Bulid regression with VDSO enabled

[Stefan Agner]

On 2015-05-09 14:41, Stefan Agner wrote:
> On 2015-05-08 23:15, Nathan Lynch wrote:
>> On 05/08/2015 11:08 AM, Stefan Agner wrote:
>>> On 2015-05-08 17:27, Nathan Lynch wrote:
>>>> On 05/08/2015 06:13 AM, Stefan Agner wrote:
>>>>> Also, I used the BFD linker by adding LD=${CROSS_COMPILE}ld.bfd to the
>>>>> build command. Interestingly thought, I had the same issue when using
>>>>> gold linker...
>>>>
>>>> When I try this, ld.gold is used regardless.
>>>>
>>>> You can tell by doing and checking for a NT_GNU_GOLD_VERSION note:
>>>>
>>>> $ readelf -n arch/arm/vdso/vdso.so.raw
>>>>
>>>> Displaying notes found at file offset 0x000001bc with length 0x00000040:
>>>>   Owner                 Data size       Description
>>>>   GNU                  0x00000009       NT_GNU_GOLD_VERSION (gold version)
>>>>   GNU                  0x00000014       NT_GNU_BUILD_ID (unique build
>>>> ID bitstring)
>>>>     Build ID: f025409550acb7f955c61d95691291da078e6688
>>>
>>> Hm, you are right. When deleting moving ld.bfd to ld, which makes the
>>> BFD linker as default, compiling works flawless.
>>>
>>> However, so far LD=${CROSS_COMPILE}ld.bfc seems to work for other build
>>> objects, at least those do not have this note. For instance time.o does
>>> not return anything:
>>>
>>> $ readelf -n arch/arm/kernel/time.o
>>
>> This is just an object file that hasn't been linked, but your general
>> point stands -- LD is honored by other parts of the build.
>>
>>
>>> So it seems to be a vdso.so.raw specific problem not using the specified
>>> linker...?
>>
>> Yes, since it's produced by an invocation of $(CC) which is expected
>> to call the linker implicitly, and this is how the toolchain's default
>> linker ends up getting used even when you set LD on the command line.
>>
>> If I'm not mistaken, implicitly performing a link through the compiler
>> seems to be 1) conventional for all VDSOs, not just ARM's, but 2) unusual
>> for other parts of the kernel build.
>>
>> I suppose it's possible to make arch/arm/vdso/Makefile honor LD, but it
>> would basically entail a rewrite.  In the meantime, using -fuse-ld=bfd
>> may suffice:
>>
>> diff --git a/arch/arm/vdso/Makefile b/arch/arm/vdso/Makefile
>> index 8aa791051029..da0ce897edde 100644
>> --- a/arch/arm/vdso/Makefile
>> +++ b/arch/arm/vdso/Makefile
>> @@ -43,6 +43,7 @@ quiet_cmd_vdsold = VDSO    $@
>>        cmd_vdsold = $(CC) $(c_flags) -Wl,-T $(filter %.lds,$^)
>> $(filter %.o,$^) \
>>                     $(call cc-ldoption, -Wl$(comma)--build-id) \
>>                     -Wl,-Bsymbolic -Wl,-z,max-page-size=4096 \
>> +                   $(call cc-option, -fuse-ld=bfd) \
>>                     -Wl,-z,common-page-size=4096 -o $@
>>
>>  quiet_cmd_vdsomunge = MUNGE   $@
> 
> 
> Thanks Nathan, your solution looks reasonable. I figured that the option
> is available since GCC 4.8, so it should work probably for most
> toolchains which try to make use of gold linker as default.
> 
> Unfortunately, I hit another problem with the configuration on my build
> host. With your patch applied, I get:
> collect2: fatal error: cannot find 'ld'
> compilation terminated.
> make[2]: *** [arch/arm/vdso/vdso.so.raw] Error 1
> make[1]: *** [arch/arm/vdso] Error 2
> make[1]: *** Waiting for unfinished jobs....

Sorry for the noise, it turns out when using no paths in CROSS_COMPILE
and instead add the location of the toolchain to PATH properly, this
second issue is fixed.

--
Stefan