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Compiler Attributes: disable __counted_by for clang < 19.1.3
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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:
llvm/llvm-project#110497

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

Fixes: c8248fa ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
Cc: [email protected] # 6.6.x: 16c31dd: Compiler Attributes: counted_by: bump min gcc version
Cc: [email protected] # 6.6.x: 2993eb7: Compiler Attributes: counted_by: fixup clang URL
Cc: [email protected] # 6.6.x: 231dc3f: lkdtm/bugs: Improve warning message for compilers without counted_by support
Cc: [email protected] # 6.6.x
Reported-by: Nathan Chancellor <[email protected]>
Closes: https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Link: https://lore.kernel.org/all/Zw8iawAF5W2uzGuh@archlinux/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b
Signed-off-by: Jan Hendrik Farr <[email protected]>
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Cydox authored and 1Naim committed Nov 4, 2024
1 parent 3698e19 commit 9509691
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Showing 5 changed files with 29 additions and 15 deletions.
2 changes: 1 addition & 1 deletion drivers/misc/lkdtm/bugs.c
Original file line number Diff line number Diff line change
Expand Up @@ -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))
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13 changes: 0 additions & 13 deletions include/linux/compiler_attributes.h
Original file line number Diff line number Diff line change
Expand Up @@ -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
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19 changes: 19 additions & 0 deletions include/linux/compiler_types.h
Original file line number Diff line number Diff line change
Expand Up @@ -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.
*/
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8 changes: 8 additions & 0 deletions init/Kconfig
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Expand Up @@ -120,6 +120,14 @@ 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)

# 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
# TODO: when gcc 15 is released remove the build test and add gcc version check
config CC_HAS_COUNTED_BY
def_bool $(success,echo 'struct flex { int count; int array[] __attribute__((__counted_by__(count))); };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)

config PAHOLE_VERSION
int
default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
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2 changes: 1 addition & 1 deletion lib/overflow_kunit.c
Original file line number Diff line number Diff line change
Expand Up @@ -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);
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