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

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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 9d7ca61b13
("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>
This commit is contained in:
Kees Cook 2025-04-18 14:32:39 -07:00
parent 5106c6506c
commit 5e88c48cb4
3 changed files with 9 additions and 4 deletions
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1
lib/Makefile
View File

@ -71,7 +71,6 @@ 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 += $(call cc-disable-warning, unused-but-set-variable)
UBSAN_SANITIZE_test_ubsan.o := y
obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o

3
mm/kasan/Makefile
View File

@ -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)
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
@ -44,6 +44,7 @@ ifndef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
CFLAGS_KASAN_TEST += -fno-builtin
endif
CFLAGS_REMOVE_kasan_test_c.o += $(call cc-option, -Wvla-larger-than=1)
CFLAGS_kasan_test_c.o := $(CFLAGS_KASAN_TEST)
RUSTFLAGS_kasan_test_rust.o := $(RUSTFLAGS_KASAN)

9
scripts/Makefile.extrawarn
View File

@ -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=1)
# disable pointer signed / unsigned warnings in gcc 4.0
KBUILD_CFLAGS += -Wno-pointer-sign