linux-kernel - Re: Live patching for powerpc

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Message-ID: <20160414192031.GB25297@packer-debian-8-amd64.digitalocean.com>
Date:	Thu, 14 Apr 2016 15:20:32 -0400
From:	Jessica Yu <jeyu@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Michael Ellerman <mpe@...erman.id.au>, linuxppc-dev@...abs.org,
	bsingharora@...il.com, duwe@....de, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, kamalesh@...ux.vnet.ibm.com, pmladek@...e.com,
	jikos@...nel.org, live-patching@...r.kernel.org
Subject: Re: Live patching for powerpc

+++ Miroslav Benes [14/04/16 15:28 +0200]:
>On Wed, 13 Apr 2016, Jessica Yu wrote:
>
>> +++ Miroslav Benes [13/04/16 15:01 +0200]:
>> > On Wed, 13 Apr 2016, Michael Ellerman wrote:
>> >
>> > > This series adds live patching support for powerpc (ppc64le only ATM).
>> > >
>> > > It's unchanged since the version I posted on March 24, with the exception
>> > > that
>> > > I've dropped the first patch, which was a testing-only patch.
>> > >
>> > > If there's no further comments I'll put this in a topic branch in the next
>> > > day
>> > > or two and Jiri & I will both merge that into next.
>> >
>> > Hi,
>> >
>> > I'll definitely give it a proper look today or tomorrow, but there is one
>> > thing that needs to be solved. The patch set from Jessica reworking
>> > relocations for live patching is now merged in our for-next branch. This
>> > means that we need to find out if there is something in struct
>> > mod_arch_specific for powerpc which needs to be preserved and do it.
>> >
>>
>> I took a look around the powerpc module.c code and it looks like the
>> mod_arch_specific stuff should be fine, since it is statically allocated
>> in the module struct (unlike the situation in s390, where
>> mod->arch.syminfo was vmalloc'd and we had to delay the free).
>> However I'm not familiar with the powerpc code so I need to dig around
>> a bit more to be 100% sure.
>
>I came to the same conclusion. There is only struct bug_entry *bug_table
>in mod_arch_specific but it looks unimportant wrt relocations.

Yeah, I think we are fine. As long as none of the values in
mod_arch_specific are "cleared," and I don't see that happening
anywhere.

>> A second concern I have is that apply_relocate_add() relies on
>> sections like .stubs and .toc (for 64-bit) and .init.plt and .plt
>> sections (for 32-bit). In order for apply_relocate_add() to work for
>> livepatch, we must make sure these sections aren't thrown away and are
>> not in init module memory since this memory will be freed at the end
>> of module load (see how INIT_OFFSET_MASK is used in kernel/module.c).
>> As long as these sections are placed in module core memory, we will be
>> OK. I need to think about this a bit more.
>
>I knew I shouldn't have opened arch/powerpc/kernel/module*.c.
>
>We could always hack sh_flags of those sections in
>module_arch_frob_sections() to make them stay.
>

I think we are fine here too. The onus would be on the patch build
tool (e.g., kpatch) to set the sh_flags to SHF_ALLOC, like we
already do to keep the klp relocation sections in memory :-)

For the 32-bit module code, I don't believe we would need to preserve
the .init.plt section for livepatch's call to apply_relocate_add(),
since relocations to init sections should've been applied during
module initialization, and we don't patch those types of functions.
Please correct me if my understanding is off.

Jessica