Currently under Clang, CC_HAS_AUTO_VAR_INIT_ZERO requires an extra
-enable flag compared to CC_HAS_AUTO_VAR_INIT_PATTERN. GCC does 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.
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Cc: llvm@xxxxxxxxxxxxxxx
Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
Suggested-by: Will Deacon <will@xxxxxxxxxx>
Link: https://lore.kernel.org/lkml/20210914102837.6172-1-will@xxxxxxxxxx/
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Makefile | 6 +++---
security/Kconfig.hardening | 5 ++++-
2 files changed, 7 insertions(+), 4 deletions(-)
diff --git a/Makefile b/Makefile
index 34a0afc3a8eb..72d165ffabdb 100644
--- a/Makefile
+++ b/Makefile
@@ -831,12 +831,12 @@ 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.
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 90cbaff86e13..ded17b8abce2 100644
--- 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
--
2.30.2
