netdev - [PATCH net] eth: fbnic: Fix counter roll-over issue

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Message-ID: <20251125211704.3222413-1-mohsin.bashr@gmail.com>
Date: Tue, 25 Nov 2025 13:17:04 -0800
From: Mohsin Bashir <mohsin.bashr@...il.com>
To: netdev@...r.kernel.org
Cc: alexanderduyck@...com,
	andrew+netdev@...n.ch,
	davem@...emloft.net,
	edumazet@...gle.com,
	horms@...nel.org,
	jacob.e.keller@...el.com,
	kernel-team@...a.com,
	kuba@...nel.org,
	lee@...ger.us,
	mohsin.bashr@...il.com,
	pabeni@...hat.com,
	sanman.p211993@...il.com
Subject: [PATCH net] eth: fbnic: Fix counter roll-over issue

Fix a potential counter roll-over issue in fbnic_mbx_alloc_rx_msgs()
when calculating descriptor slots. The issue occurs when head - tail
results in a large positive value (unsigned) and the compiler interprets
head - tail - 1 as a signed value.

Since FBNIC_IPC_MBX_DESC_LEN is a power of two, use a masking operation,
which is a common way of avoiding this problem when dealing with these
sort of ring space calculations.

Fixes: da3cde08209e ("eth: fbnic: Add FW communication mechanism")
Signed-off-by: Mohsin Bashir <mohsin.bashr@...il.com>
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
 drivers/net/ethernet/meta/fbnic/fbnic_fw.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_fw.c b/drivers/net/ethernet/meta/fbnic/fbnic_fw.c
index c87cb9ed09e7..fcd9912e7ad3 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_fw.c
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_fw.c
@@ -201,7 +201,7 @@ static int fbnic_mbx_alloc_rx_msgs(struct fbnic_dev *fbd)
 		return -ENODEV;
 
 	/* Fill all but 1 unused descriptors in the Rx queue. */
-	count = (head - tail - 1) % FBNIC_IPC_MBX_DESC_LEN;
+	count = (head - tail - 1) & (FBNIC_IPC_MBX_DESC_LEN - 1);
 	while (!err && count--) {
 		struct fbnic_tlv_msg *msg;
 
-- 
2.47.3