rtnetlink: add needed_{head,tail}room attributes

Various network interface types make use of needed_{head,tail}room values
to efficiently reserve buffer space for additional encapsulation headers,
such as VXLAN, Geneve, IPSec, etc. However, it is not currently possible
to query these values in a generic way.

Introduce ability to query the needed_{head,tail}room values of a network
device via rtnetlink, such that applications that may wish to use these
values can do so.

For example, Cilium agent iterates over present devices based on user config
(direct routing, vxlan, geneve, wireguard etc.) and in future will configure
netkit in order to expose the needed_{head,tail}room into K8s pods. See
b9ed315d3c ("netkit: Allow for configuring needed_{head,tail}room").

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alasdair McWilliam <alasdair@mcwilliam.dev>
Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/20250917095543.14039-1-alasdair@mcwilliam.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/rt-link.yaml

@@ -1057,6 +1057,12 @@ attribute-sets:
       -
         name: netns-immutable
         type: u8
+      -
+        name: headroom
+        type: u16
+      -
+        name: tailroom
+        type: u16
   -
     name: prop-list-link-attrs
     subset-of: link-attrs

include/uapi/linux/if_link.h

@@ -379,6 +379,8 @@ enum {
 	IFLA_DPLL_PIN,
 	IFLA_MAX_PACING_OFFLOAD_HORIZON,
 	IFLA_NETNS_IMMUTABLE,
+	IFLA_HEADROOM,
+	IFLA_TAILROOM,
 	__IFLA_MAX
 };
 

net/core/rtnetlink.c

@@ -1326,6 +1326,8 @@ static noinline size_t if_nlmsg_size(const struct net_device *dev,
 	       + rtnl_devlink_port_size(dev)
 	       + rtnl_dpll_pin_size(dev)
 	       + nla_total_size(8)  /* IFLA_MAX_PACING_OFFLOAD_HORIZON */
+	       + nla_total_size(2)  /* IFLA_HEADROOM */
+	       + nla_total_size(2)  /* IFLA_TAILROOM */
 	       + 0;
 }
 
@@ -2091,7 +2093,11 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
 	    nla_put_u32(skb, IFLA_CARRIER_UP_COUNT,
 			atomic_read(&dev->carrier_up_count)) ||
 	    nla_put_u32(skb, IFLA_CARRIER_DOWN_COUNT,
-			atomic_read(&dev->carrier_down_count)))
+			atomic_read(&dev->carrier_down_count)) ||
+	    nla_put_u16(skb, IFLA_HEADROOM,
+			READ_ONCE(dev->needed_headroom)) ||
+	    nla_put_u16(skb, IFLA_TAILROOM,
+			READ_ONCE(dev->needed_tailroom)))
 		goto nla_put_failure;
 
 	if (rtnl_fill_proto_down(skb, dev))
@@ -2243,6 +2249,8 @@ static const struct nla_policy ifla_policy[IFLA_MAX+1] = {
 	[IFLA_GSO_IPV4_MAX_SIZE]	= NLA_POLICY_MIN(NLA_U32, MAX_TCP_HEADER + 1),
 	[IFLA_GRO_IPV4_MAX_SIZE]	= { .type = NLA_U32 },
 	[IFLA_NETNS_IMMUTABLE]	= { .type = NLA_REJECT },
+	[IFLA_HEADROOM]		= { .type = NLA_REJECT },
+	[IFLA_TAILROOM]		= { .type = NLA_REJECT },
 };
 
 static const struct nla_policy ifla_info_policy[IFLA_INFO_MAX+1] = {