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

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

[Mohsin Bashir]

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@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.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

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 25 Nov 2025 13:17:04 -0800 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net] eth: fbnic: Fix counter roll-over issue
    https://git.kernel.org/netdev/net/c/6d66e093e074

You are awesome, thank you!
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