used as a source port.
> 
> [...]

Here is the summary with links:
  - [net-next] net: fix off-by-one in udp_flow_src_port() / psp_write_headers()
    https://git.kernel.org/netdev/net-next/c/c26b8c4e291c

You are awesome, thank you!
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