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

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

[Konstantin Khorenko]

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

[PATCH 1/2] 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

Re: [PATCH 1/2] net: fix skb_ext_total_length() BUILD_BUG_ON with CONFIG_GCOV_PROFILE_ALL

[Vasileios Almpanis]

Reviewed-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>

On 4/2/26 4:05 PM, Konstantin Khorenko wrote:
> 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(),

Re: [PATCH 1/2] net: fix skb_ext_total_length() BUILD_BUG_ON with CONFIG_GCOV_PROFILE_ALL

[Paolo Abeni]

On 4/2/26 4:05 PM, Konstantin Khorenko wrote:
> 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>

No empty lines in the tags area.

Also given the commit description, isn't the introduction of the 5th skb
extension a better fixes tag?

> 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

Sashiko notes that there could be still build breakage:

https://sashiko.dev/#/patchset/20260402140558.1437002-1-khorenko%40virtuozzo.com

Could you please double check the above?

I think a 'noinline' in skb_extensions_init() would address any
complains on patch 2/2

/P

>  
>  	skbuff_ext_cache = kmem_cache_create("skbuff_ext_cache",
>  					     SKB_EXT_ALIGN_VALUE * skb_ext_total_length(),

[PATCH 2/2] net: add __no_profile to skb_extensions_init() for GCOV compatibility

[Konstantin Khorenko]

With -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.

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 47c7f0ab6e84..99704d6832e2 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5156,7 +5156,7 @@ static __always_inline __no_profile unsigned int skb_ext_total_length(void)
 	return l;
 }
 
-static void skb_extensions_init(void)
+static 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