[PATCH 1/4] wifi: mt76: mt7915: fix potential tx_retries underflow

[Ryder Lee]

When FIELD_GET returns 0 for the retry count, subtracting 1 causes
an unsigned integer underflow, resulting in tx_retries becoming a
very large value (0xFFFFFFFF for u32).

Fix by checking if count is non-zero before subtracting 1.

Fixes: 943e4fb96e6f ("wifi: mt76: mt7915: report tx retries/failed counts for non-WED path")
Signed-off-by: Ryder Lee <ryder.lee at mediatek.com>
---
 drivers/net/wireless/mediatek/mt76/mt7915/mac.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7915/mac.c b/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
index cec2c4208..334c19ab2 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
@@ -912,16 +912,16 @@ mt7915_mac_tx_free(struct mt7915_dev *dev, void *data, int len)
 		}
 
 		if (!mtk_wed_device_active(&mdev->mmio.wed) && wcid) {
-			u32 tx_retries = 0, tx_failed = 0;
+			u32 tx_retries = 0, tx_failed = 0, count;
 
 			if (v3 && (info & MT_TX_FREE_MPDU_HEADER_V3)) {
-				tx_retries =
-					FIELD_GET(MT_TX_FREE_COUNT_V3, info) - 1;
+				count = FIELD_GET(MT_TX_FREE_COUNT_V3, info);
+				tx_retries = count ? count - 1 : 0;
 				tx_failed = tx_retries +
 					!!FIELD_GET(MT_TX_FREE_STAT_V3, info);
 			} else if (!v3 && (info & MT_TX_FREE_MPDU_HEADER)) {
-				tx_retries =
-					FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
+				count = FIELD_GET(MT_TX_FREE_COUNT, info);
+				tx_retries = count ? count - 1 : 0;
 				tx_failed = tx_retries +
 					!!FIELD_GET(MT_TX_FREE_STAT, info);
 			}
-- 
2.45.2