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

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Date:   Mon, 22 Feb 2021 07:18:17 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [PATCH] gcc-plugins: Disable GCC_PLUGIN_CYC_COMPLEXITY for s390

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