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

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Message-ID: <878ti08oru.fsf@concordia.ellerman.id.au>
Date:   Thu, 31 Aug 2017 12:16:53 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Joe Stringer <joe@....org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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.

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.

cheers