From: Nathan Chancellor <nathan@kernel.org>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Michal Marek <michal.lkml@markovi.net>, Tom Rix <trix@redhat.com>,
Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
clang-built-linux <llvm@lists.linux.dev>, X86 ML <x86@kernel.org>,
Dmitrii Bundin <dmitrii.bundin.a@gmail.com>,
Fangrui Song <maskray@google.com>,
Alexey Alexandrov <aalexand@google.com>,
Bill Wendling <morbo@google.com>,
Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH v2 2/5] Makefile.compiler: Use KBUILD_AFLAGS for as-option
Date: Tue, 6 Sep 2022 09:08:12 -0700 [thread overview]
Message-ID: <YxdwbKA5ThYJcPBP@dev-arch.thelio-3990X> (raw)
In-Reply-To: <CAK7LNASQJ-B2kRGXea-dQt+1BgEsp_aLEPS_uJb1R6FSOj1Khg@mail.gmail.com>
On Mon, Sep 05, 2022 at 06:09:28PM +0900, Masahiro Yamada wrote:
> On Thu, Sep 1, 2022 at 4:53 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > On Wed, Aug 31, 2022 at 11:44:05AM -0700, Nick Desaulniers wrote:
> > > as-instr uses KBUILD_AFLAGS, but as-option uses KBUILD_CFLAGS. This can
> > > cause as-option to fail unexpectedly because clang will emit
> > > -Werror,-Wunused-command-line-argument for various -m and -f flags for
> > > assembler sources.
> >
> > Now that I am looking closer at it, where does that '-Werror' come from?
>
> The related commit is
> c3f0d0bc5b01ad90c45276952802455750444b4f
>
> The previous discussion with Arnd is
> https://lore.kernel.org/linux-kbuild/20170314213724.3836900-1-arnd@arndb.de/
Right, although this is for cc-option, not as-option, no?
> > For cc-option, we add it to elevate clang's warnings about unused '-f',
> > '-m', and '-W' flags to errors so that we do not add those flags.
> > However, I do not see '-Werror' in as-option. I am going to assume it
> > came from CONFIG_WERROR, as I believe Android has that turned on by
> > default.
>
> CONFIG_WERROR is added to CFLAGS.
> But, I guess it is more correct to do likewise for others.
> (https://patchwork.kernel.org/project/linux-kbuild/patch/20220905083619.672091-1-masahiroy@kernel.org/)
Ack.
> > I think that is the real problem: without '-Werror', the only
> > error that should come from as-option is when an option isn't supported
> > by the assembler, as clang will still warn but those will not be fatal
> > but with '-Werror', those warnings turn fatal, causing all subsequent
> > as-option calls to fail.
>
> Presumably, it is correct to add -Werror to as-option as well.
> We have no reason to add it to cc-option, but not to as-option.
>
> I also believe '-x assembler' should be changed to
> '-x assembler-with-cpp'.
>
> As I mentioned somewhere before, our assembly code (*.S) is always
> preprocessed. There is no *.s file in the kernel source tree.
>
> So, '-x assembler-with-cpp' matches the real situation.
All good points, I think that is a good fix as well.
> One interesting thing is, clang does not warn
> [-Wunused-command-line-argument] for *.S files.
>
> $ clang -fomit-frame-pointer -c -x assembler /dev/null -o /dev/null
> clang: warning: argument unused during compilation:
> '-fomit-frame-pointer' [-Wunused-command-line-argument]
>
> $ clang -fomit-frame-pointer -c -x assembler-with-cpp /dev/null -o /dev/null
Interesting... I suspect that is likely intentional, as some compiler
flags could be used during preprocessing (it's come up before:
https://github.com/llvm/llvm-project/issues/55656) but I was not able to
figure out exactly where in clang the flags were consumed but Driver.cpp
is quite large and I didn't look too hard :)
More importantly, it still allows us to catch invalid assembler
arguments:
$ clang -c -x assembler-with-cpp /dev/null -o /dev/null -Wa,-foo
clang-16: error: unsupported argument '-foo' to option '-Wa,'
$ clang -c -x assembler-with-cpp /dev/null -o /dev/null -Wa,--noexecstack
> The root cause is we are using '-x assembler', which
> never happens in the kernel tree.
>
> To sum up, the code I think correct is:
>
> as-option = $(call try-run,\
> $(CC) -Werror $(KBUILD_AFLAGS) $(1) -c -x assembler-with-cpp
> /dev/null -o "$$TMP",$(1),$(2))
>
Agreed. Thank you as always for your feedback!
Cheers,
Nathan
next prev parent reply other threads:[~2022-09-06 16:08 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-31 18:44 [PATCH v2 0/5] fix debug info for asm and DEBUG_INFO_SPLIT Nick Desaulniers
2022-08-31 18:44 ` [PATCH v2 1/5] x86/boot/compressed: prefer cc-option for CFLAGS additions Nick Desaulniers
2022-08-31 19:41 ` Nathan Chancellor
2022-08-31 18:44 ` [PATCH v2 2/5] Makefile.compiler: Use KBUILD_AFLAGS for as-option Nick Desaulniers
2022-08-31 19:53 ` Nathan Chancellor
2022-09-05 9:09 ` Masahiro Yamada
2022-09-06 16:08 ` Nathan Chancellor [this message]
2022-09-07 4:14 ` Masahiro Yamada
2022-09-07 3:12 ` Nick Desaulniers
2022-09-07 4:27 ` Masahiro Yamada
2022-08-31 18:44 ` [PATCH v2 3/5] Makefile.compiler: replace cc-ifversion with compiler-specific macros Nick Desaulniers
2022-08-31 20:08 ` Nathan Chancellor
2022-09-05 6:44 ` Masahiro Yamada
2022-08-31 18:44 ` [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files Nick Desaulniers
2022-08-31 20:22 ` Nathan Chancellor
2022-09-05 7:49 ` Masahiro Yamada
2022-09-07 4:27 ` Nick Desaulniers
2022-08-31 18:44 ` [PATCH v2 5/5] Makefile.debug: set -g unconditional on CONFIG_DEBUG_INFO_SPLIT Nick Desaulniers
2022-08-31 20:23 ` Nathan Chancellor
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