net/mlx4_en: Introduce flexible array to silence overflow warning

The call "skb_copy_from_linear_data(skb, inl + 1, spc)" triggers a FORTIFY
memcpy() warning on ppc64 platform:

In function ‘fortify_memcpy_chk’,
    inlined from ‘skb_copy_from_linear_data’ at ./include/linux/skbuff.h:4029:2,
    inlined from ‘build_inline_wqe’ at drivers/net/ethernet/mellanox/mlx4/en_tx.c:722:4,
    inlined from ‘mlx4_en_xmit’ at drivers/net/ethernet/mellanox/mlx4/en_tx.c:1066:3:
./include/linux/fortify-string.h:513:25: error: call to ‘__write_overflow_field’ declared with
attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()?
[-Werror=attribute-warning]
  513 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Same behaviour on x86 you can get if you use "__always_inline" instead of
"inline" for skb_copy_from_linear_data() in skbuff.h

The call here copies data into inlined tx destricptor, which has 104
bytes (MAX_INLINE) space for data payload. In this case "spc" is known
in compile-time but the destination is used with hidden knowledge
(real structure of destination is different from that the compiler
can see). That cause the fortify warning because compiler can check
bounds, but the real bounds are different.  "spc" can't be bigger than
64 bytes (MLX4_INLINE_ALIGN), so the data can always fit into inlined
tx descriptor. The fact that "inl" points into inlined tx descriptor is
determined earlier in mlx4_en_xmit().

Avoid confusing the compiler with "inl + 1" constructions to get to past
the inl header by introducing a flexible array "data" to the struct so
that the compiler can see that we are not dealing with an array of inl
structs, but rather, arbitrary data following the structure. There are
no changes to the structure layout reported by pahole, and the resulting
machine code is actually smaller.
Reported-by: Josef Oskera <joskera@redhat.com>
Link: https://lore.kernel.org/lkml/20230217094541.2362873-1-joskera@redhat.com
Fixes: f68f2ff9 ("fortify: Detect struct member overflows in memcpy() at compile-time")
Cc: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20230218183842.never.954-kees@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlx4/en_tx.c

@@ -699,32 +699,32 @@ static void build_inline_wqe(struct mlx4_en_tx_desc *tx_desc,
 			inl->byte_count = cpu_to_be32(1 << 31 | skb->len);
 		} else {
 			inl->byte_count = cpu_to_be32(1 << 31 | MIN_PKT_LEN);
-			memset(((void *)(inl + 1)) + skb->len, 0,
+			memset(inl->data + skb->len, 0,
 			       MIN_PKT_LEN - skb->len);
 		}
-		skb_copy_from_linear_data(skb, inl + 1, hlen);
+		skb_copy_from_linear_data(skb, inl->data, hlen);
 		if (shinfo->nr_frags)
-			memcpy(((void *)(inl + 1)) + hlen, fragptr,
+			memcpy(inl->data + hlen, fragptr,
 			       skb_frag_size(&shinfo->frags[0]));
 
 	} else {
 		inl->byte_count = cpu_to_be32(1 << 31 | spc);
 		if (hlen <= spc) {
-			skb_copy_from_linear_data(skb, inl + 1, hlen);
+			skb_copy_from_linear_data(skb, inl->data, hlen);
 			if (hlen < spc) {
-				memcpy(((void *)(inl + 1)) + hlen,
+				memcpy(inl->data + hlen,
 				       fragptr, spc - hlen);
 				fragptr +=  spc - hlen;
 			}
-			inl = (void *) (inl + 1) + spc;
-			memcpy(((void *)(inl + 1)), fragptr, skb->len - spc);
+			inl = (void *)inl->data + spc;
+			memcpy(inl->data, fragptr, skb->len - spc);
 		} else {
-			skb_copy_from_linear_data(skb, inl + 1, spc);
-			inl = (void *) (inl + 1) + spc;
-			skb_copy_from_linear_data_offset(skb, spc, inl + 1,
+			skb_copy_from_linear_data(skb, inl->data, spc);
+			inl = (void *)inl->data + spc;
+			skb_copy_from_linear_data_offset(skb, spc, inl->data,
 							 hlen - spc);
 			if (shinfo->nr_frags)
-				memcpy(((void *)(inl + 1)) + hlen - spc,
+				memcpy(inl->data + hlen - spc,
 				       fragptr,
 				       skb_frag_size(&shinfo->frags[0]));
 		}

include/linux/mlx4/qp.h

@@ -446,6 +446,7 @@ enum {
 
 struct mlx4_wqe_inline_seg {
 	__be32			byte_count;
+	__u8			data[];
 };
 
 enum mlx4_update_qp_attr {