net/core/skbuff.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)
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): https://lore.kernel.org/lkml/20260401142020.1434243-1-khorenko@virtuozzo.com/T/#t That combined series was split per subsystem as requested by Jakub. The companion patches are: - iommu: disable GCOV for iommu_amdv1.o (sent to iommu maintainers) - gcov: add -fprofile-update=atomic globally (sent to gcov/kbuild maintainers, depends on this series and the iommu patch) 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. This is v3 of a previously posted standalone fix: https://lore.kernel.org/lkml/20260331165125.959833-1-khorenko@virtuozzo.com/T/#t 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 needs __no_profile as well. Konstantin Khorenko (2): net: fix skb_ext_total_length() BUILD_BUG_ON with CONFIG_GCOV_PROFILE_ALL net: add __no_profile to skb_extensions_init() for GCOV compatibility net/core/skbuff.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) -- 2.43.5
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
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):
https://lore.kernel.org/lkml/20260401142020.1434243-1-khorenko@virtuozzo.com/T/#t
That combined series was split per subsystem as requested by Jakub.
The companion patches are:
- iommu: use __always_inline for amdv1pt_install_leaf_entry()
(sent to iommu maintainers)
- gcov: add -fprofile-update=atomic globally (sent to gcov/kbuild
maintainers, depends on this series and the iommu patch)
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 v2 -> v3:
- Patch 2/2: added __init since skb_extensions_init() is only called
from __init skb_init() - without it noinline would keep the function
in permanent .text, wasting memory after boot.
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.
v2: https://lore.kernel.org/lkml/20260402140558.1437002-1-khorenko@virtuozzo.com/T/#t
v1: https://lore.kernel.org/lkml/20260401142020.1434243-1-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 __init __no_profile to skb_extensions_init() for
GCOV compatibility
net/core/skbuff.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
--
2.43.5
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>
---
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 43ee86dcf2ea..59fb4b2bb821 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5142,7 +5142,7 @@ 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;
@@ -5156,9 +5156,7 @@ 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(),
--
2.43.5With -fprofile-update=atomic in global CFLAGS_GCOV, GCC still cannot
constant-fold the skb_ext_total_length() loop when it is inlined into a
profiled caller. The existing __no_profile on skb_ext_total_length()
itself is insufficient because after __always_inline expansion the code
resides in the caller's body, which still carries GCOV instrumentation.
Mark skb_extensions_init() with __no_profile so the BUILD_BUG_ON checks
can be evaluated at compile time. Also mark it noinline to prevent the
compiler from inlining it into skb_init() (which lacks __no_profile),
which would re-expose the function body to GCOV instrumentation.
Add __init since skb_extensions_init() is only called from __init
skb_init(). Previously it was implicitly inlined into the .init.text
section; with noinline it would otherwise remain in permanent .text,
wasting memory after boot.
Build-tested with both CONFIG_GCOV_PROFILE_ALL=y and
CONFIG_KCOV_INSTRUMENT_ALL=y.
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
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 59fb4b2bb821..0978f526acf4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5153,7 +5153,7 @@ static __always_inline __no_profile unsigned int skb_ext_total_length(void)
return l;
}
-static void skb_extensions_init(void)
+static noinline void __init __no_profile skb_extensions_init(void)
{
BUILD_BUG_ON(SKB_EXT_NUM > 8);
BUILD_BUG_ON(skb_ext_total_length() > 255);
--
2.43.5When 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>
---
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 43ee86dcf2ea..59fb4b2bb821 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5142,7 +5142,7 @@ 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;
@@ -5156,9 +5156,7 @@ 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(),
--
2.43.5With -fprofile-update=atomic in global CFLAGS_GCOV, GCC still cannot
constant-fold the skb_ext_total_length() loop when it is inlined into a
profiled caller. The existing __no_profile on skb_ext_total_length()
itself is insufficient because after __always_inline expansion the code
resides in the caller's body, which still carries GCOV instrumentation.
Mark skb_extensions_init() with __no_profile so the BUILD_BUG_ON checks
can be evaluated at compile time. Also mark it noinline to prevent the
compiler from inlining it into skb_init() (which lacks __no_profile),
which would re-expose the function body to GCOV instrumentation.
Build-tested with both CONFIG_GCOV_PROFILE_ALL=y and
CONFIG_KCOV_INSTRUMENT_ALL=y.
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
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 59fb4b2bb821..8d75e352e3b1 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5153,7 +5153,7 @@ static __always_inline __no_profile unsigned int skb_ext_total_length(void)
return l;
}
-static void skb_extensions_init(void)
+static noinline void __no_profile skb_extensions_init(void)
{
BUILD_BUG_ON(SKB_EXT_NUM > 8);
BUILD_BUG_ON(skb_ext_total_length() > 255);
--
2.43.5© 2016 - 2026 Red Hat, Inc.