From: Oliver Hartkopp <socketcan@hartkopp.net>

The skb extension ids range from 0 .. 7 to fit their bits as flags into
a single byte. The ids are automatically enumnerated in enum skb_ext_id
in skbuff.h, where SKB_EXT_NUM is defined as the last value.

When having 8 skb extension ids (0 .. 7), SKB_EXT_NUM becomes 8 which is
a valid value for SKB_EXT_NUM.

This patch intentionally lacks a Fixes tag. Backporting is pointless as
former kernels did not use the entire range of skb extension ids.

Link: https://lore.kernel.org/netdev/aXoMqaA7b2CqJZNA@strlen.de/
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
---
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 648c20e19038..609851d70173 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5156,11 +5156,11 @@ static __always_inline unsigned int skb_ext_total_length(void)
 	return l;
 }
 
 static void skb_extensions_init(void)
 {
-	BUILD_BUG_ON(SKB_EXT_NUM >= 8);
+	BUILD_BUG_ON(SKB_EXT_NUM > 8);
 #if !IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL)
 	BUILD_BUG_ON(skb_ext_total_length() > 255);
 #endif
 
 	skbuff_ext_cache = kmem_cache_create("skbuff_ext_cache",

---
base-commit: 021718d2cc1a2df2f53b06968fa89280199371bd
change-id: 20260205-skb_ext-a04c61cc985e

Best regards,
--  
Oliver Hartkopp <socketcan@hartkopp.net>