[PATCH net] mlx4: supress fortify for inlined xmit

[Josef Oskera]

This call "skb_copy_from_linear_data(skb, inl + 1, spc)" triggers 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().

Fixes: f68f2ff91512c1 fortify: Detect struct member overflows in memcpy() at compile-time
Signed-off-by: Josef Oskera <joskera@xxxxxxxxxx>
---
 drivers/net/ethernet/mellanox/mlx4/en_tx.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
index c5758637b7bed6..f30ca9fe90e5b4 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
@@ -719,7 +719,16 @@ static void build_inline_wqe(struct mlx4_en_tx_desc *tx_desc,
 			inl = (void *) (inl + 1) + spc;
 			memcpy(((void *)(inl + 1)), fragptr, skb->len - spc);
 		} else {
-			skb_copy_from_linear_data(skb, inl + 1, spc);
+			unsafe_memcpy(inl + 1, skb->data, spc,
+					/* This copies data into inlined tx descriptor, which has
+					 * 104 bytes (MAX_INLINE) space for data.
+					 * Real structure of destination is in this case hidden for
+					 * the compiler
+					 * "spc" is compile-time known variable and can't be bigger
+					 * than 64 (MLX4_INLINE_ALIGN).
+					 * Bounds and other conditions are checked in current
+					 * function and earlier in mlx4_en_xmit()
+					 */);
 			inl = (void *) (inl + 1) + spc;
 			skb_copy_from_linear_data_offset(skb, spc, inl + 1,
 							 hlen - spc);
-- 
2.39.0