Re: [PATCH] kbuild: Switch from -Wvla to -Wvla-larger-than=0

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Apr 18, 2025 at 02:32:39PM -0700, Kees Cook wrote:
> Variable Length Arrays (VLAs) on the stack must not be used in the kernel.
> Function parameter VLAs[1] should be usable, but -Wvla will warn for
> those. For example, this will produce a warning but it is not using a
> stack VLA:
> 
>     int something(size_t n, int array[n]) { ...
> 
> Clang has no way yet to distinguish between the VLA types[2], so
> depend on GCC for now to keep stack VLAs out of the tree by using GCC's
> -Wvla-larger-than=0 option (though GCC may split -Wvla[3] similarly to
> how Clang is planning to).
> 
> Switch to -Wvla-larger-than=0 and adjust the two VLA-checking selftests
> to disable the updated option name.
> 
> Link: https://en.cppreference.com/w/c/language/array [1]
> Link: https://github.com/llvm/llvm-project/issues/57098 [2]
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98217 [3]
> Signed-off-by: Kees Cook <kees@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

>  lib/Makefile               | 2 +-
>  mm/kasan/Makefile          | 2 +-
>  scripts/Makefile.extrawarn | 9 +++++++--
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/Makefile b/lib/Makefile
> index f07b24ce1b3f..37b6e5782ecb 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -71,7 +71,7 @@ CFLAGS_test_bitops.o += -Werror
>  obj-$(CONFIG_TEST_SYSCTL) += test_sysctl.o
>  obj-$(CONFIG_TEST_IDA) += test_ida.o
>  obj-$(CONFIG_TEST_UBSAN) += test_ubsan.o
> -CFLAGS_test_ubsan.o += $(call cc-disable-warning, vla)
> +CFLAGS_test_ubsan.o += $(call cc-option, -Wno-vla-larger-than)
>  CFLAGS_test_ubsan.o += $(call cc-disable-warning, unused-but-set-variable)
>  UBSAN_SANITIZE_test_ubsan.o := y
>  obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
> diff --git a/mm/kasan/Makefile b/mm/kasan/Makefile
> index 1a958e7c8a46..0e326116a70b 100644
> --- a/mm/kasan/Makefile
> +++ b/mm/kasan/Makefile
> @@ -35,7 +35,7 @@ CFLAGS_shadow.o := $(CC_FLAGS_KASAN_RUNTIME)
>  CFLAGS_hw_tags.o := $(CC_FLAGS_KASAN_RUNTIME)
>  CFLAGS_sw_tags.o := $(CC_FLAGS_KASAN_RUNTIME)
>  
> -CFLAGS_KASAN_TEST := $(CFLAGS_KASAN) $(call cc-disable-warning, vla)
> +CFLAGS_KASAN_TEST := $(CFLAGS_KASAN) $(call cc-option, -Wno-vla-larger-than)
>  ifndef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
>  # If compiler instruments memintrinsics by prefixing them with __asan/__hwasan,
>  # we need to treat them normally (as builtins), otherwise the compiler won't
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index d75897559d18..0229b10c5d81 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -45,8 +45,13 @@ endif
>  # These result in bogus false positives
>  KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
>  
> -# Variable Length Arrays (VLAs) should not be used anywhere in the kernel
> -KBUILD_CFLAGS += -Wvla
> +# Stack Variable Length Arrays (VLAs) must not be used in the kernel.
> +# Function array parameters should, however, be usable, but -Wvla will
> +# warn for those. Clang has no way yet to distinguish between the VLA
> +# types, so depend on GCC for now to keep stack VLAs out of the tree.
> +# https://github.com/llvm/llvm-project/issues/57098
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98217
> +KBUILD_CFLAGS += $(call cc-option,-Wvla-larger-than=0)
>  
>  # disable pointer signed / unsigned warnings in gcc 4.0
>  KBUILD_CFLAGS += -Wno-pointer-sign
> -- 
> 2.34.1
>