Re: VDSO ELF header

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 26/03/2021 à 11:46, Michael Ellerman a écrit :
> Laurent Dufour <ldufour@linux.ibm.com> writes:
>> Le 25/03/2021 à 17:56, Laurent Dufour a écrit :
>>> Le 25/03/2021 à 17:46, Christophe Leroy a écrit :
>>>> Le 25/03/2021 à 17:11, Laurent Dufour a écrit :
>>>>> 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.
> 
> Many of the other architectures have separate vm_special_mapping's for
> the data page and the vdso binary, where the former is called "vvar".
> 
> eg, s390:
> 
> static struct vm_special_mapping vvar_mapping = {
> 	.name = "[vvar]",
> 	.fault = vvar_fault,
> };
> 
> static struct vm_special_mapping vdso_mapping = {
> 	.name = "[vdso]",
> 	.mremap = vdso_mremap,
> };
> 
> 
> I guess we probably should be doing that too.
> 

Dmitry proposed the same, see 
https://github.com/0x7f454c46/linux/commit/783c7a2532d2219edbcf555cc540eab05f698d2a

Discussion at https://github.com/checkpoint-restore/criu/issues/1417

Christophe