[PATCH v2 0/1] Fix kernel build failure with CONFIG_GCOV_PROFILE_ALL=y

[PATCH v2 0/1] Fix kernel build failure with CONFIG_GCOV_PROFILE_ALL=y

[Konstantin Khorenko]

This patch fixes a build failure in net/core/skbuff.c that occurs when
CONFIG_GCOV_PROFILE_ALL=y is enabled.

v2: use __no_profile instead of preprocessor guards, as suggested
    by Thomas Weißschuh.  This solves the root cause (GCOV counters
    preventing constant folding) and also removes the existing
    CONFIG_KCOV_INSTRUMENT_ALL guard which is no longer needed.

Build failure without this patch
---------------------------------

With the following configuration:

  CONFIG_GCOV_KERNEL=y
  CONFIG_GCOV_PROFILE_ALL=y
  # CONFIG_GCOV_PROFILE_FTRACE is not set
  CONFIG_GCOV_PROFILE_RDS=y
  CONFIG_GCOV_PROFILE_URING=y

The kernel fails to build with:

  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
    706 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |                                             ^
  ././include/linux/compiler_types.h:687:25: note: in definition of macro '__compiletime_assert'
    687 |                         prefix ## suffix();                             \
        |                         ^~~~~~
  ././include/linux/compiler_types.h:706:9: note: in expansion of macro '_compiletime_assert'
    706 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |         ^~~~~~~~~~~~~~~~~~~
  ./include/linux/build_bug.h:40:37: note: in expansion of macro 'compiletime_assert'
     40 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
        |                                     ^~~~~~~~~~~~~~~~~~
  ./include/linux/build_bug.h:51:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
     51 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
        |         ^~~~~~~~~~~~~~~~
  net/core/skbuff.c:5163:9: note: in expansion of macro 'BUILD_BUG_ON'
   5163 |         BUILD_BUG_ON(skb_ext_total_length() > 255);
        |         ^~~~~~~~~~~~
  make[4]: *** [scripts/Makefile.build:289: net/core/skbuff.o] Error 1

Testing
-------

Tested with GCC 11.4.1 and GCC 16.0.1 20260327 (experimental) - both
exhibit the same build failure without the patch.  Both compile
successfully with the patch applied.

Konstantin Khorenko (1):
  net: fix skb_ext_total_length() BUILD_BUG_ON with CONFIG_GCOV_PROFILE_ALL

 net/core/skbuff.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

-- 
2.43.5

[PATCH v2 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, 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, as __no_profile handles both GCOV and KCOV
instrumentation at the root cause level rather than just disabling the
check.

Fixes: 5d21d0a65b57 ("net: generalize calculation of skb extensions length")
Fixes: d6e5794b06c0 ("net: avoid build bug in skb extension length calculation")

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
---
 net/core/skbuff.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 0e217041958a..47c7f0ab6e84 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5145,7 +5145,7 @@ 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;
@@ -5159,9 +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)
 	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(),
-- 
2.43.5