Re: [PATCH 10/23] stack-protector: test compiler capability in Kconfig and drop AUTO mode
From: Masahiro Yamada
Date: Tue Feb 20 2018 - 23:40:58 EST
2018-02-17 3:38 GMT+09:00 Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>:
> Add CC_HAS_STACKPROTECTOR(_STRONG) to test if the compiler supports
> -fstack-protector(-strong) option.
>
> X86 has additional shell scripts in case the compiler supports the
> option, but generates broken code. I added CC_HAS_SANE_STACKPROTECTOR
> to test this. I had to add -m32 to gcc-x86_32-has-stack-protector.sh
> to make it work correctly.
>
> If the compiler does not support the option, the menu is automatically
> hidden. If _STRONG is not supported, it will fall back to _REGULAR.
> This means, _AUTO is implicitly supported in the dependency solver of
> Kconfig, hence removed.
>
> I also turned the 'choice' into only two boolean symbols. The use of
> 'choice' is not a good idea here, because all of all{yes,mod,no}config
> would choose the first visible value, while we want allnoconfig to
> disable as many features as possible.
>
> I did not add CC_HAS_STACKPROTECTOR_NONE in the hope that GCC versions
> we support will recognize -fno-stack-protector.
>
> If this turns out to be a problem, it will be possible to do this:
>
> stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
> stackp-flags-$(CONFIG_CC_STACKPROTECTOR) := -fstack-protector
> stackp-flags-$(CONFIG_CC_STACKPROTECTOR_STRONG) := -fstack-protector-strong
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> ---
>
> Makefile | 93 ++-----------------------------
> arch/Kconfig | 37 ++++++------
> arch/x86/Kconfig | 8 ++-
> scripts/gcc-x86_32-has-stack-protector.sh | 7 +--
> scripts/gcc-x86_64-has-stack-protector.sh | 5 --
> 5 files changed, 30 insertions(+), 120 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 9a8c689..e9fc7c9 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -675,55 +675,11 @@ ifneq ($(CONFIG_FRAME_WARN),0)
> KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
> endif
>
> -# This selects the stack protector compiler flag. Testing it is delayed
> -# until after .config has been reprocessed, in the prepare-compiler-check
> -# target.
> -ifdef CONFIG_CC_STACKPROTECTOR_AUTO
> - stackp-flag := $(call cc-option,-fstack-protector-strong,$(call cc-option,-fstack-protector))
> - stackp-name := AUTO
> -else
> -ifdef CONFIG_CC_STACKPROTECTOR_REGULAR
> - stackp-flag := -fstack-protector
> - stackp-name := REGULAR
> -else
> -ifdef CONFIG_CC_STACKPROTECTOR_STRONG
> - stackp-flag := -fstack-protector-strong
> - stackp-name := STRONG
> -else
> - # If either there is no stack protector for this architecture or
> - # CONFIG_CC_STACKPROTECTOR_NONE is selected, we're done, and $(stackp-name)
> - # is empty, skipping all remaining stack protector tests.
> - #
> - # Force off for distro compilers that enable stack protector by default.
> - KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
> -endif
> -endif
> -endif
> -# Find arch-specific stack protector compiler sanity-checking script.
> -ifdef stackp-name
> -ifneq ($(stackp-flag),)
> - stackp-path := $(srctree)/scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh
> - stackp-check := $(wildcard $(stackp-path))
> - # If the wildcard test matches a test script, run it to check functionality.
> - ifdef stackp-check
> - ifneq ($(shell $(CONFIG_SHELL) $(stackp-check) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
> - stackp-broken := y
> - endif
> - endif
> - ifndef stackp-broken
> - # If the stack protector is functional, enable code that depends on it.
> - KBUILD_CPPFLAGS += -DCONFIG_CC_STACKPROTECTOR
> - # Either we've already detected the flag (for AUTO) or we'll fail the
> - # build in the prepare-compiler-check rule (for specific flag).
> - KBUILD_CFLAGS += $(stackp-flag)
> - else
> - # We have to make sure stack protector is unconditionally disabled if
> - # the compiler is broken (in case we're going to continue the build in
> - # AUTO mode).
> - KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
> - endif
> -endif
> -endif
> +stackp-flags-y := -fno-stack-protector
> +stackp-flags-$(CONFIG_CC_STACKPROTECTOR) := -fstack-protector
> +stackp-flags-$(CONFIG_CC_STACKPROTECTOR_STRONG) := -fstack-protector-strong
> +
> +KBUILD_CFLAGS += $(stackp-flags-y)
>
> ifeq ($(cc-name),clang)
> KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
> @@ -1079,7 +1035,7 @@ endif
> # prepare2 creates a makefile if using a separate output directory.
> # From this point forward, .config has been reprocessed, so any rules
> # that need to depend on updated CONFIG_* values can be checked here.
> -prepare2: prepare3 prepare-compiler-check outputmakefile asm-generic
> +prepare2: prepare3 outputmakefile asm-generic
>
> prepare1: prepare2 $(version_h) include/generated/utsrelease.h \
> include/config/auto.conf
> @@ -1105,43 +1061,6 @@ uapi-asm-generic:
> PHONY += prepare-objtool
> prepare-objtool: $(objtool_target)
>
> -# Check for CONFIG flags that require compiler support. Abort the build
> -# after .config has been processed, but before the kernel build starts.
> -#
> -# For security-sensitive CONFIG options, we don't want to fallback and/or
> -# silently change which compiler flags will be used, since that leads to
> -# producing kernels with different security feature characteristics
> -# depending on the compiler used. (For example, "But I selected
> -# CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")
> -PHONY += prepare-compiler-check
> -prepare-compiler-check: FORCE
> -# Make sure compiler supports requested stack protector flag.
> -ifdef stackp-name
> - # Warn about CONFIG_CC_STACKPROTECTOR_AUTO having found no option.
> - ifeq ($(stackp-flag),)
> - @echo CONFIG_CC_STACKPROTECTOR_$(stackp-name): \
> - Compiler does not support any known stack-protector >&2
> - else
> - # Fail if specifically requested stack protector is missing.
> - ifeq ($(call cc-option, $(stackp-flag)),)
> - @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \
> - $(stackp-flag) not supported by compiler >&2 && exit 1
> - endif
> - endif
> -endif
> -# Make sure compiler does not have buggy stack-protector support. If a
> -# specific stack-protector was requested, fail the build, otherwise warn.
> -ifdef stackp-broken
> - ifeq ($(stackp-name),AUTO)
> - @echo CONFIG_CC_STACKPROTECTOR_$(stackp-name): \
> - $(stackp-flag) available but compiler is broken: disabling >&2
> - else
> - @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \
> - $(stackp-flag) available but compiler is broken >&2 && exit 1
> - endif
> -endif
> - @:
> -
> # Generate some files
> # ---------------------------------------------------------------------------
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 76c0b54..9b7a628 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -535,13 +535,21 @@ config HAVE_CC_STACKPROTECTOR
> bool
> help
> An arch should select this symbol if:
> - - its compiler supports the -fstack-protector option
> - it has implemented a stack canary (e.g. __stack_chk_guard)
>
> -choice
> - prompt "Stack Protector buffer overflow detection"
> +config CC_HAS_STACKPROTECTOR
> + bool
> + default $(cc-option -fstack-protector)
> +
> +config CC_HAS_STACKPROTECTOR_STRONG
> + bool
> + default $(cc-option -fstack-protector-strong)
> +
> +config CC_STACKPROTECTOR
> + bool "Stack Protector buffer overflow detection"
> depends on HAVE_CC_STACKPROTECTOR
> - default CC_STACKPROTECTOR_AUTO
> + depends on CC_HAS_STACKPROTECTOR
> + default y
> help
CC_HAS_STACKPROTECTOR is not mandatory in this case
because we can directly describe $(cc-option ...)
in 'depends on' context, like this:
config CC_STACKPROTECTOR
bool "Stack Protector buffer overflow detection"
depends on HAVE_CC_STACKPROTECTOR
depends on $(cc-option -fstack-protector-strong)
default y
The difference is CONFIG_CC_HAS_STACKPROTECTOR
will not be written into the .config file.
Maybe, is this useful information?
You can check .config in case
"BTW, can my compiler support this flag?"
I will keep CC_HAS_ symbols,
but I am open to this item.
--
Best Regards
Masahiro Yamada