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Unverified Commit fdb64c84 authored Sep 14, 2021 by Kees Cook Committed by Nathan Chancellor Oct 14, 2022

UPSTREAM: hardening: Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO

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;a=commitdiff;h=a25e0b5e6ac8a77a71c229e0a7b744603365b0e9



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>
(cherry picked from commit f02003c860d921171be4a27e2893766eb3bc6871)
Change-Id: I572f9d1763dbc7b53f2ad0dc87ba074313f556eb
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

parent 09ea0309

Makefile

+3 −3

Original line number	Original line	Diff line number	Diff line
	@@ -759,12 +759,12 @@ endif

	# Initialize all stack variables with a zero value.		# Initialize all stack variables with a zero value.
	ifdef CONFIG_INIT_STACK_ALL_ZERO		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		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		KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
	endif		endif
			endif

	KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments)		KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments)

security/Kconfig.hardening

+4 −1

Original line number	Original line	Diff line number	Diff line
	@@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)		def_bool $(cc-option,-ftrivial-auto-var-init=pattern)

	config CC_HAS_AUTO_VAR_INIT_ZERO		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)		def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)

	choice		choice
	prompt "Initialize kernel stack variables at function entry"		prompt "Initialize kernel stack variables at function entry"
	default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS		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_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		default INIT_STACK_NONE
	help		help
	This option enables initialization of stack variables at		This option enables initialization of stack variables at