net: Document sockaddr safety in ARP and routing UAPI structures

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

---
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Simon Horman <[email protected]>
Cc: <[email protected]>

---
 include/uapi/linux/if_arp.h | 3 ++-
 include/uapi/linux/route.h  | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

index 4783af9fe520..4164cc6e8aa5 100644
@@ -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		 */

index a0de9a7331a2..a2955e25d7ee 100644
@@ -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

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