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

[Masahiro Yamada <yamada.masahiro@socionext.com>]

2017-10-29 0:00 GMT+09:00 Masahiro Yamada <yamada.masahiro@socionext.com>:
> 2017-10-28 5:13 GMT+09:00 Nick Desaulniers <ndesaulniers@google.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@socionext.com>
>> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>> Reviewed-by: Matthias Kaehlcke <mka@chromium.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
>>
>> --
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>
>
> 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))




-- 
Best Regards
Masahiro Yamada