net: ipv4: fix stat increase when udp early demux drops the packet

udp_v4_early_demux now returns drop reasons as it either returns 0 or
ip_mc_validate_source, which returns itself a drop reason. However its
use was not converted in ip_rcv_finish_core and the drop reason is
ignored, leading to potentially skipping increasing LINUX_MIB_IPRPFILTER
if the drop reason is SKB_DROP_REASON_IP_RPFILTER.

This is a fix and we're not converting udp_v4_early_demux to explicitly
return a drop reason to ease backports; this can be done as a follow-up.

Fixes: d46f827016d8 ("net: ip: make ip_mc_validate_source() return drop reason")
Cc: Menglong Dong <menglong8.dong@gmail.com>
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250701074935.144134-1-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 30a5e9460d006de306b5bac49c92f9b9bf21f2f5..5a49eb99e5c48000caa6f7d5f60651d68fc6bca6 100644 (file)
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -319,8 +319,8 @@ static int ip_rcv_finish_core(struct net *net,
                              const struct sk_buff *hint)
 {
        const struct iphdr *iph = ip_hdr(skb);
-       int err, drop_reason;
        struct rtable *rt;
+       int drop_reason;
 
        if (ip_can_use_hint(skb, iph, hint)) {
                drop_reason = ip_route_use_hint(skb, iph->daddr, iph->saddr,
@@ -345,9 +345,10 @@ static int ip_rcv_finish_core(struct net *net,
                        break;
                case IPPROTO_UDP:
                        if (READ_ONCE(net->ipv4.sysctl_udp_early_demux)) {
-                               err = udp_v4_early_demux(skb);
-                               if (unlikely(err))
+                               drop_reason = udp_v4_early_demux(skb);
+                               if (unlikely(drop_reason))
                                        goto drop_error;
+                               drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
                                /* must reload iph, skb->head might have changed */
                                iph = ip_hdr(skb);