Re: [RFC PATCH 4/7] kconfig: support new special property shell=

From: Masahiro Yamada
Date: Sat Feb 10 2018 - 02:13:16 EST

2018-02-10 14:48 GMT+09:00 Ulf Magnusson <ulfalizer@xxxxxxxxx>:
> On Fri, Feb 09, 2018 at 12:46:54PM -0800, Kees Cook wrote:
>> On Fri, Feb 9, 2018 at 4:46 AM, Ulf Magnusson <ulfalizer@xxxxxxxxx> wrote:
>> > One thing that makes Kconfig confusing (though it works well enough in
>> > practice) is that .config files both record user selections (the saved
>> > configuration) and serve as a configuration output format for make.
>> >
>> > It becomes easier to think about .config files once you realize that
>> > assignments to promptless symbols never have an effect on Kconfig
>> > itself: They're just configuration output, intermixed with the saved
>> > user selections.
>> >
>> > Assume 'option env' symbols got written out for example:
>> >
>> > - For a non-user-assignable symbol, the entry in the .config
>> > file is just configuration output and ignored by Kconfig,
>> > which will fetch the value from the environment instead.
>> >
>> > - For an assignable 'option env' symbol, the entry in the
>> > .config file is a saved user selection (as well as
>> > configuration output), and will be respected by Kconfig.
>>
>> In the stack-protector case, this becomes quite important, since the
>> goal is to record the user's selection regardless of compiler
>> capability. For example, if someone selects _REGULAR, it shouldn't
>> "upgrade" to _STRONG. (Similarly for _NONE.) Having _AUTO provides a
>> way to pick "best possible for this compiler", though. If a user had
>> previously selected _STRONG but they're doing builds with an older
>> compiler (or a misconfigured newer compiler) without support, the goal
>> is to _fail_ to build, not silently select _REGULAR.
>>
>> So, in this case, what's gained is the logic for _AUTO, and the logic
>> to not show, say, _STRONG when it's not available in the compiler. But
>> we must still fail to build if _STRONG was in the .config. It can't
>> silently rewrite it to _REGULAR because the compiler support for
>> _STRONG regressed.
>>
>> -Kees
>>
>> --
>> Kees Cook
>> Pixel Security
>
> Provided that would be the desired behavior:
>
> What about changing the meaning of the choice symbols from e.g. "select
> -fstack-protector-strong" to "want -fstack-protector-strong"? Then the
> user preference would always be remembered, regardless of what's
> available.
>
> Here's a proof-of-concept. I realized that the fancy new 'imply' keyword
> fits pretty well here, since it works like a dependency-respecting
> select.
>
> config CC_HAS_STACKPROTECTOR_STRONG
> bool
> option shell="$CC -Werror -fstack-protector-strong -c -x c /dev/null"
>
> config CC_HAS_STACKPROTECTOR
> bool
> option shell="$CC -Werror -fstack-protector -c -x c /dev/null"
>
>
> choice
> prompt "Stack Protector buffer overflow detection"
> default WANT_CC_STACKPROTECTOR_STRONG
>
> config WANT_CC_STACKPROTECTOR_STRONG
> bool "Strong"
> imply CC_STACKPROTECTOR_STRONG
>
> config WANT_CC_STACKPROTECTOR_REGULAR
> bool "Regular"
> imply CC_STACKPROTECTOR_REGULAR
>
> config WANT_CC_STACKPROTECTOR_NONE
> bool "None"
> imply CC_STACKPROTECTOR_NONE
>
> endchoice
>
>
> config CC_STACKPROTECTOR_STRONG
> bool
> depends on CC_HAS_STACKPROTECTOR_STRONG


Do you mean

config CC_STACKPROTECTOR_STRONG
bool
depends on CC_HAS_STACKPROTECTOR_STRONG && \
WANT_CC_STACKPROTECTOR_STRONG

or, maybe


config CC_STACKPROTECTOR_STRONG
bool
depends on CC_HAS_STACKPROTECTOR_STRONG
default WANT_CC_STACKPROTECTOR_STRONG

?





> config CC_STACKPROTECTOR_REGULAR
> bool
> depends on CC_HAS_STACKPROTECTOR_REGULAR
>
> config CC_STACKPROTECTOR_NONE
> bool
>
> This version has the drawback of always showing all the options, even if
> some they wouldn't be available. Kconfig comments could be added to warn
> if an option isn't available at least:
>
> comment "Warning: Your compiler does not support -fstack-protector-strong"
> depends on !CC_HAS_STACKPROTECTOR_STRONG
>
> config WANT_CC_STACKPROTECTOR_STRONG
> ...
>
>
> comment "Warning: Your compiler does not support -fstack-protector"
> depends on !CC_HAS_STACKPROTECTOR_REGULAR
>
> config WANT_CC_STACKPROTECTOR_REGULAR
> ...
>
> This final comment might be nice to have too:
>
> comment "Warning: Selected stack protector not available"
> depends on !(CC_STACKPROTECTOR_STRONG ||
> CC_STACKPROTECTOR_REGULAR ||
> CC_STACKPROTECTOR_NONE)
>
> Should probably introduce a clear warning that tells the user what they
> need to change in Kconfig if they build with a broken selection too.
>
>
> CC_STACKPROTECTOR_AUTO could be added to the choice in a slightly kludgy
> way too. Maybe there's something neater.
>
> config CC_STACKPROTECTOR_AUTO
> bool "Automatic"
> imply CC_STACKPROTECTOR_STRONG
> imply CC_STACKPROTECTOR_REGULAR if !CC_HAS_STACKPROTECTOR_STRONG
> imply CC_STACKPROTECTOR_NONE if !CC_HAS_STACKPROTECTOR_STRONG && \
> !CC_HAS_STACKPROTECTOR_REGULAR
>
>
> Another drawback of this approach is that it breaks existing .config
> files (the CC_STACKPROTECTOR_* settings are ignored, since they just
> look like "configuration output" to Kconfig now). If that'd be a
> problem, the old names could be used instead of
> WANT_CC_STACKPROTECTOR_STRONG, etc., and new names introduced instead,
> though it'd look a bit cryptic.
>
> Ideas?
>



FWIW, the following is what I was playing with.
(The idea for emitting warnings is Ulf's idea)


------------------>8-------------------
config CC
string
option env="CC"

config CC_HAS_STACKPROTECTOR
bool
option shell="$CC -Werror -fstack-protector -c -x c /dev/null"

config CC_HAS_STACKPROTECTOR_STRONG
bool
option shell="$CC -Werror -fstack-protector-strong -c -x c /dev/null"

config CC_HAS_STACKPROTECTOR_NONE
bool
option shell="$CC -Werror -fno-stack-protector -c -x c /dev/null"

config CC_STACKPROTECTOR
bool

choice
prompt "Stack Protector buffer overflow detection"

config CC_STACKPROTECTOR_AUTO
bool "Auto"
select CC_STACKPROTECTOR if (CC_HAS_STACKPROTECTOR || \
CC_HAS_STACKPROTECTOR_STRONG)

config CC_STACKPROTECTOR_REGULAR
bool "Regular"
select CC_STACKPROTECTOR

config CC_STACKPROTECTOR_STRONG
bool "Strong"
select CC_STACKPROTECTOR

config CC_STACKPROTECTOR_NONE
bool "None"

endchoice


comment "(WARNING) stackprotecter was chosen, but your compile does
not support it. Build will fail"
depends on CC_STACKPROTECTOR_REGULAR && \
!CC_HAS_STACKPROTECTOR

comment "(WARNING) stackprotecter-strong was chosen, but your compile
does not support it. Build will fail"
depends on CC_STACKPROTECTOR_STRONG && \
!CC_HAS_STACKPROTECTOR_STRONG
------------------------->8---------------------------------





BTW, setting option flags in Makefile is dirty, like follows:


ccflags-$(CONFIG_CC_STACKPROTECTOR_STRONG) += -fstack-protector-strong
ccflags-$(CONFIG_CC_STACKPROTECTOR_REGULAR) += -fstack-protector

if ($(CONFIG_CC_STACKPROTECTOR_AUTO),y)
ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR) += -fstack-protector
ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_STRONG) += -fstack-protector-strong
ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) += -fno-stack-protector
endif

if ($(CONFIG_CC_STACKPROTECTOR_NONE),y)
ccflags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) += -fno-stack-protector
endif




One idea could be to calculate the compiler option in Kconfig.

config CC_OPT_STACKPROTECTOR
string
default "-fstack-protector-strong" if CC_STACKPROTECTOR_STRONG || \
(CC_STACKPROTECTOR_AUTO && \
CC_HAS_STACKPROTECTOR_STRONG)
default "-fstack-protector" if CC_STACKPROTECTOR_REGULAR || \
(CC_STACKPROTECTOR_AUTO && \
CC_HAS_STACKPROTECTOR)
default "-fno-stack-protector" if CC_HAS_STACKPROTECTOR_NONE



Makefile will become clean.
Of course, this is at the cost of ugliness in Kconfig.




--
Best Regards
Masahiro Yamada