[PATCH v2 0/2] net: fix skb_ext BUILD_BUG_ON failures with GCOV

[Konstantin Khorenko <khorenko@virtuozzo.com>]

This mini-series fixes build failures in net/core/skbuff.c when the
kernel is built with CONFIG_GCOV_PROFILE_ALL=y.

This is part of a larger effort to add -fprofile-update=atomic to
global CFLAGS_GCOV (posted earlier as a combined series) [1].

The companion patch:
 - gcov: add -fprofile-update=atomic globally (sent to gcov/kbuild
   maintainers, depends on this series) [2]

There was another companion patch here, but i appeared to be already
applied recently: 8b72aa5704c7 ("iommupt/amdv1: mark
amdv1pt_install_leaf_entry as __always_inline").

Patch 1/2 fixes a pre-existing build failure with CONFIG_GCOV_PROFILE_ALL:
GCOV counters prevent GCC from constant-folding the skb_ext_total_length()
loop.  It also removes the CONFIG_KCOV_INSTRUMENT_ALL preprocessor guard
from d6e5794b06c0: that guard was a precaution in case KCOV instrumentation
also prevented constant folding, but KCOV's -fsanitize-coverage=trace-pc
does not interfere with GCC's constant folding (verified experimentally
with GCC 14.2 and GCC 16.0.1), so the guard is unnecessary.

Patch 2/2 is an additional fix needed when -fprofile-update=atomic is
added to CFLAGS_GCOV: __no_profile on the __always_inline function alone
is insufficient because after inlining, the code resides in the caller's
profiled body.  The caller (skb_extensions_init) needs __no_profile and
noinline to prevent re-exposure to GCOV instrumentation.

Changes v1 -> v2:
 - Patch 1/2: expanded the commit message to explain why removing the
   CONFIG_KCOV_INSTRUMENT_ALL guard is safe (KCOV's
   -fsanitize-coverage=trace-pc does not inhibit constant folding,
   unlike GCOV's -fprofile-arcs + -fno-tree-loop-im).
   Changed Fixes tag to point at the commit that introduced the 5th
   SKB extension type which triggered the failure.
   Removed empty lines in tags area.

 - Patch 2/2: added noinline to skb_extensions_init() to prevent
   the compiler from inlining it into skb_init(), which would
   re-expose the function body to GCOV instrumentation.

v1: https://lore.kernel.org/lkml/20260402140558.1437002-1-khorenko@virtuozzo.com/T/#t

[1] https://lore.kernel.org/lkml/20260401142020.1434243-1-khorenko@virtuozzo.com/T/#t
[2] https://lore.kernel.org/lkml/20260402141831.1437357-1-khorenko@virtuozzo.com/T/#t

Tested with:
 - GCC 14.2.1, CONFIG_GCOV_PROFILE_ALL=y
 - GCC 14.2.1, CONFIG_KCOV_INSTRUMENT_ALL=y (GCOV disabled)
 - GCC 16.0.1 20260327 (experimental), CONFIG_GCOV_PROFILE_ALL=y

Konstantin Khorenko (2):
  net: fix skb_ext_total_length() BUILD_BUG_ON with
    CONFIG_GCOV_PROFILE_ALL
  net: add noinline __no_profile to skb_extensions_init() for GCOV
    compatibility

 net/core/skbuff.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

-- 
2.43.5