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Subject: Re: VDSO ELF header
To: Laurent Dufour <ldufour@linux.ibm.com>,
 "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
References: <c45ae4f8-1cbc-c687-b6a2-9a431fafc85c@linux.ibm.com>
From: Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <9366c258-127f-f105-abd1-6baa9a6745c5@csgroup.eu>
Date: Thu, 25 Mar 2021 17:46:30 +0100
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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 ?

> 
> 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.

> 
>  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


Christophe