From: Kees Cook keescook@chromium.org
commit f02003c860d921171be4a27e2893766eb3bc6871 upstream.
Currently under Clang, CC_HAS_AUTO_VAR_INIT_ZERO requires an extra -enable flag compared to CC_HAS_AUTO_VAR_INIT_PATTERN. GCC 12[1] will not, and will happily ignore the Clang-specific flag. However, its presence on the command-line is both cumbersome and confusing. Due to GCC's tolerant behavior, though, we can continue to use a single Kconfig cc-option test for the feature on both compilers, but then drop the Clang-specific option in the Makefile.
In other words, this patch does not change anything other than making the compiler command line shorter once GCC supports -ftrivial-auto-var-init=zero.
[1] https://gcc.gnu.org/git/?p=gcc.git%3Ba=commitdiff%3Bh=a25e0b5e6ac8a77a71c229...
Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Masahiro Yamada masahiroy@kernel.org Cc: llvm@lists.linux.dev Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init") Suggested-by: Will Deacon will@kernel.org Link: https://lore.kernel.org/lkml/20210914102837.6172-1-will@kernel.org/ Reviewed-by: Nick Desaulniers ndesaulniers@google.com Reviewed-by: Nathan Chancellor nathan@kernel.org Acked-by: Will Deacon will@kernel.org Signed-off-by: Kees Cook keescook@chromium.org Signed-off-by: Nathan Chancellor nathan@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- Makefile | 6 +++--- security/Kconfig.hardening | 5 ++++- 2 files changed, 7 insertions(+), 4 deletions(-)
--- a/Makefile +++ b/Makefile @@ -844,12 +844,12 @@ endif
# Initialize all stack variables with a zero value. ifdef CONFIG_INIT_STACK_ALL_ZERO -# Future support for zero initialization is still being debated, see -# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being -# renamed or dropped. KBUILD_CFLAGS += -ftrivial-auto-var-init=zero +ifdef CONFIG_CC_IS_CLANG +# https://bugs.llvm.org/show_bug.cgi?id=45497 KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang endif +endif
# While VLAs have been removed, GCC produces unreachable stack probes # for the randomize_kstack_offset feature. Disable it for all compilers. --- a/security/Kconfig.hardening +++ b/security/Kconfig.hardening @@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
config CC_HAS_AUTO_VAR_INIT_ZERO + # GCC ignores the -enable flag, so we can test for the feature with + # a single invocation using the flag, but drop it as appropriate in + # the Makefile, depending on the presence of Clang. def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
choice prompt "Initialize kernel stack variables at function entry" default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO default INIT_STACK_NONE help This option enables initialization of stack variables at