linux-kernel - Re: [PATCH v2] kbuild: fix linker feature test macros when cross compiling with Clang

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Date:   Mon, 30 Oct 2017 09:13:49 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Pirama Arumuga Nainar <pirama@...gle.com>,
        Douglas Anderson <dianders@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Marcin Nowakowski <marcin.nowakowski@...tec.com>,
        Mark Charlebois <charlebm@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kbuild: fix linker feature test macros when cross
 compiling with Clang

That's safe to do for now.  Here's the 0-day failure thread:
https://lists.01.org/pipermail/lkp/2017-October/007427.html  If I can
sort out the issue, I'll submit a v3.

On Mon, Oct 30, 2017 at 8:46 AM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> 2017-10-30 15:50 GMT+09:00 Masahiro Yamada <yamada.masahiro@...ionext.com>:
>> 2017-10-29 0:00 GMT+09:00 Masahiro Yamada <yamada.masahiro@...ionext.com>:
>>> 2017-10-28 5:13 GMT+09:00 Nick Desaulniers <ndesaulniers@...gle.com>:
>>>> I was not seeing my linker flags getting added when using ld-option when
>>>> cross compiling with Clang. Upon investigation, this seems to be due to
>>>> a difference in how GCC vs Clang handle cross compilation.
>>>>
>>>> GCC is configured at build time to support one backend, that is implicit
>>>> when compiling.  Clang is explicit via the use of `-target <triple>` and
>>>> ships with all supported backends by default.
>>>>
>>>> GNU Make feature test macros that compile then link will always fail
>>>> when cross compiling with Clang unless Clang's triple is passed along to
>>>> the compiler. For example:
>>>>
>>>> $ clang -x c /dev/null -c -o temp.o
>>>> $ aarch64-linux-android/bin/ld -E temp.o
>>>> aarch64-linux-android/bin/ld:
>>>> unknown architecture of input file `temp.o' is incompatible with
>>>> aarch64 output
>>>> aarch64-linux-android/bin/ld:
>>>> warning: cannot find entry symbol _start; defaulting to
>>>> 0000000000400078
>>>> $ echo $?
>>>> 1
>>>>
>>>> $ clang -target aarch64-linux-android- -x c /dev/null -c -o temp.o
>>>> $ aarch64-linux-android/bin/ld -E temp.o
>>>> aarch64-linux-android/bin/ld:
>>>> warning: cannot find entry symbol _start; defaulting to 00000000004002e4
>>>> $ echo $?
>>>> 0
>>>>
>>>> This causes conditional checks that invoke $(CC) without the target
>>>> triple, then $(LD) on the result, to always fail.
>>>>
>>>> Suggested-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>>>> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>>>> Reviewed-by: Matthias Kaehlcke <mka@...omium.org>
>>>> ---
>>>> Changes since v1:
>>>> * base patch off of
>>>>   git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
>>>>   kbuild branch, per Masahiro.
>>>> * Use $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) rather than $(CLANG_TRIPLE), per
>>>>   Masahiro.
>>>>
>>>>  scripts/Kbuild.include | 5 +++--
>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
>>>> index 064f477dfdca..0f09e4508554 100644
>>>> --- a/scripts/Kbuild.include
>>>> +++ b/scripts/Kbuild.include
>>>> @@ -228,12 +228,13 @@ cc-if-fullversion = $(shell [ $(cc-fullversion) $(1) $(2) ] && echo $(3) || echo
>>>>  # cc-ldoption
>>>>  # Usage: ldflags += $(call cc-ldoption, -Wl$(comma)--hash-style=both)
>>>>  cc-ldoption = $(call try-run-cached,\
>>>> -       $(CC) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
>>>> +       $(CC) $(1) $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
>>>>
>>>>  # ld-option
>>>>  # Usage: LDFLAGS += $(call ld-option, -X)
>>>>  ld-option = $(call try-run-cached,\
>>>> -       $(CC) -x c /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
>>>> +       $(CC) $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -x c /dev/null -c -o "$$TMPO"; \
>>>> +       $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
>>>>
>>>>  # ar-option
>>>>  # Usage: KBUILD_ARFLAGS := $(call ar-option,D)
>>>> --
>>>> 2.15.0.rc2.357.g7e34df9404-goog
>>>>
>>>> --
>>>> To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in
>>>> the body of a message to majordomo@...r.kernel.org
>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>
>>>
>>> Applied to linux-kbuild/kbuild.  Thanks!
>>
>>
>>
>> I do not know the cause of the problem reported by the 0-day bot, but
>> if the problem happens in the following line,
>>
>> ifeq ($(CONFIG_X86_32),y)
>> LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
>> else
>>
>>
>>
>> Does the following solve the issue?    (adding $(LDFLAGS))
>>
>> ld-option = $(call try-run,\
>>         $(CC) $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -x c /dev/null -c
>> -o "$$TMPO"; \
>>         $(LD) $(LDFLAGS) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
>>
>>
>
>
> I dropped this patch for now.
>
> --
> Best Regards
> Masahiro Yamada



-- 
Thanks,
~Nick Desaulniers