Re: [PATCH v2 11/21] stack-protector: test compiler capability in Kconfig and drop AUTO mode

[Masahiro Yamada]

2018-03-28 20:18 GMT+09:00 Kees Cook <keescook@xxxxxxxxxxxx>:
> On Mon, Mar 26, 2018 at 10:29 PM, Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>> Move the test for -fstack-protector(-strong) option to Kconfig.
>>
>> If the compiler does not support the option, the corresponding menu
>> is automatically hidden.  If _STRONG is not supported, it will fall
>> back to _REGULAR.  If _REGULAR is not supported, it will be disabled.
>> This means, _AUTO is implicitly handled by 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.
>>
>> 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.
>>
>> Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>
> This looks really good. Notes below...
>
>> ---
>>
>> Changes in v2:
>>   - Describe $(cc-option ...) directly in depends on context
>>
>>  Makefile                                  | 93 ++-----------------------------
>>  arch/Kconfig                              | 29 +++-------
>>  arch/x86/Kconfig                          |  8 ++-
>>  scripts/gcc-x86_32-has-stack-protector.sh |  7 +--
>>  scripts/gcc-x86_64-has-stack-protector.sh |  5 --
>>  5 files changed, 22 insertions(+), 120 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 5c395ed..5cadffa 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -675,55 +675,11 @@ ifneq ($(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).
>
> Let's keep this comment (slightly rewritten) since the reason for
> setting this flag isn't obvious.

Will move this to help of arch/x86/Kconfig


>> -    KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
>> -  endif
>> -endif
>> -endif
>> +stackp-flags-y                                 := -fno-stack-protector
>
> This is a (minor?) regression in my testing. Making this unconditional
> may break for a compiler built without stack-protector. It should be
> rare, but it's technically possible. Perhaps:
>
> stackp-flags-y := ($call cc-option, -fno-stack-protector)

Will add CONFIG_CC_HAS_STACKPROTECTOR_NONE


>> +stackp-flags-$(CONFIG_CC_STACKPROTECTOR)       := -fstack-protector
>> +stackp-flags-$(CONFIG_CC_STACKPROTECTOR_STRONG)        := -fstack-protector-strong
>> +
>> +KBUILD_CFLAGS += $(stackp-flags-y)
>> [...]
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 8e0d665..b42378d 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -535,13 +535,13 @@ config HAVE_CC_STACKPROTECTOR
>>         bool
>>         help
>>           An arch should select this symbol if:
>> -         - its compiler supports the -fstack-protector option
>
> Please leave this note: it's still valid. An arch must still have
> compiler support for this to be sensible.
>

No.

"its compiler supports the -fstack-protector option"
is tested by $(cc-option -fstack-protector)

ARCH does not need to know the GCC support level.




>>           - it has implemented a stack canary (e.g. __stack_chk_guard)

-- 
Best Regards
Masahiro Yamada