net: fix skb_ext_total_length() BUILD_BUG_ON with CONFIG_GCOV_PROFILE_ALL

When CONFIG_GCOV_PROFILE_ALL=y is enabled, the kernel fails to build:

  In file included from <command-line>:
  In function 'skb_extensions_init',
      inlined from 'skb_init' at net/core/skbuff.c:5214:2:
  ././include/linux/compiler_types.h:706:45: error: call to
    '__compiletime_assert_1490' declared with attribute error:
    BUILD_BUG_ON failed: skb_ext_total_length() > 255

CONFIG_GCOV_PROFILE_ALL adds -fprofile-arcs -ftest-coverage
-fno-tree-loop-im to CFLAGS globally. GCC inserts branch profiling
counters into the skb_ext_total_length() loop and, combined with
-fno-tree-loop-im (which disables loop invariant motion), cannot
constant-fold the result.
BUILD_BUG_ON requires a compile-time constant and fails.

The issue manifests in kernels with 5+ SKB extension types enabled
(e.g., after addition of SKB_EXT_CAN, SKB_EXT_PSP). With 4 extensions
GCC can still unroll and fold the loop despite GCOV instrumentation;
with 5+ it gives up.

Mark skb_ext_total_length() with __no_profile to prevent GCOV from
inserting counters into this function. Without counters the loop is
"clean" and GCC can constant-fold it even with -fno-tree-loop-im active.
This allows BUILD_BUG_ON to work correctly while keeping GCOV profiling
for the rest of the kernel.

This also removes the CONFIG_KCOV_INSTRUMENT_ALL preprocessor guard
introduced by d6e5794b06c0. That guard was added as a precaution because
KCOV instrumentation was also suspected of inhibiting constant folding.
However, KCOV uses -fsanitize-coverage=trace-pc, which inserts
lightweight trace callbacks that do not interfere with GCC's constant
folding or loop optimization passes. Only GCOV's -fprofile-arcs combined
with -fno-tree-loop-im actually prevents the compiler from evaluating
the loop at compile time. The guard is therefore unnecessary and can be
safely removed.

Fixes: 96ea3a1e2d31 ("can: add CAN skb extension infrastructure")
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Reviewed-by: Thomas Weissschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20260410162150.3105738-2-khorenko@virtuozzo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 1 insertions, 3 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c5441154795c..87e7d689ab25 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5118,7 +5118,7 @@ static const u8 skb_ext_type_len[] = {
 #endif
 };
 
-static __always_inline unsigned int skb_ext_total_length(void)
+static __always_inline __no_profile unsigned int skb_ext_total_length(void)
 {
 	unsigned int l = SKB_EXT_CHUNKSIZEOF(struct skb_ext);
 	int i;
@@ -5132,9 +5132,7 @@ 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)
 	BUILD_BUG_ON(skb_ext_total_length() > 255);
-#endif
 
 	skbuff_ext_cache = kmem_cache_create("skbuff_ext_cache",
 					     SKB_EXT_ALIGN_VALUE * skb_ext_total_length(),