linux-kernel - Re: [PATCH] compiler: Don't perform compiletime

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Date:   Wed, 30 Aug 2017 23:52:39 -0700
From:   Joe Stringer <joe@....org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Abbott <abbotti@....co.uk>, Arnd Bergmann <arnd@...db.de>,
        Michal Nazarewicz <mina86@...a86.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] compiler: Don't perform compiletime_assert with -O0.

On 30 August 2017 at 19:16, Michael Ellerman <mpe@...erman.id.au> wrote:
> Joe Stringer <joe@....org> writes:
>
>> On 30 August 2017 at 15:59, Andrew Morton <akpm@...ux-foundation.org> wrote:
>>> On Tue, 29 Aug 2017 16:01:14 -0700 Joe Stringer <joe@....org> wrote:
>>>
>>>> Recent changes[0] to make use of __compiletime_assert() from
>>>> container_of() increased the usage of this macro, allowing developers to
>>>> notice type conflicts in usage of container_of() at compile time.
>>>> However, the implementation of __compiletime_assert relies on compiler
>>>> optimizations to report an error. This means that if a developer uses
>>>> "-O0" with any code that performs container_of(), the compiler will
>>>> always report an error regardless of whether there is an actual problem
>>>> in the code.
>>>>
>>>> This patch disables compile_time_assert when optimizations are disabled
>>>> to allow such code to compile with CFLAGS="-O0".
>>>
>>> I'm wondering if we should backport this into -stable.  Probably not,
>>> as I doubt if many people use -O0 - it's a pretty weird thing to do.  I
>>> used to use it a bit because it makes the ".lst" files (intermingled .c
>>> and .s files) make more sense.  In fact I'm wondering how you even
>>> noticed this?
>>
>> Local debugging, was trying to get a better understanding of the
>> underlying assembly and the code I was using just happened to use
>> container_of().
>
> Does the kernel actually build with -O0? I didn't think it actually
> worked.

I haven't tried the whole kernel, but you can set these CFLAGS on
specific files with a one-liner in a makefile:

CFLAGS_foo.o = -O0