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From:   Kees Cook <keescook@google.com>
Date:   Wed, 4 Apr 2018 17:20:14 -0700
Message-ID: <CAGXu5jLebFAFTmu0r4jZLUvWi_6ki2JL598M-tCXeTnWo5vWPQ@mail.gmail.com>
Subject: Re: [GIT PULL] x86/build changes for v4.17
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Matthias Kaehlcke <mka@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        James Y Knight <jyknight@google.com>,
        Chandler Carruth <chandlerc@google.com>,
        Stephen Hines <srhines@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Guenter Roeck <groeck@chromium.org>,
        Greg Hackmann <ghackmann@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Wed, Apr 4, 2018 at 5:05 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Apr 4, 2018 at 4:31 PM, Matthias Kaehlcke <mka@chromium.org> wrote:
>>
>> From some experiments it looks like clang, in difference to gcc, does
>> not treat constant values passed as parameters to inline function as
>> constants.
>
> Yeah, I think gcc used to have those semantics a long time ago too.
>
> Many of our __builtin_constant_p() uses are indeed just in macros, but
> certainly not all.
>
> Other examples are found in our "fortified" string functions.
>
> There a clang build will likely simply miss some of the build-time
> fortification checks, and trigger them at runtime instead.
>
> Of course, we hopefully don't *have* any build-time failures, because
> gcc will have caught them, so you won't care as long as clang is a
> secondary compiler, but long-term they'd be good.

Yeah, it's used in inline functions in a lot of places. Some quickly
jump out: kmalloc, crypto, bitmaps, networking, uaccess, kvm, etc from
doing a dumb grep as:

git grep -B5 __builtin_constant_p | grep -A5 inline

FWIW, I prefer inline functions over macros just to keep type checking
a some level of sanity when reading build warnings/errors. ;)

-Kees

-- 
Kees Cook
Pixel Security