C vdso

[Christophe Leroy]

Le 23/10/2020 à 15:24, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy at csgroup.eu> writes:
>> Le 24/09/2020 à 15:17, Christophe Leroy a écrit :
>>> Le 17/09/2020 à 14:33, Michael Ellerman a écrit :
>>>> Christophe Leroy <christophe.leroy at csgroup.eu> writes:
>>>>>
>>>>> What is the status with the generic C vdso merge ?
>>>>> In some mail, you mentionned having difficulties getting it working on
>>>>> ppc64, any progress ? What's the problem ? Can I help ?
>>>>
>>>> Yeah sorry I was hoping to get time to work on it but haven't been able
>>>> to.
>>>>
>>>> It's causing crashes on ppc64 ie. big endian.
> ...
>>>
>>> Can you tell what defconfig you are using ? I have been able to setup a full glibc PPC64 cross
>>> compilation chain and been able to test it under QEMU with success, using Nathan's vdsotest tool.
>>
>> What config are you using ?
> 
> ppc64_defconfig + guest.config
> 
> Or pseries_defconfig.
> 
> I'm using Ubuntu GCC 9.3.0 mostly, but it happens with other toolchains too.
> 
> At a minimum we're seeing relocations in the output, which is a problem:
> 
>    $ readelf -r build\~/arch/powerpc/kernel/vdso64/vdso64.so
>    
>    Relocation section '.rela.dyn' at offset 0x12a8 contains 8 entries:
>      Offset          Info           Type           Sym. Value    Sym. Name + Addend
>    000000001368  000000000016 R_PPC64_RELATIVE                     7c0
>    000000001370  000000000016 R_PPC64_RELATIVE                     9300
>    000000001380  000000000016 R_PPC64_RELATIVE                     970
>    000000001388  000000000016 R_PPC64_RELATIVE                     9300
>    000000001398  000000000016 R_PPC64_RELATIVE                     a90
>    0000000013a0  000000000016 R_PPC64_RELATIVE                     9300
>    0000000013b0  000000000016 R_PPC64_RELATIVE                     b20
>    0000000013b8  000000000016 R_PPC64_RELATIVE                     9300

Looks like it's due to the OPD and relation between the function() and .function()

By using DOTSYM() in the 'bl' call, that's directly the dot function which is called and the OPD is 
not used anymore, it can get dropped.

Now I get .rela.dyn full of 0, don't know if we should drop it explicitely.

Christophe