VDSO ELF header

[Laurent Dufour]

Le 25/03/2021 à 17:56, Laurent Dufour a écrit :
> Le 25/03/2021 à 17:46, Christophe Leroy a écrit :
>> Hi Laurent
>>
>> Le 25/03/2021 à 17:11, Laurent Dufour a écrit :
>>> Hi Christophe,
>>>
>>> Since v5.11 and the changes you made to the VDSO code, it no more exposing 
>>> the ELF header at the beginning of the VDSO mapping in user space.
>>>
>>> This is confusing CRIU which is checking for this ELF header cookie 
>>> (https://github.com/checkpoint-restore/criu/issues/1417).
>>
>> How does it do on other architectures ?
> 
> Good question, I'll double check the CRIU code.

On x86, there are 2 VDSO entries:
7ffff7fcb000-7ffff7fce000 r--p 00000000 00:00 0                          [vvar]
7ffff7fce000-7ffff7fcf000 r-xp 00000000 00:00 0                          [vdso]

And the VDSO is starting with the ELF header.

> 
>>
>>>
>>> I'm not an expert in loading and ELF part and reading the change you made, I 
>>> can't identify how this could work now as I'm expecting the loader to need 
>>> that ELF header to do the relocation.
>>
>> I think the loader is able to find it at the expected place.
> 
> Actually, it seems the loader relies on the AUX vector AT_SYSINFO_EHDR. I guess 
> CRIU should do the same.
> 
>>>
>>>  From my investigation it seems that the first bytes of the VDSO area are now 
>>> the vdso_arch_data.
>>>
>>> Is the ELF header put somewhere else?
>>> How could the loader process the VDSO without that ELF header?
>>>
>>
>> Like most other architectures, we now have the data section as first page and 
>> the text section follows. So you will likely find the elf header on the second 
>> page.

I'm wondering if the data section you're refering to is the vvar section I can 
see on x86.


>>
>> Done in this commit: 
>> https://github.com/linuxppc/linux/commit/511157ab641eb6bedd00d62673388e78a4f871cf
> 
> I'll double check on x86, but anyway, I think CRIU should rely on 
> AT_SYSINFO_EHDR and not assume that the ELF header is at the beginning of VDSO 
> mapping.
> 
> Thanks for your help.
> Laurent.
>