xsk: Try to make xdp_umem_reg extension a bit more future-proof

We recently found out that extending xsk_umem_reg might be a bit
complicated due to not enforcing padding to be zero [0]. Add
a couple of things to make it less error-prone:
1. Remove xdp_umem_reg_v2 since its sizeof is the same as xdp_umem_reg
2. Add BUILD_BUG_ON that checks that the size of xdp_umem_reg_v1 is less
   than xdp_umem_reg; presumably, when we get to v2, there is gonna
   be a similar line to enforce that sizeof(v2) > sizeof(v1)
3. Add BUILD_BUG_ON to make sure the last field plus its size matches
   the overall struct size. The intent is to demonstrate that we don't
   have any lingering padding.

0: https://lore.kernel.org/bpf/ZqI29QE+5JnkdPmE@boxer/T/#me03113f7c2458fd08f3c4114a7a9472ac3646c98Reported-by: Julian Schindel <mail@arctic-alpaca.de>
Cc: Magnus Karlsson <magnus.karlsson@gmail.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/r/20240726222048.1397869-1-sdf@fomichev.meSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

net/xdp/xsk.c

@@ -1320,14 +1320,6 @@ struct xdp_umem_reg_v1 {
 	__u32 headroom;
 };
 
-struct xdp_umem_reg_v2 {
-	__u64 addr; /* Start of packet data area */
-	__u64 len; /* Length of packet data area */
-	__u32 chunk_size;
-	__u32 headroom;
-	__u32 flags;
-};
-
 static int xsk_setsockopt(struct socket *sock, int level, int optname,
 			  sockptr_t optval, unsigned int optlen)
 {
@@ -1371,10 +1363,19 @@ static int xsk_setsockopt(struct socket *sock, int level, int optname,
 
 		if (optlen < sizeof(struct xdp_umem_reg_v1))
 			return -EINVAL;
-		else if (optlen < sizeof(struct xdp_umem_reg_v2))
-			mr_size = sizeof(struct xdp_umem_reg_v1);
 		else if (optlen < sizeof(mr))
-			mr_size = sizeof(struct xdp_umem_reg_v2);
+			mr_size = sizeof(struct xdp_umem_reg_v1);
+
+		BUILD_BUG_ON(sizeof(struct xdp_umem_reg_v1) >= sizeof(struct xdp_umem_reg));
+
+		/* Make sure the last field of the struct doesn't have
+		 * uninitialized padding. All padding has to be explicit
+		 * and has to be set to zero by the userspace to make
+		 * struct xdp_umem_reg extensible in the future.
+		 */
+		BUILD_BUG_ON(offsetof(struct xdp_umem_reg, tx_metadata_len) +
+			     sizeof_field(struct xdp_umem_reg, tx_metadata_len) !=
+			     sizeof(struct xdp_umem_reg));
 
 		if (copy_from_sockptr(&mr, optval, mr_size))
 			return -EFAULT;