Re: [PATCH net-next] net: Document sockaddr safety in ARP and routing UAPI structures

[Andrew Lunn]

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