Re: Live patching for powerpc

[Miroslav Benes]

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.

> 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.

Miroslav

> 
> Third and unrelated comment: the klp_write_module_reloc stub isn't
> needed anymore :-)
> 
> Thanks,
> Jessica
>