[PATCH net] net: dsa: fix off-by-one in maximum bridge ID determination

[PATCH net] net: dsa: fix off-by-one in maximum bridge ID determination

[Vladimir Oltean]

Prior to the blamed commit, the bridge_num range was from
0 to ds->max_num_bridges - 1. After the commit, it is from
1 to ds->max_num_bridges.

So this check:
	if (bridge_num >= max)
		return 0;
must be updated to:
	if (bridge_num > max)
		return 0;

in order to allow the last bridge_num value (==max) to be used.

This is easiest visible when a driver sets ds->max_num_bridges=1.
The observed behaviour is that even the first created bridge triggers
the netlink extack "Range of offloadable bridges exceeded" warning, and
is handled in software rather than being offloaded.

Fixes: 3f9bb0301d50 ("net: dsa: make dp->bridge_num one-based")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 net/dsa/dsa.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
index 99ede37698ac..35ce3941fae3 100644
--- a/net/dsa/dsa.c
+++ b/net/dsa/dsa.c
@@ -158,7 +158,7 @@ unsigned int dsa_bridge_num_get(const struct net_device *bridge_dev, int max)
 		bridge_num = find_next_zero_bit(&dsa_fwd_offloading_bridges,
 						DSA_MAX_NUM_OFFLOADING_BRIDGES,
 						1);
-		if (bridge_num >= max)
+		if (bridge_num > max)
 			return 0;
 
 		set_bit(bridge_num, &dsa_fwd_offloading_bridges);
-- 
2.43.0

Re: [PATCH net] net: dsa: fix off-by-one in maximum bridge ID determination

[patchwork-bot+netdevbpf]

Hello:

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

On Tue, 20 Jan 2026 23:10:39 +0200 you wrote:
> Prior to the blamed commit, the bridge_num range was from
> 0 to ds->max_num_bridges - 1. After the commit, it is from
> 1 to ds->max_num_bridges.
> 
> So this check:
> 	if (bridge_num >= max)
> 		return 0;
> must be updated to:
> 	if (bridge_num > max)
> 		return 0;
> 
> [...]

Here is the summary with links:
  - [net] net: dsa: fix off-by-one in maximum bridge ID determination
    https://git.kernel.org/netdev/net/c/dfca045cd4d0

You are awesome, thank you!
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