linux-kernel - Re: [PATCH net-next] compiler: Document behavior compiling with -O0

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Message-ID: <CAPWQB7FgS3gyODdmvx1QHmYp5zr34e6OQKwRxfh64D6cKowsDQ@mail.gmail.com>
Date:   Thu, 24 Aug 2017 09:49:13 -0700
From:   Joe Stringer <joe@....org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Ian Abbott <abbotti@....co.uk>, Arnd Bergmann <arnd@...db.de>,
        Michal Nazarewicz <mina86@...a86.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH net-next] compiler: Document behavior compiling with -O0

On 24 August 2017 at 09:45, Joe Stringer <joe@....org> wrote:
> Recent changes[0] to make use of __compiletime_assert() from container_of()
> increased the scope of this macro, resulting in a wider set of
> situations where developers cannot compile their code using "-O0". I
> noticed this when making use of the macro in my own development, and
> spent more time than I'd like to admit tracking the problem down. This
> patch documents the behavior in lieu of a compile-time assertion
> implementation that does not rely on optimizations.
>
> Example compilation failure:
>
> ./include/linux/compiler.h:547:38: error: call to ‘__compiletime_assert_94’ declared with attribute error: pointer type mismatch in container_of()
>   _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>                                       ^
> ./include/linux/compiler.h:530:4: note: in definition of macro ‘__compiletime_assert’
>     prefix ## suffix();    \
>     ^~~~~~
> ./include/linux/compiler.h:547:2: note: in expansion of macro ‘_compiletime_assert’
>   _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>   ^~~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:46:37: note: in expansion of macro ‘compiletime_assert’
>  #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>                                      ^~~~~~~~~~~~~~~~~~
> ./include/linux/kernel.h:860:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
>   BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \
>   ^~~~~~~~~~~~~~~~
>
> [0] http://lkml.kernel.org/r/20170525120316.24473-7-abbotti@mev.co.uk
>
> Signed-off-by: Joe Stringer <joe@....org>

This patch was written against the net-next tree, but is not targeted
at the net-next tree. My usual submissions go there so my formatting
scripts just inserted that. I can strip out and resend against a
particular tree if this is a problem.