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

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2017-10-30 15:50 GMT+09:00 Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>:
> 2017-10-29 0:00 GMT+09:00 Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>:
>> 2017-10-28 5:13 GMT+09:00 Nick Desaulniers <ndesaulniers@xxxxxxxxxx>:
>>> 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@xxxxxxxxxxxxx>
>>> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>>> Reviewed-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
>>> ---
>>> 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 := $(call ar-option,D)
>>> --
>>> 2.15.0.rc2.357.g7e34df9404-goog
>>>
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in
>>> the body of a message to majordomo@xxxxxxxxxxxxxxx
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>
>> Applied to linux-kbuild/kbuild.  Thanks!
>
>
>
> I do not know the cause of the problem reported by the 0-day bot, but
> if the problem happens in the following line,
>
> ifeq ($(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
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