[PATCH 0/1] disable __counted_by for clang < 19.1.3

[PATCH 0/1] disable __counted_by for clang < 19.1.3

[Jan Hendrik Farr]

Hi Kees,

Bill's PR to disable __counted_by for "whole struct" __bdos cases has now
been merged into 19.1.3 [1], so here's the patch to disable __counted_by
for clang versions < 19.1.3 in the kernel.

Hopefully in the near future __counted_by for whole struct __bdos can be
enabled once again in coordination between the kernel, gcc, and clang.
There has been recent progress on this in [2] thanks to Tavian.

Also see previous discussion on the mailing list [3]

Thanks to everyone for moving this issue along. In particular, Bill for
his PR to clang/llvm, Kees and Thorsten for reproducers of the two issues,
Nathan for Kconfig-ifying this patch, and Miguel for reviewing.


Info for the stable team:

This patch should be backported to kernels >= 6.6 to make sure that those
build correctly with the effected clang versions. This patch cherry-picks
cleanly onto linux-6.11.y. For linux-6.6.y three prerequiste commits are
neded:

16c31dd7fdf6: Compiler Attributes: counted_by: bump min gcc version
2993eb7a8d34: Compiler Attributes: counted_by: fixup clang URL
231dc3f0c936: lkdtm/bugs: Improve warning message for compilers without counted_by support

There are still two merge conflicts even with those prerequistes.
Here's the correct resolution:

1. include/linux/compiler_types.h:
	use the incoming change until before (but not including) the
        "Apply __counted_by() when the Endianness matches to increase test coverage."
        comment

2. lib/overflow_kunit.c: 
	HEAD is correct

[1] https://github.com/llvm/llvm-project/pull/112786
[2] https://github.com/llvm/llvm-project/pull/112636
[3] https://lore.kernel.org/lkml/3E304FB2-799D-478F-889A-CDFC1A52DCD8@toblux.com/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b

Best Regards
Jan

Jan Hendrik Farr (1):
  Compiler Attributes: disable __counted_by for clang < 19.1.3

 drivers/misc/lkdtm/bugs.c           |  2 +-
 include/linux/compiler_attributes.h | 13 -------------
 include/linux/compiler_types.h      | 19 +++++++++++++++++++
 init/Kconfig                        |  9 +++++++++
 lib/overflow_kunit.c                |  2 +-
 5 files changed, 30 insertions(+), 15 deletions(-)

-- 
2.47.0

[PATCH 1/1] Compiler Attributes: disable __counted_by for clang < 19.1.3

[Jan Hendrik Farr]

This patch disables __counted_by for clang versions < 19.1.3 because
of the two issues listed below. It does this by introducing
CONFIG_CC_HAS_COUNTED_BY.

1. clang < 19.1.2 has a bug that can lead to __bdos returning 0:
https://github.com/llvm/llvm-project/pull/110497

2. clang < 19.1.3 has a bug that can lead to __bdos being off by 4:
https://github.com/llvm/llvm-project/pull/112636

Fixes: c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
Cc: stable@vger.kernel.org # 6.6.x: 16c31dd7fdf6: Compiler Attributes: counted_by: bump min gcc version
Cc: stable@vger.kernel.org # 6.6.x: 2993eb7a8d34: Compiler Attributes: counted_by: fixup clang URL
Cc: stable@vger.kernel.org # 6.6.x: 231dc3f0c936: lkdtm/bugs: Improve warning message for compilers without counted_by support
Cc: stable@vger.kernel.org # 6.6.x
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202409260949.a1254989-oliver.sang@intel.com
Link: https://lore.kernel.org/all/Zw8iawAF5W2uzGuh@archlinux/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jan Hendrik Farr <kernel@jfarr.cc>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
---
 drivers/misc/lkdtm/bugs.c           |  2 +-
 include/linux/compiler_attributes.h | 13 -------------
 include/linux/compiler_types.h      | 19 +++++++++++++++++++
 init/Kconfig                        |  9 +++++++++
 lib/overflow_kunit.c                |  2 +-
 5 files changed, 30 insertions(+), 15 deletions(-)

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 62ba01525479..376047beea3d 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -445,7 +445,7 @@ static void lkdtm_FAM_BOUNDS(void)
 
 	pr_err("FAIL: survived access of invalid flexible array member index!\n");
 
-	if (!__has_attribute(__counted_by__))
+	if (!IS_ENABLED(CONFIG_CC_HAS_COUNTED_BY))
 		pr_warn("This is expected since this %s was built with a compiler that does not support __counted_by\n",
 			lkdtm_kernel_info);
 	else if (IS_ENABLED(CONFIG_UBSAN_BOUNDS))
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 32284cd26d52..c16d4199bf92 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -94,19 +94,6 @@
 # define __copy(symbol)
 #endif
 
-/*
- * Optional: only supported since gcc >= 15
- * Optional: only supported since clang >= 18
- *
- *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
- * clang: https://github.com/llvm/llvm-project/pull/76348
- */
-#if __has_attribute(__counted_by__)
-# define __counted_by(member)		__attribute__((__counted_by__(member)))
-#else
-# define __counted_by(member)
-#endif
-
 /*
  * Optional: not supported by gcc
  * Optional: only supported since clang >= 14.0
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1a957ea2f4fe..639be0f30b45 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -323,6 +323,25 @@ struct ftrace_likely_data {
 #define __no_sanitize_or_inline __always_inline
 #endif
 
+/*
+ * Optional: only supported since gcc >= 15
+ * Optional: only supported since clang >= 18
+ *
+ *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
+ * clang: https://github.com/llvm/llvm-project/pull/76348
+ *
+ * __bdos on clang < 19.1.2 can erroneously return 0:
+ * https://github.com/llvm/llvm-project/pull/110497
+ *
+ * __bdos on clang < 19.1.3 can be off by 4:
+ * https://github.com/llvm/llvm-project/pull/112636
+ */
+#ifdef CONFIG_CC_HAS_COUNTED_BY
+# define __counted_by(member)		__attribute__((__counted_by__(member)))
+#else
+# define __counted_by(member)
+#endif
+
 /*
  * Apply __counted_by() when the Endianness matches to increase test coverage.
  */
diff --git a/init/Kconfig b/init/Kconfig
index 530a382ee0fe..92f106cf5572 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -116,6 +116,15 @@ config CC_HAS_ASM_INLINE
 config CC_HAS_NO_PROFILE_FN_ATTR
 	def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
 
+config CC_HAS_COUNTED_BY
+	# TODO: when gcc 15 is released remove the build test and add
+	# a gcc version check
+	def_bool $(success,echo 'struct flex { int count; int array[] __attribute__((__counted_by__(count))); };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
+	# clang needs to be at least 19.1.3 to avoid __bdos miscalculations
+	# https://github.com/llvm/llvm-project/pull/110497
+	# https://github.com/llvm/llvm-project/pull/112636
+	depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index 2abc78367dd1..5222c6393f11 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -1187,7 +1187,7 @@ static void DEFINE_FLEX_test(struct kunit *test)
 {
 	/* Using _RAW_ on a __counted_by struct will initialize "counter" to zero */
 	DEFINE_RAW_FLEX(struct foo, two_but_zero, array, 2);
-#if __has_attribute(__counted_by__)
+#ifdef CONFIG_CC_HAS_COUNTED_BY
 	int expected_raw_size = sizeof(struct foo);
 #else
 	int expected_raw_size = sizeof(struct foo) + 2 * sizeof(s16);
-- 
2.47.0

Re: [PATCH 1/1] Compiler Attributes: disable __counted_by for clang < 19.1.3

[Thorsten Blum]

On 29. Oct 2024, at 15:00, Jan Hendrik Farr wrote:
> 
> This patch disables __counted_by for clang versions < 19.1.3 because
> of the two issues listed below. It does this by introducing
> CONFIG_CC_HAS_COUNTED_BY.
> 
> 1. clang < 19.1.2 has a bug that can lead to __bdos returning 0:
> https://github.com/llvm/llvm-project/pull/110497
> 
> 2. clang < 19.1.3 has a bug that can lead to __bdos being off by 4:
> https://github.com/llvm/llvm-project/pull/112636
> 
> Fixes: c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
> Cc: stable@vger.kernel.org # 6.6.x: 16c31dd7fdf6: Compiler Attributes: counted_by: bump min gcc version
> Cc: stable@vger.kernel.org # 6.6.x: 2993eb7a8d34: Compiler Attributes: counted_by: fixup clang URL
> Cc: stable@vger.kernel.org # 6.6.x: 231dc3f0c936: lkdtm/bugs: Improve warning message for compilers without counted_by support
> Cc: stable@vger.kernel.org # 6.6.x
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Closes: https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202409260949.a1254989-oliver.sang@intel.com
> Link: https://lore.kernel.org/all/Zw8iawAF5W2uzGuh@archlinux/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Jan Hendrik Farr <kernel@jfarr.cc>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Tested-by: Nathan Chancellor <nathan@kernel.org>
> Reviewed-by: Miguel Ojeda <ojeda@kernel.org>

Thanks for fixing this and your work on the Clang issues. Feel free to add:

Reviewed-by: Thorsten Blum <thorsten.blum@linux.dev>

Re: [PATCH 1/1] Compiler Attributes: disable __counted_by for clang < 19.1.3

[Kees Cook]

On Tue, 29 Oct 2024 15:00:36 +0100, Jan Hendrik Farr wrote:
> This patch disables __counted_by for clang versions < 19.1.3 because
> of the two issues listed below. It does this by introducing
> CONFIG_CC_HAS_COUNTED_BY.
> 
> 1. clang < 19.1.2 has a bug that can lead to __bdos returning 0:
> https://github.com/llvm/llvm-project/pull/110497
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] Compiler Attributes: disable __counted_by for clang < 19.1.3
      https://git.kernel.org/kees/c/f06e108a3dc5

Take care,

-- 
Kees Cook

Re: [PATCH 1/1] Compiler Attributes: disable __counted_by for clang < 19.1.3

[Dafna Hirschfeld]

On 29.10.2024 15:00, Jan Hendrik Farr wrote:
>This patch disables __counted_by for clang versions < 19.1.3 because
>of the two issues listed below. It does this by introducing
>CONFIG_CC_HAS_COUNTED_BY.
>
>1. clang < 19.1.2 has a bug that can lead to __bdos returning 0:
>https://github.com/llvm/llvm-project/pull/110497
>
>2. clang < 19.1.3 has a bug that can lead to __bdos being off by 4:
>https://github.com/llvm/llvm-project/pull/112636
>
>Fixes: c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
>Cc: stable@vger.kernel.org # 6.6.x: 16c31dd7fdf6: Compiler Attributes: counted_by: bump min gcc version
>Cc: stable@vger.kernel.org # 6.6.x: 2993eb7a8d34: Compiler Attributes: counted_by: fixup clang URL
>Cc: stable@vger.kernel.org # 6.6.x: 231dc3f0c936: lkdtm/bugs: Improve warning message for compilers without counted_by support
>Cc: stable@vger.kernel.org # 6.6.x
>Reported-by: Nathan Chancellor <nathan@kernel.org>
>Closes: https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/
>Reported-by: kernel test robot <oliver.sang@intel.com>
>Closes: https://lore.kernel.org/oe-lkp/202409260949.a1254989-oliver.sang@intel.com
>Link: https://lore.kernel.org/all/Zw8iawAF5W2uzGuh@archlinux/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b
>Suggested-by: Nathan Chancellor <nathan@kernel.org>
>Signed-off-by: Jan Hendrik Farr <kernel@jfarr.cc>
>Reviewed-by: Nathan Chancellor <nathan@kernel.org>
>Tested-by: Nathan Chancellor <nathan@kernel.org>
>Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
>---
> drivers/misc/lkdtm/bugs.c           |  2 +-
> include/linux/compiler_attributes.h | 13 -------------
> include/linux/compiler_types.h      | 19 +++++++++++++++++++
> init/Kconfig                        |  9 +++++++++
> lib/overflow_kunit.c                |  2 +-
> 5 files changed, 30 insertions(+), 15 deletions(-)
>
>diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
>index 62ba01525479..376047beea3d 100644
>--- a/drivers/misc/lkdtm/bugs.c
>+++ b/drivers/misc/lkdtm/bugs.c
>@@ -445,7 +445,7 @@ static void lkdtm_FAM_BOUNDS(void)
>
> 	pr_err("FAIL: survived access of invalid flexible array member index!\n");
>
>-	if (!__has_attribute(__counted_by__))
>+	if (!IS_ENABLED(CONFIG_CC_HAS_COUNTED_BY))
> 		pr_warn("This is expected since this %s was built with a compiler that does not support __counted_by\n",
> 			lkdtm_kernel_info);
> 	else if (IS_ENABLED(CONFIG_UBSAN_BOUNDS))
>diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
>index 32284cd26d52..c16d4199bf92 100644
>--- a/include/linux/compiler_attributes.h
>+++ b/include/linux/compiler_attributes.h
>@@ -94,19 +94,6 @@
> # define __copy(symbol)
> #endif
>
>-/*
>- * Optional: only supported since gcc >= 15
>- * Optional: only supported since clang >= 18
>- *
>- *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
>- * clang: https://github.com/llvm/llvm-project/pull/76348
>- */
>-#if __has_attribute(__counted_by__)
>-# define __counted_by(member)		__attribute__((__counted_by__(member)))
>-#else
>-# define __counted_by(member)
>-#endif
>-

Why is the define of __counted_by moved from here (compiler_attributes.h)
to a different location (compiler_types.h) ?
I am asking this because I try to compile an out of tree kernel module with ofed headers.
in the ofed header of compiler_types.h there is an added include of types.h:

#include_next <linux/compiler_attributes.h>
#include <linux/types.h>

so what happen is that __counted_by is defined in types.h, and then I get a compilation error:

././include/linux/compiler_types.h:323: error: "__counted_by" redefined [-Werror]

I am not sure yet how this should be fixed.
Thank you,
Dafna





> /*
>  * Optional: not supported by gcc
>  * Optional: only supported since clang >= 14.0
>diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
>index 1a957ea2f4fe..639be0f30b45 100644
>--- a/include/linux/compiler_types.h
>+++ b/include/linux/compiler_types.h
>@@ -323,6 +323,25 @@ struct ftrace_likely_data {
> #define __no_sanitize_or_inline __always_inline
> #endif
>
>+/*
>+ * Optional: only supported since gcc >= 15
>+ * Optional: only supported since clang >= 18
>+ *
>+ *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
>+ * clang: https://github.com/llvm/llvm-project/pull/76348
>+ *
>+ * __bdos on clang < 19.1.2 can erroneously return 0:
>+ * https://github.com/llvm/llvm-project/pull/110497
>+ *
>+ * __bdos on clang < 19.1.3 can be off by 4:
>+ * https://github.com/llvm/llvm-project/pull/112636
>+ */
>+#ifdef CONFIG_CC_HAS_COUNTED_BY
>+# define __counted_by(member)		__attribute__((__counted_by__(member)))
>+#else
>+# define __counted_by(member)
>+#endif
>+
> /*
>  * Apply __counted_by() when the Endianness matches to increase test coverage.
>  */
>diff --git a/init/Kconfig b/init/Kconfig
>index 530a382ee0fe..92f106cf5572 100644
>--- a/init/Kconfig
>+++ b/init/Kconfig
>@@ -116,6 +116,15 @@ config CC_HAS_ASM_INLINE
> config CC_HAS_NO_PROFILE_FN_ATTR
> 	def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
>
>+config CC_HAS_COUNTED_BY
>+	# TODO: when gcc 15 is released remove the build test and add
>+	# a gcc version check
>+	def_bool $(success,echo 'struct flex { int count; int array[] __attribute__((__counted_by__(count))); };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
>+	# clang needs to be at least 19.1.3 to avoid __bdos miscalculations
>+	# https://github.com/llvm/llvm-project/pull/110497
>+	# https://github.com/llvm/llvm-project/pull/112636
>+	depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)
>+
> config PAHOLE_VERSION
> 	int
> 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
>diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
>index 2abc78367dd1..5222c6393f11 100644
>--- a/lib/overflow_kunit.c
>+++ b/lib/overflow_kunit.c
>@@ -1187,7 +1187,7 @@ static void DEFINE_FLEX_test(struct kunit *test)
> {
> 	/* Using _RAW_ on a __counted_by struct will initialize "counter" to zero */
> 	DEFINE_RAW_FLEX(struct foo, two_but_zero, array, 2);
>-#if __has_attribute(__counted_by__)
>+#ifdef CONFIG_CC_HAS_COUNTED_BY
> 	int expected_raw_size = sizeof(struct foo);
> #else
> 	int expected_raw_size = sizeof(struct foo) + 2 * sizeof(s16);
>-- 
>2.47.0
>

Re: [PATCH 1/1] Compiler Attributes: disable __counted_by for clang < 19.1.3

[Miguel Ojeda]

On Mon, Jun 9, 2025 at 3:14 PM Dafna Hirschfeld
<dafna.hirschfeld@intel.com> wrote:
>
> Why is the define of __counted_by moved from here (compiler_attributes.h)
> to a different location (compiler_types.h) ?

`compiler_attributes.h` is supposed to have only things that do not
depend on the kernel configuration and so on.

It sounds like those out-of-tree headers should be fixed.

I hope that helps.

Cheers,
Miguel