linux-kernel - Re: [PATCH v4 07/12] ppc64/kexec_file: add support to relocate purgatory

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Date:   Wed, 22 Jul 2020 23:03:00 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel test robot <lkp@...el.com>, Pingfan Liu <piliu@...hat.com>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Dave Young <dyoung@...hat.com>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v4 07/12] ppc64/kexec_file: add support to relocate
 purgatory

On 22/07/20 9:55 am, Michael Ellerman 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
>> sha256 digest, relocations may have to be applied to the purgatory.
>> So, add support to relocate purgatory in kexec_file_load system call
>> by setting up TOC pointer and applying RELA relocations as needed.
>>
>> Reported-by: kernel test robot <lkp@...el.com>
>> [lkp: In v1, 'struct mem_sym' was declared in parameter list]
>> Signed-off-by: Hari Bathini <hbathini@...ux.ibm.com>
>> ---
>>
>> * Michael, can you share your opinion on the below:
>>     - https://lore.kernel.org/patchwork/patch/1272027/
>>     - My intention in cover note.
> 
> It seems like a lot of complexity for little benefit.
> 
> AFAICS your final purgatory_64.c is only 36 lines, and all it does is a
> single (open coded) memcpy().
> 
> It seems like we could write that in not many more lines of assembler
> and avoid all this code.

Hi Michael,

I am not sure if you would agree with me on this, but I am looking at the
purgatory code as work in progress. As mentioned in the cover note, I intend
to add log messaging, sha256 verification into purgatory. And also change it
to position independent executable after moving common purgatory code (sha256
verification) to arch-independent code.

When I initially took this up, I wanted to add all the above changes too, but
cut down on it, in the interest of time, first to get kdump (kexec -s -p)
working in v5.9 merge window.

But as the logic in patches 07/12 & 08/12 has been tested in kexec-tools code
a lot of times and there are unlikely to be any changes to them except for
__kexec_do_relocs() function (afaics), when -PIE would be used, I submitted them.
With patch 09/12, I tried for a change that uses relocations while is minimal
for now.

Would you prefer it to be absolutely minimal by dropping patches 7 & 8 for
now and writing the backup data copy code in assembler?

Thanks
Hari