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

[Kees Cook <kees@kernel.org>]

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>
---
Cc: Christoph Hellwig <hch@lst.de>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas.schier@linux.dev>
Cc: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: <kasan-dev@googlegroups.com>
Cc: <linux-mm@kvack.org>
Cc: <linux-kbuild@vger.kernel.org>
Cc: <llvm@lists.linux.dev>
---
 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