Re: VDSO ELF header

[Michael Ellerman <mpe@ellerman.id.au>]

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.

cheers