[PATCH] net: xgene: fix signedness bug in xgene_enet_get_fpsel()

[Anas Iqbal <mohd.abd.6602@gmail.com>]

xgene_enet_get_fpsel() returns a u8 but can compute a negative
value when xgene_enet_ring_bufnum(id) is less than
RING_BUFNUM_BUFPOOL. This leads to an implicit conversion of a
negative value to u8, resulting in a large unintended value.

This can cause incorrect behavior when the result is used in
bit operations such as BIT(), potentially leading to undefined
behavior.

Fix this by validating the value before subtraction to avoid
underflow.

Fixes: 2c839337520b ("drivers: net: xgene: Add helper function")
Signed-off-by: Anas Iqbal <mohd.abd.6602@gmail.com>
---
 drivers/net/ethernet/apm/xgene/xgene_enet_hw.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/apm/xgene/xgene_enet_hw.h b/drivers/net/ethernet/apm/xgene/xgene_enet_hw.h
index 2f534f9d4416..fe563c396773 100644
--- a/drivers/net/ethernet/apm/xgene/xgene_enet_hw.h
+++ b/drivers/net/ethernet/apm/xgene/xgene_enet_hw.h
@@ -405,10 +405,16 @@ static inline bool xgene_enet_is_bufpool(u16 id)
 
 static inline u8 xgene_enet_get_fpsel(u16 id)
 {
-	if (xgene_enet_is_bufpool(id))
-		return xgene_enet_ring_bufnum(id) - RING_BUFNUM_BUFPOOL;
+	u16 val;
 
-	return 0;
+	if (!xgene_enet_is_bufpool(id))
+		return 0;
+
+	val = xgene_enet_ring_bufnum(id);
+	if (val < RING_BUFNUM_BUFPOOL)
+		return 0;
+
+	return val - RING_BUFNUM_BUFPOOL;
 }
 
 static inline u16 xgene_enet_get_numslots(u16 id, u32 size)
-- 
2.43.0