linux-kernel - Re: Live patching for powerpc

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Date:	Fri, 15 Apr 2016 10:28:58 +0200 (CEST)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Jessica Yu <jeyu@...hat.com>
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

On Thu, 14 Apr 2016, Jessica Yu wrote:

> +++ Miroslav Benes [14/04/16 15:28 +0200]:
> > On Wed, 13 Apr 2016, Jessica Yu wrote:
> 
> > > 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 :-)

Yes, this is probably the best way.

> 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.

I think you are right, but I also think we don't have to worry about 
32-bit powerpc anyway. This patch set supports ppc64le only so we can 
leave it for now.

Miroslav