Date:   Tue, 5 Dec 2017 11:25:13 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-kbuild <linux-kbuild@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Douglas Anderson <dianders@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Matthew Wilcox <matthew@....cx>,
        Matthias Kaehlcke <mka@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Gideon Israel Dsouza <gidisrael@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] [RFC] kbuild: add macro for controlling warnings to linux/compiler.h

On Tue, Dec 5, 2017 at 7:32 AM, Arnd Bergmann <arnd@...db.de> wrote:
> I have occasionally run into a situation where it would make sense to
> control a compiler warning from a source file rather than doing so from
> a Makefile using the $(cc-disable-warning, ...) or $(cc-option, ...)
> helpers.
>
> The approach here is similar to what glibc uses, using __diag() and
> related macros to encapsulate a _Pragma("GCC diagnostic ...") statement
> that gets turned into the respective "#pragma GCC diagnostic ..." by
> the preprocessor when the macro gets expanded.
>
> Like glibc, I also have an argument to pass the affected compiler
> version, but decided to actually evaluate that one. For now, this
> supports GCC_4_6, GCC_4_7, GCC_4_8, GCC_4_9, GCC_5, GCC_6, GCC_7,
> GCC_8 and GCC_9. Adding support for CLANG_5 and other interesting
> versions is straightforward here. GNU compilers starting with gcc-4.2
> could support it in principle, but "#pragma GCC diagnostic push"
> was only added in gcc-4.6, so it seems simpler to not deal with those
> at all. The same versions show a large number of warnings already,
> so it seems easier to just leave it at that and not do a more
> fine-grained control for them.
>
> The use cases I found so far include:
>
> - turning off the gcc-8 -Wattribute-alias warning inside of the
>   SYSCALL_DEFINEx() macro without having to do it globally.
>
> - Reducing the build time for a simple re-make after a change,
>   once we move the warnings from ./Makefile and
>   ./scripts/Makefile.extrawarn into linux/compiler.h
>
> - More control over the warnings based on other configurations,
>   using preprocessor syntax instead of Makefile syntax. This should make
>   it easier for the average developer to understand and change things.
>
> - Adding an easy way to turn the W=1 option on unconditionally
>   for a subdirectory or a specific file. This has been requested
>   by several developers in the past that want to have their subsystems
>   W=1 clean.
>
> - Integrating clang better into the build systems. Clang supports
>   more warnings than GCC, and we probably want to classify them
>   as default, W=1, W=2 etc, but there are cases in which the
>   warnings should be classified differently due to excessive false
>   positives from one or the other compiler.
>
> - Adding a way to turn the default warnings into errors (e.g. using
>   a new "make E=0" tag) while not also turning the W=1 warnings into
>   errors.
>
> This patch for now just adds the minimal infrastructure in order to
> do the first of the list above. As the #pragma GCC diagnostic
> takes precedence over command line options, the next step would be
> to convert a lot of the individual Makefiles that set nonstandard
> options to use __diag() instead.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> I'm marking this RFC for now, as I'd like to get consensus about
> whether we want to go down this particular route first, and maybe
> if someone can come up with better naming for the macros.

I like this. I wonder if it would be a good idea to add an additional
argument that forces documentation of the reason for adding a diag
marking? Something like:

__diag_warn(GCC_7, vla, "No VLAs should be used in this code");

-Kees

> ---
>  include/linux/compiler-gcc.h   | 64 ++++++++++++++++++++++++++++++++++++++++++
>  include/linux/compiler_types.h | 10 +++++++
>  2 files changed, 74 insertions(+)
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index 2272ded07496..5d595cfdb2c4 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -329,3 +329,67 @@
>   * code
>   */
>  #define uninitialized_var(x) x = x
> +
> +/*
> + * turn individual warnings and errors on and off locally, depending
> + * on version.
> + */
> +#if GCC_VERSION >= 40600
> +#define __diag_str1(s) #s
> +#define __diag_str(s) __diag_str1(s)
> +#define __diag(s) _Pragma(__diag_str(GCC diagnostic s))
> +
> +/* compilers before gcc-4.6 do not understand "#pragma GCC diagnostic push" */
> +#define __diag_GCC_4_6(s) __diag(s)
> +#else
> +#define __diag(s)
> +#define __diag_GCC_4_6(s)
> +#endif
> +
> +#if GCC_VERSION >= 40700
> +#define __diag_GCC_4_7(s) __diag(s)
> +#else
> +#define __diag_GCC_4_7(s)
> +#endif
> +
> +#if GCC_VERSION >= 40800
> +#define __diag_GCC_4_8(s) __diag(s)
> +#else
> +#define __diag_GCC_4_8(s)
> +#endif
> +
> +#if GCC_VERSION >= 40900
> +#define __diag_GCC_4_9(s) __diag(s)
> +#else
> +#define __diag_GCC_4_9(s)
> +#endif
> +
> +#if GCC_VERSION >= 50000
> +#define __diag_GCC_5(s) __diag(s)
> +#else
> +#define __diag_GCC_5(s)
> +#endif
> +
> +#if GCC_VERSION >= 60000
> +#define __diag_GCC_6(s) __diag(s)
> +#else
> +#define __diag_GCC_6(s)
> +#endif
> +
> +#if GCC_VERSION >= 70000
> +#define __diag_GCC_7(s) __diag(s)
> +#else
> +#define __diag_GCC_7(s)
> +#endif
> +
> +#if GCC_VERSION >= 80000
> +#define __diag_GCC_8(s) __diag(s)
> +#else
> +#define __diag_GCC_8(s)
> +#endif
> +
> +#if GCC_VERSION >= 90000
> +#define __diag_GCC_9(s) __diag(s)
> +#else
> +#define __diag_GCC_9(s)
> +#endif
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 6b79a9bba9a7..7e7664d57adb 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -271,4 +271,14 @@ struct ftrace_likely_data {
>  # define __native_word(t) (sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
>  #endif
>
> +#ifndef __diag
> +#define __diag(string)
> +#endif
> +
> +#define __diag_push()                  __diag(push)
> +#define __diag_ignore(version, option) __diag_ ## version (ignored option)
> +#define __diag_warn(version, option)   __diag_ ## version (warning option)
> +#define __diag_error(version, option)  __diag_ ## version (error   option)
> +#define __diag_pop()                   __diag(pop)
> +
>  #endif /* __LINUX_COMPILER_TYPES_H */
> --
> 2.9.0
>



-- 
Kees Cook
Pixel Security

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