linux-kernel - Re: [PATCH v3 07/12] ppc64/kexec_file: add support to relocate purgatory

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Date:   Fri, 17 Jul 2020 02:41:42 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Cc:     kernel test robot <lkp@...el.com>, Pingfan Liu <piliu@...hat.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 07/12] ppc64/kexec_file: add support to relocate
 purgatory



On 16/07/20 5:50 am, Thiago Jung Bauermann wrote:
> 
> Hari Bathini <hbathini@...ux.ibm.com> writes:
> 
>> Right now purgatory implementation is only minimal. But if purgatory
>> code is to be enhanced to copy memory to the backup region and verify
> 
> Can't the memcpy be done in asm? We have arch/powerpc/lib/memcpy_64.S
> for example, perhaps it could be linked in with the purgatory?

I wanted to avoid touching common code to make it work for purgatory
for now.

> 
>> sha256 digest, relocations may have to be applied to the purgatory.
> 
> Do we want to do the sha256 verification? My original patch series for
> kexec_file_load() had a purgatory in C from kexec-tools which did the
> sha256 verification but Michael Ellerman thought it was unnecessary and
> decided to use the simpler purgatory in asm from kexec-lite.

kexec_file_load could as well be used without IMA or secureboot. With sha256 digest
calculated anyway, verifying it would make sense to accommodate that case as well.

> 
>> So, add support to relocate purgatory in kexec_file_load system call
>> by setting up TOC pointer and applying RELA relocations as needed.
> 
> If we do want to use a C purgatory, Michael Ellerman had suggested
> building it as a Position Independent Executable, which greatly reduces
> the number and types of relocations that are needed. See patches 4 and 9
> here:
> 
> https://lore.kernel.org/linuxppc-dev/1478748449-3894-1-git-send-email-bauerman@linux.vnet.ibm.com/
> 
> In the series above I hadn't converted x86 to PIE. If I had done that,
> possibly Dave Young's opinion would have been different. :-)
> 
> If that's still not desirable, he suggested in that discussion lifting
> some code from x86 to generic code, which I implemented and would
> simplify this patch as well:
> 
> https://lore.kernel.org/linuxppc-dev/5009580.5GxAkTrMYA@morokweng/
> 

Agreed. But I prefer to work on PIE and/or moving common relocation_add code
for x86 & s390 to generic code later when I try to build on these purgatory
changes. So, a separate series later to rework purgatory with the things you
mentioned above sounds ok?

Thanks
Hari