netfilter: rpfilter: Unmask upper DSCP bits

The rpfilter match performs a reverse path filter test on a packet by
performing a FIB lookup with the source and destination addresses
swapped.

Unmask the upper DSCP bits of the DS field of the tested packet so that
in the future the FIB lookup could be performed according to the full
DSCP value.

No functional changes intended since the upper DSCP bits are masked when
comparing against the TOS selectors in FIB rules and routes.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240821125251.1571445-5-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/netfilter/ipt_rpfilter.c

@@ -8,6 +8,7 @@
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/netdevice.h>
+#include <net/inet_dscp.h>
 #include <linux/ip.h>
 #include <net/ip.h>
 #include <net/ip_fib.h>
@@ -75,7 +76,7 @@ static bool rpfilter_mt(const struct sk_buff *skb, struct xt_action_param *par)
 	flow.daddr = iph->saddr;
 	flow.saddr = rpfilter_get_saddr(iph->daddr);
 	flow.flowi4_mark = info->flags & XT_RPFILTER_VALID_MARK ? skb->mark : 0;
-	flow.flowi4_tos = iph->tos & IPTOS_RT_MASK;
+	flow.flowi4_tos = iph->tos & INET_DSCP_MASK;
 	flow.flowi4_scope = RT_SCOPE_UNIVERSE;
 	flow.flowi4_l3mdev = l3mdev_master_ifindex_rcu(xt_in(par));
 	flow.flowi4_uid = sock_net_uid(xt_net(par), NULL);