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kbuild: Switch from -Wvla to -Wvla-larger-than=1

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=N option (though GCC may split -Wvla similarly[3] to
how Clang is planning to).

While GCC 8+ supports -Wvla-larger-than, only 9+ supports ...=0[4],
so use -Wvla-larger-than=1. Adjust mm/kasan/Makefile to remove it from
CFLAGS (GCC <9 appears unable to disable the warning correctly[5]).

The VLA usage in lib/test_ubsan.c was removed in commit 9d7ca61b1366
("lib/test_ubsan.c: VLA no longer used in kernel") so the lib/Makefile
disabling of VLA checking can be entirely removed.

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]
Link: https://lore.kernel.org/lkml/7780883c-0ac8-4aaa-b850-469e33b50672@linux.ibm.com/ [4]
Link: https://lore.kernel.org/r/202505071331.4iOzqmuE-lkp@intel.com/ [5]
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Link: https://lore.kernel.org/r/20250418213235.work.532-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Kees Cook 5e88c48c 5106c650

+9 -4
-1
lib/Makefile
··· 71 71 obj-$(CONFIG_TEST_SYSCTL) += test_sysctl.o 72 72 obj-$(CONFIG_TEST_IDA) += test_ida.o 73 73 obj-$(CONFIG_TEST_UBSAN) += test_ubsan.o 74 - CFLAGS_test_ubsan.o += $(call cc-disable-warning, vla) 75 74 CFLAGS_test_ubsan.o += $(call cc-disable-warning, unused-but-set-variable) 76 75 UBSAN_SANITIZE_test_ubsan.o := y 77 76 obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
+2 -1
mm/kasan/Makefile
··· 35 35 CFLAGS_hw_tags.o := $(CC_FLAGS_KASAN_RUNTIME) 36 36 CFLAGS_sw_tags.o := $(CC_FLAGS_KASAN_RUNTIME) 37 37 38 - CFLAGS_KASAN_TEST := $(CFLAGS_KASAN) $(call cc-disable-warning, vla) 38 + CFLAGS_KASAN_TEST := $(CFLAGS_KASAN) 39 39 ifndef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX 40 40 # If compiler instruments memintrinsics by prefixing them with __asan/__hwasan, 41 41 # we need to treat them normally (as builtins), otherwise the compiler won't ··· 44 44 CFLAGS_KASAN_TEST += -fno-builtin 45 45 endif 46 46 47 + CFLAGS_REMOVE_kasan_test_c.o += $(call cc-option, -Wvla-larger-than=1) 47 48 CFLAGS_kasan_test_c.o := $(CFLAGS_KASAN_TEST) 48 49 RUSTFLAGS_kasan_test_rust.o := $(RUSTFLAGS_KASAN) 49 50
+7 -2
scripts/Makefile.extrawarn
··· 45 45 # These result in bogus false positives 46 46 KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer) 47 47 48 - # Variable Length Arrays (VLAs) should not be used anywhere in the kernel 49 - KBUILD_CFLAGS += -Wvla 48 + # Stack Variable Length Arrays (VLAs) must not be used in the kernel. 49 + # Function array parameters should, however, be usable, but -Wvla will 50 + # warn for those. Clang has no way yet to distinguish between the VLA 51 + # types, so depend on GCC for now to keep stack VLAs out of the tree. 52 + # https://github.com/llvm/llvm-project/issues/57098 53 + # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98217 54 + KBUILD_CFLAGS += $(call cc-option,-Wvla-larger-than=1) 50 55 51 56 # disable pointer signed / unsigned warnings in gcc 4.0 52 57 KBUILD_CFLAGS += -Wno-pointer-sign