[PATCH 1/1] net: fix skb_ext_total_length() BUILD_BUG_ON with CONFIG_GCOV_PROFILE_ALL

[Konstantin Khorenko]

When CONFIG_GCOV_PROFILE_ALL=y is enabled, GCC inserts branch profiling
counters into skb_ext_total_length() and, combined with -fno-tree-loop-im
from GCOV, prevents the compiler from constant-folding the loop that
sums skb_ext_type_len[] array elements.  This causes the compile-time
BUILD_BUG_ON(skb_ext_total_length() > 255) check to fail, even though
the actual computed value is well below 255.

The kernel already has a guard for CONFIG_KCOV_INSTRUMENT_ALL, which
causes the same problem.  Add a similar guard for GCOV.

The number of loop iterations matters: with 4 extension types (as in
earlier kernels), GCC 11 can still constant-fold despite GCOV. With 5+
types (after SKB_EXT_CAN and other additions), it gives up.  This is
why the issue only manifests in recent kernels with GCOV enabled.

Tested with GCC 11.4.1 and GCC 16.0.1 20260327 (experimental) - both
exhibit the same behavior.

Note that skb_ext_total_length() is still correct at runtime; this
change only allows the build to succeed when GCOV_PROFILE_ALL is
enabled for coverage analysis.

Fixes: 5d21d0a65b57 ("net: generalize calculation of skb extensions length")

Signed-off-by: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
---
 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 0e217041958a..98c3d4e63219 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5159,7 +5159,7 @@ static __always_inline unsigned int skb_ext_total_length(void)
 static void skb_extensions_init(void)
 {
 	BUILD_BUG_ON(SKB_EXT_NUM > 8);
-#if !IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL)
+#if !IS_ENABLED(CONFIG_KCOV_INSTRUMENT_ALL) && !IS_ENABLED(CONFIG_GCOV_PROFILE_ALL)
 	BUILD_BUG_ON(skb_ext_total_length() > 255);
 #endif
 
-- 
2.43.5