Re: VDSO ELF header

[Laurent Dufour <ldufour@linux.ibm.com>]

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.

> 
>>
>> 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.
> 
> 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.