linux-hardening - Re: [PATCH] gcc-plugins: Disable GCC_PLUGIN_CYC

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Message-ID: <8d3de6fc-0991-9cef-d5fd-032fdbe2e85e@roeck-us.net>
Date:   Mon, 22 Feb 2021 08:03:31 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Heiko Carstens <hca@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH] gcc-plugins: Disable GCC_PLUGIN_CYC_COMPLEXITY for s390

On 2/22/21 7:45 AM, Masahiro Yamada wrote:
> On Tue, Feb 23, 2021 at 12:18 AM Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> On 2/22/21 4:05 AM, Heiko Carstens wrote:
>>> On Sun, Feb 21, 2021 at 02:56:50PM -0800, Guenter Roeck wrote:
>>>> Commit 334ef6ed06fa ("init/Kconfig: make COMPILE_TEST depend on !S390") disabled
>>>> COMPILE_TEST for s390. At the same time, "make allmodconfig/allyesconfig" for
>>>> s390 is still supported. However, it generates thousands of compiler
>>>> messages such as the following, making it highly impractical to run.
>>>>
>>>> Cyclomatic Complexity 1 scripts/mod/devicetable-offsets.c:main
>>>> Cyclomatic Complexity 1 scripts/mod/devicetable-offsets.c:_GLOBAL__sub_I_00100_0_main
>>>>
>>>> Since GCC_PLUGIN_CYC_COMPLEXITY is primarily used for testing, disable it
>>>> when building s390 images.
>>>>
>>>> Cc: Arnd Bergmann <arnd@...nel.org>
>>>> Cc: Heiko Carstens <hca@...ux.ibm.com>
>>>> Fixes: 334ef6ed06fa ("init/Kconfig: make COMPILE_TEST depend on !S390")
>>>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>>>> ---
>>>>  scripts/gcc-plugins/Kconfig | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
>>>> index ab9eb4cbe33a..5e9bb500f443 100644
>>>> --- a/scripts/gcc-plugins/Kconfig
>>>> +++ b/scripts/gcc-plugins/Kconfig
>>>> @@ -21,7 +21,7 @@ if GCC_PLUGINS
>>>>
>>>>  config GCC_PLUGIN_CYC_COMPLEXITY
>>>>      bool "Compute the cyclomatic complexity of a function" if EXPERT
>>>> -    depends on !COMPILE_TEST        # too noisy
>>>> +    depends on !COMPILE_TEST && !S390       # too noisy
>>>
>>> I don't see a reason to disable this in general for s390. COMPILE_TEST
>>> was only disabled for s390 because a lot of irrelevant configs didn't
>>> compile and it would cause a lot of unnecessary work to fix that.
>>>
>>> However the !COMPILE_TEST dependency here looks more like it was
>>> misused in lack of a possibility to detect if the config was generated
>>> with allyesconfig/allmodconfig. Maybe that could be added somehow to
>>> Kconfig?
>>>
>>
>> That is what COMPILE_TEST used to be for. Not setting it for s390 doesn't
>> just cause this config to be disabled. Lots of other compile tests
>> for s390 are now disabled as well, and code which should be disabled
>> is now enabled. Looking at it, I'd be a bit surprised if allmodconfig/
>> allyesconfig with COMPILE_TEST=y still builds for s390.
>>
>> I agree that COMPILE_TEST it is being misused. However, I think what
>> you'd be looking for is something like COMPILE_TEST_RANDCONFIG.
>> FWIW, in my opinion, randconfig should not enable COMPILE_TEST
>> to start with. But that is a different discussion.
>>
>> Anyway, I can disable GCC_PLUGIN_CYC_COMPLEXITY manually in my own
>> build tests, so this isn't that much of a problem for me, so feel
>> free to drop this patch.
>>
>> Guenter
> 
> 
> Maybe, we can add something like CONFIG_SUPPRESS_NOISY_TESTS,
> which is set to y by all{yes,mod}config.
> 
> This is self-documenting, so we do not need the '# too noisy' comment.
> 
> 
> 
> config SUPPRESS_NOISY_TESTS
>        bool "suppress noisy test"
> 
> 
> config GCC_PLUGIN_CYC_COMPLEXITY
>         bool "Compute the cyclomatic complexity of a function" if EXPERT
>         depends on !SUPPRESS_NOISY_TESTS
> 

Good idea. Downside would be that it won't solve the real problem
for s390 (which is lack of allmodconfig/allyesconfig compile test
coverage because COMPILE_TEST isn't set anymore), but that is a
different problem anyway, and my original patch doesn't solve
that either.

On the other side, if that flag would be set explicitly by
all{yes,mod}config, it would really beg for being misused. We
might then as well add a new flag that is explicitly associated
with all{yes,mod}config, but not with randconfig.

Thanks,
Guenter