[PATCH net-next v2] net: generalize calculation of skb extensions length

[Thomas Weißschuh]

Remove the necessity to modify skb_ext_total_length() when new extension
types are added.
Also reduces the line count a bit.

With optimizations enabled the function is folded down to the same
constant value as before during compilation.
This has been validated on x86 with GCC 6.5.0 and 13.2.1.
Also a similar construct has been validated on godbolt.org with GCC 5.1.
In any case the compiler has to be able to evaluate the construct at
compile-time for the BUILD_BUG_ON() in skb_extensions_init().

Even if not evaluated at compile-time this function would only ever
be executed once at run-time, so the overhead would be very minuscule.

Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
---
Changes in v2:
- Add more proof for identical behavior to the commit log
- Link to v1: https://lore.kernel.org/r/20230822-skb_ext-simplify-v1-1-9dd047340ab5@xxxxxxxxxxxxxx
---
 net/core/skbuff.c | 24 +++++++-----------------
 1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index faa6c86da2a5..45707059082f 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4785,23 +4785,13 @@ static const u8 skb_ext_type_len[] = {
 
 static __always_inline unsigned int skb_ext_total_length(void)
 {
-	return SKB_EXT_CHUNKSIZEOF(struct skb_ext) +
-#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
-		skb_ext_type_len[SKB_EXT_BRIDGE_NF] +
-#endif
-#ifdef CONFIG_XFRM
-		skb_ext_type_len[SKB_EXT_SEC_PATH] +
-#endif
-#if IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
-		skb_ext_type_len[TC_SKB_EXT] +
-#endif
-#if IS_ENABLED(CONFIG_MPTCP)
-		skb_ext_type_len[SKB_EXT_MPTCP] +
-#endif
-#if IS_ENABLED(CONFIG_MCTP_FLOWS)
-		skb_ext_type_len[SKB_EXT_MCTP] +
-#endif
-		0;
+	unsigned int l = SKB_EXT_CHUNKSIZEOF(struct skb_ext);
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(skb_ext_type_len); i++)
+		l += skb_ext_type_len[i];
+
+	return l;
 }
 
 static void skb_extensions_init(void)

---
base-commit: 90308679c297ffcbb317c715ef434e9fb3c881dc
change-id: 20230822-skb_ext-simplify-3d93ceb95f62

Best regards,
-- 
Thomas Weißschuh <linux@xxxxxxxxxxxxxx>