Currently, ip_extract_route_hint uses RTN_BROADCAST to decide
whether to use the route dst hint mechanism.

This check is too strict, as it prevents directed broadcast
routes from using the hint, resulting in poor performance
during bursts of directed broadcast traffic.

Fix this in ip_extract_route_hint and modify ip_route_use_hint
to preserve the intended behaviour.

Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
---
 net/ipv4/ip_input.c | 11 +++++++----
 net/ipv4/route.c    |  2 +-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index fc323994b1fa..a09aca2c8567 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -587,9 +587,13 @@ static void ip_sublist_rcv_finish(struct list_head *head)
 }
 
 static struct sk_buff *ip_extract_route_hint(const struct net *net,
-					     struct sk_buff *skb, int rt_type)
+					     struct sk_buff *skb)
 {
-	if (fib4_has_custom_rules(net) || rt_type == RTN_BROADCAST ||
+	const struct iphdr *iph = ip_hdr(skb);
+
+	if (fib4_has_custom_rules(net) ||
+	    ipv4_is_lbcast(iph->daddr) ||
+	    ipv4_is_zeronet(iph->daddr) ||
 	    IPCB(skb)->flags & IPSKB_MULTIPATH)
 		return NULL;
 
@@ -618,8 +622,7 @@ static void ip_list_rcv_finish(struct net *net, struct list_head *head)
 
 		dst = skb_dst(skb);
 		if (curr_dst != dst) {
-			hint = ip_extract_route_hint(net, skb,
-						     dst_rtable(dst)->rt_type);
+			hint = ip_extract_route_hint(net, skb);
 
 			/* dispatch old sublist */
 			if (!list_empty(&sublist))
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index f639a2ae881a..1f212b2ce4c6 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2210,7 +2210,7 @@ ip_route_use_hint(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 		goto martian_source;
 	}
 
-	if (rt->rt_type != RTN_LOCAL)
+	if (!(rt->rt_flags & RTCF_LOCAL))
 		goto skip_validate_source;
 
 	reason = fib_validate_source_reason(skb, saddr, daddr, dscp, 0, dev,
-- 
2.39.5

On 8/19/25 11:46 AM, Oscar Maes wrote:
> Currently, ip_extract_route_hint uses RTN_BROADCAST to decide
> whether to use the route dst hint mechanism.
> 
> This check is too strict, as it prevents directed broadcast
> routes from using the hint, resulting in poor performance
> during bursts of directed broadcast traffic.
> 
> Fix this in ip_extract_route_hint and modify ip_route_use_hint
> to preserve the intended behaviour.
> 
> Signed-off-by: Oscar Maes <oscmaes92@gmail.com>
> ---
>  net/ipv4/ip_input.c | 11 +++++++----
>  net/ipv4/route.c    |  2 +-
>  2 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
> index fc323994b1fa..a09aca2c8567 100644
> --- a/net/ipv4/ip_input.c
> +++ b/net/ipv4/ip_input.c
> @@ -587,9 +587,13 @@ static void ip_sublist_rcv_finish(struct list_head *head)
>  }
>  
>  static struct sk_buff *ip_extract_route_hint(const struct net *net,
> -					     struct sk_buff *skb, int rt_type)
> +					     struct sk_buff *skb)
>  {
> -	if (fib4_has_custom_rules(net) || rt_type == RTN_BROADCAST ||
> +	const struct iphdr *iph = ip_hdr(skb);
> +
> +	if (fib4_has_custom_rules(net) ||
> +	    ipv4_is_lbcast(iph->daddr) ||
> +	    ipv4_is_zeronet(iph->daddr) ||
>  	    IPCB(skb)->flags & IPSKB_MULTIPATH)
>  		return NULL;
>  

seems ok to me.

Reviewed-by: David Ahern <dsahern@kernel.org>