[PATCH v6 6/6] net/mlx5: fix LTO stringop-overflow warning

[Stephen Hemminger <stephen@networkplumber.org>]

When LTO inlines mlx5dr_action_prepare_decap_l3_data() into
its callers, GCC sees the 64-byte stack buffer 'mh_data' but
cannot prove num_of_actions is bounded, producing a false
-Wstringop-overflow warning.

Add __rte_assume(num_of_actions <= DECAP_L3_NUM_ACTIONS_W_VLAN)
so GCC can verify the loop stays within buffer bounds.

Bugzilla ID: 1710
Fixes: f8c8a6d8440d ("net/mlx5/hws: add action object")
Cc: stable@dpdk.org

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 drivers/net/mlx5/hws/mlx5dr_action.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/mlx5/hws/mlx5dr_action.c b/drivers/net/mlx5/hws/mlx5dr_action.c
index b35bf07c3c..afb638250d 100644
--- a/drivers/net/mlx5/hws/mlx5dr_action.c
+++ b/drivers/net/mlx5/hws/mlx5dr_action.c
@@ -3620,6 +3620,17 @@ mlx5dr_action_prepare_decap_l3_data(uint8_t *src, uint8_t *dst,
 	uint8_t *e_src;
 	int i;
 
+	/*
+	 * When LTO inlines this into mlx5dr_action_handle_tunnel_l3_to_l2(),
+	 * GCC sees the 64-byte mh_data buffer but cannot prove num_of_actions
+	 * is bounded, causing false -Wstringop-overflow warnings.
+	 *
+	 * Valid num_of_actions values are DECAP_L3_NUM_ACTIONS_W_NO_VLAN (6)
+	 * or DECAP_L3_NUM_ACTIONS_W_VLAN (7). This check gives GCC the proof
+	 * it needs that the loop iterations stay within buffer bounds.
+	 */
+	__rte_assume(num_of_actions <= DECAP_L3_NUM_ACTIONS_W_VLAN);
+
 	/* num_of_actions = remove l3l2 + 4/5 inserts + remove extra 2 bytes
 	 * copy from end of src to the start of dst.
 	 * move to the end, 2 is the leftover from 14B or 18B
-- 
2.51.0