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

[Konstantin Khorenko <khorenko@virtuozzo.com>]

This patch fixes a build failure in net/core/skbuff.c that occurs when
CONFIG_GCOV_PROFILE_ALL=y is enabled.  The issue manifests as a
BUILD_BUG_ON compile-time assertion failure, even though the actual
runtime value satisfies the constraint.

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

With the following configuration enabled:

  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

Root cause
----------

CONFIG_GCOV_PROFILE_ALL adds -fprofile-arcs -ftest-coverage -fno-tree-loop-im
to all kernel compilation units.  The skb_ext_total_length() function uses
a __always_inline loop over skb_ext_type_len[] array.  GCOV instrumentation
inserts branch counters, and -fno-tree-loop-im prevents loop optimization,
breaking the compiler's ability to constant-fold the result.  BUILD_BUG_ON
requires a compile-time constant and fails.

The problem is more pronounced in recent kernels (7.0+) where the number of
SKB extensions grew from 4 to 5+ (with addition of SKB_EXT_CAN, SKB_EXT_PSP,
etc.).  With 4 elements, GCC can still unroll and fold; with 5+, it gives up.

This affects both older and newer GCC versions (tested with GCC 11.4.1 and
GCC 16.0.1 20260327 experimental).

Solution
--------

Add a CONFIG_GCOV_PROFILE_ALL guard to the BUILD_BUG_ON, following the
existing pattern for CONFIG_KCOV_INSTRUMENT_ALL which has the same issue.

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

 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.43.5