Add documentation clarifying that ARP and routing UAPI structures are
constrained to IPv4-only usage, making them safe for the coming fixed-size
sockaddr conversion (with the 14-byte struct sockaddr::sa_data). These
are fine as-is, but their use was non-obvious to me, so I figured they
could use a little more documentation:

- struct arpreq: ARP protocol is IPv4-only by design
- struct rtentry: Legacy IPv4 routing API, IPv6 uses different structures

Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: <netdev@vger.kernel.org>
---
 include/uapi/linux/if_arp.h | 3 ++-
 include/uapi/linux/route.h  | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/if_arp.h b/include/uapi/linux/if_arp.h
index 4783af9fe520..4164cc6e8aa5 100644
--- a/include/uapi/linux/if_arp.h
+++ b/include/uapi/linux/if_arp.h
@@ -113,7 +113,7 @@
 #define	ARPOP_NAK	10		/* (ATM)ARP NAK			*/
 
 
-/* ARP ioctl request. */
+/* ARP ioctl request; IPv4-only protocol. */
 struct arpreq {
 	struct sockaddr	arp_pa;		/* protocol address		 */
 	struct sockaddr	arp_ha;		/* hardware address		 */
@@ -122,6 +122,7 @@ struct arpreq {
 	char		arp_dev[IFNAMSIZ];
 };
 
+/* Legacy ARP ioctl request; IPv4-only protocol. */
 struct arpreq_old {
 	struct sockaddr	arp_pa;		/* protocol address		 */
 	struct sockaddr	arp_ha;		/* hardware address		 */
diff --git a/include/uapi/linux/route.h b/include/uapi/linux/route.h
index a0de9a7331a2..a2955e25d7ee 100644
--- a/include/uapi/linux/route.h
+++ b/include/uapi/linux/route.h
@@ -27,7 +27,7 @@
 #include <linux/if.h>
 #include <linux/compiler.h>
 
-/* This structure gets passed by the SIOCADDRT and SIOCDELRT calls. */
+/* This IPv4-only structure gets passed by the SIOCADDRT and SIOCDELRT calls. */
 struct rtentry {
 	unsigned long	rt_pad1;
 	struct sockaddr	rt_dst;		/* target address		*/
-- 
2.34.1

On Tue, Jul 22, 2025 at 09:58:37AM -0700, Kees Cook wrote:
> Add documentation clarifying that ARP and routing UAPI structures are
> constrained to IPv4-only usage, making them safe for the coming fixed-size
> sockaddr conversion (with the 14-byte struct sockaddr::sa_data). These
> are fine as-is, but their use was non-obvious to me, so I figured they
> could use a little more documentation:
> 
> - struct arpreq: ARP protocol is IPv4-only by design
> - struct rtentry: Legacy IPv4 routing API, IPv6 uses different structures

I'm not sure this second statement is strictly true:

https://elixir.bootlin.com/linux/v6.15.7/source/net/appletalk/ddp.c#L918
atrtr_ioctl() ->
  atrtr_create()->
	struct sockaddr_at *ta = (struct sockaddr_at *)&r->rt_dst;

where:

struct sockaddr_at {
	__kernel_sa_family_t sat_family;
	__u8		  sat_port;
	struct atalk_addr sat_addr;
	char		  sat_zero[8];
};

This is not an IPv4 address.

Maybe this does fit with a 14 byte sockaddr::sa_data, so it is not an
issue, but the comment and commit message should be expanded to
explain this. And i only looked at the first SIOCADDRT i came across,
there are many other protocols using this ioctl.

	Andrew