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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 23 Oct 2018 11:40:37 -0700
Message-ID: <CAKwvOdmsKbW9v9or+u-W9V7T+LrF4ODY9cgUStkjkmsbMcqJ2g@mail.gmail.com>
Subject: Re: [PATCH RFC] x86: Don't include '-Wa,-' when building with Clang
To:     namit@vmware.com
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>
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On Mon, Oct 22, 2018 at 10:11 PM Nadav Amit <namit@vmware.com> wrote:
>
> at 5:37 PM, Nathan Chancellor <natechancellor@gmail.com> wrote:
>
> Commit 77b0bf55bc67 ("kbuild/Makefile: Prepare for using macros in
> inline assembly code to work around asm() related GCC inlining bugs")
> added this flag to KBUILD_CFLAGS, where it works perfectly fine with
> GCC. However, when building with Clang, all of the object files compile
> fine but the build hangs indefinitely at init/main.o, right before the
> linking stage. Don't include this flag when building with Clang.
>
> The kernel builds and boots to a shell in QEMU with both GCC and Clang
> with this patch applied.
>
> Link: https://na01.safelinks.protection.outlook.com/?url=3Dhttps%3A%2F%2F=
github.com%2FClangBuiltLinux%2Flinux%2Fissues%2F213&amp;data=3D02%7C01%7Cna=
mit%40vmware.com%7C871daebc2ca44947d28d08d638811fb5%7Cb39138ca3cee4b4aa4d6c=
d83d9dd62f0%7C1%7C0%7C636758524579997650&amp;sdata=3DshuxW81QRrO3TSqbgf462w=
gZYdLeAKeQEdGRxmnUX30%3D&amp;reserved=3D0
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>
> The reason this patch is labeled RFC is while I can verify that this
> fixes the issue, I'm not entirely sure why the '-Wa,-' works for GCC
> and not Clang. I looked into what the flag means and I couldn't really
> find anything so I just assume it's taking input from stdin? The issue
> could stem from how GCC forks gas versus how Clang does it. If this
> isn't of concern and the maintainers are happy with this patch as is,
> feel free to take it.
>
> arch/x86/Makefile | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 5b562e464009..4736dcc1caec 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -239,7 +239,10 @@ archheaders:
> archmacros:
> $(Q)$(MAKE) $(build)=3Darch/x86/kernel arch/x86/kernel/macros.s
>
> -ASM_MACRO_FLAGS =3D -Wa,arch/x86/kernel/macros.s -Wa,-
> +ASM_MACRO_FLAGS =3D -Wa,arch/x86/kernel/macros.s
> +ifneq ($(cc-name),clang)
> +ASM_MACRO_FLAGS +=3D -Wa,-
> +endif
> export ASM_MACRO_FLAGS
> KBUILD_CFLAGS +=3D $(ASM_MACRO_FLAGS)
>
>
> The '-Wa,-=E2=80=98 was indeed used to take the input from stdin when the=
 =E2=80=98-pipe=E2=80=99
> option is used in GCC. Perhaps clang ignores the =E2=80=98-pipe=E2=80=99 =
flag.
>

Clang documents support for `-pipe`.
https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-=
pipe

$ clang hello_world.c -no-integrated-as -Wa,-

is simple enough to reproduce the hang (notice no `-pipe`).
`strace`'ing the above with and without `-Wa,-` shows clang hung in a
`wait4` call after spawning as, while without `-Wa,-` shows it
receiving a SIGCHLD.

I've filed a bug against Clang to investigate:
https://bugs.llvm.org/show_bug.cgi?id=3D39410

In the meantime, I think Nathan's patch is the correct way to work
around this, if it is indeed a bug in Clang.

Reviewed-and-Tested-by: Nick Desaulniers <ndesaulniers@google.com>

https://stackoverflow.com/questions/1512933/when-should-i-use-gccs-pipe-opt=
ion
has some interesting perspectives on the use of `-pipe`.
Thanks for the patch Nathan, and more info Nadav.
--=20
Thanks,
~Nick Desaulniers