[PATCH net-next v2 1/2] rtnetlink: add needed_{head,tail}room attributes

[PATCH net-next v2 1/2] rtnetlink: add needed_{head,tail}room attributes

[Alasdair McWilliam]

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
b9ed315d3c4c ("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>
---
 v1 -> v2:
 - Corrected attribute types to IFA_REJECT in validation policy.
 - Added additional context in commit log.

 include/uapi/linux/if_link.h |  2 ++
 net/core/rtnetlink.c         | 10 +++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 45f56c9f95d9..3b491d96e52e 100644
--- a/include/uapi/linux/if_link.h
+++ b/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
 };
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 094b085cff20..d9e68ca84926 100644
--- a/net/core/rtnetlink.c
+++ b/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] = {
-- 
2.47.3

[PATCH net-next v2 2/2] rtnetlink: specs: Add {head,tail}room to rt-link.yaml

[Alasdair McWilliam]

Add {head,tail}room attributes to rt-link.yaml spec file.

Example:

$ tools/net/ynl/pyynl/cli.py \
  --spec Documentation/netlink/specs/rt-link.yaml \
  --do getlink --json '{"ifname":"enp0s1"}' --output-json \
  | jq -r '{ifname,mtu,headroom,tailroom}'
{
  "ifname": "enp0s1",
  "mtu": 1500,
  "headroom": 12,
  "tailroom": 0
}

Signed-off-by: Alasdair McWilliam <alasdair@mcwilliam.dev>
Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
---
 Documentation/netlink/specs/rt-link.yaml | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/netlink/specs/rt-link.yaml b/Documentation/netlink/specs/rt-link.yaml
index 6ab31f86854d..2a23e9699c0b 100644
--- a/Documentation/netlink/specs/rt-link.yaml
+++ b/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
-- 
2.47.3

Re: [PATCH net-next v2 2/2] rtnetlink: specs: Add {head,tail}room to rt-link.yaml

[Jakub Kicinski]

On Wed, 17 Sep 2025 10:55:43 +0100 Alasdair McWilliam wrote:
> Add {head,tail}room attributes to rt-link.yaml spec file.

Not sure this makes sense as a separate commit.

Squashed and applied, thanks!