net: Add struct sockaddr_unspec for sockaddr of unknown length

Add flexible sockaddr structure to support addresses longer than the
traditional 14-byte struct sockaddr::sa_data limitation without
requiring the full 128-byte sa_data of struct sockaddr_storage. This
allows the network APIs to pass around a pointer to an object that
isn't lying to the compiler about how big it is, but must be accompanied
by its actual size as an additional parameter.

It's possible we may way to migrate to including the size with the
struct in the future, e.g.:

struct sockaddr_unspec {
	u16 sa_data_len;
	u16 sa_family;
	u8  sa_data[] __counted_by(sa_data_len);
};

Signed-off-by: Kees Cook <[email protected]>

Author: kees

include/linux/socket.h

@@ -40,6 +40,23 @@ struct sockaddr {
 	};
 };
 
+/**
+ * struct sockaddr_unspec - Unspecified size sockaddr for callbacks
+ * @sa_family: Address family (AF_UNIX, AF_INET, AF_INET6, etc.)
+ * @sa_data: Flexible array for address data
+ *
+ * This structure is designed for callback interfaces where the
+ * total size is known via the sockaddr_len parameter. Unlike struct
+ * sockaddr which has a fixed 14-byte sa_data limit or struct
+ * sockaddr_storage which has a fixed 128-byte sa_data limit, this
+ * structure can accommodate addresses of any size, but must be used
+ * carefully.
+ */
+struct sockaddr_unspec {
+	__kernel_sa_family_t	sa_family;	/* address family, AF_xxx */
+	char			sa_data[];	/* flexible address data */
+};
+
 struct linger {
 	int		l_onoff;	/* Linger active		*/
 	int		l_linger;	/* How long to linger for	*/