linux-kernel - Re: [PATCH] kbuild: Check for unknown options with cc-option and clang in Kbuild

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Message-ID: <20190725051857.GA53904@archlinux-threadripper>
Date:   Wed, 24 Jul 2019 22:18:57 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Peter Smith <peter.smith@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH] kbuild: Check for unknown options with cc-option and
 clang in Kbuild

Hi Stephen,

Was the second Kbuild in the subject line supposed to be Kconfig?

On Wed, Jul 24, 2019 at 04:50:30PM -0700, Stephen Boyd wrote:
> If the particular version of clang a user has doesn't enable
> -Werror=unknown-warning-option by default, even though it is the
> default[1], then make sure to pass the option to the Kconfig cc-option

Hmmm interesting, I did not even know that was possible... Is that a
clang configuration option or an out of tree patch? Looks like it has
been on by default since clang 3.2: https://godbolt.org/z/mOmusu

> command so that testing options from Kconfig files works properly.
> Otherwise, depending on the default values setup in the clang toolchain
> we will silently assume options such as -Wmaybe-uninitialized are
> supported by clang, when they really aren't.
> 
> This issue only started happening for me once commit 589834b3a009
> ("kbuild: Add -Werror=unknown-warning-option to CLANG_FLAGS") was
> applied on top of commit b303c6df80c9 ("kbuild: compute false-positive
> -Wmaybe-uninitialized cases in Kconfig"). This leads kbuild to try and

Prior to 589834b3a009, how did cc-option work at all if
-Wunknown-warning-option wasn't enabled by default? I assume that clang
would just eat any unknown flags while returning zero so it looked like
the flag was supported?

> test for the existence of the -Wmaybe-uninitialized flag with the
> cc-option command in scripts/Kconfig.include, and it doesn't see an
> error returned from the option test so it sets the config value to Y.

It might be worth explicitly saying somewhere in here that clang will
not error on unknown flags without -Werror + -Wunknown-warning-option.

> Then the makefile tries to pass the unknown option on the command line
> and -Werror=unknown-warning-option catches the invalid option and breaks
> the build.
> 
> Note: this doesn't change the cc-option tests in Makefiles, because
> those use a different rule that includes KBUILD_CFLAGS by default, and
> the KBUILD_CFLAGS already has -Werror=unknown-warning-option. Thanks to
> Doug for pointing out the different rule.
> 
> [1] https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option
> Cc: Peter Smith <peter.smith@...aro.org>
> Cc: Nathan Chancellor <natechancellor@...il.com>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Douglas Anderson <dianders@...omium.org>
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> 
> Change-Id: I3bb69d45bb062d1306acbf19bc0adfb60f706442

I assume that shouldn't be there?

Overall, seems okay to me (took me a sec to understand the bug,
certainly a very specific one). It might make sense to explicitly add
somewhere in the commit message that this syncs cc-option behavior
between Kconfig and Kbuild as a whole, as I didn't understand that at
first. Thanks for the triage and sorry for the breakage!

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>