Re: [PATCH 0/2] Fix issue with alternatives/paravirt patches

[Petr Mladek]

On Tue 2016-07-05 22:34:58, Jessica Yu wrote:
> Hi,
> 
> A few months ago, Chris Arges reported a bug involving alternatives/paravirt
> patching that was discussed here [1] and here [2]. To briefly summarize the
> bug, patch modules that contained .altinstructions or .parainstructions
> sections would break because these alternative/paravirt patches would be
> applied first by the module loader (see x86 module_finalize()), then
> livepatch would later clobber these patches when applying per-object
> relocations. This lead to crashes and unpredictable behavior.
> 
> One conclusion we reached from our last discussion was that we will
> need to introduce some arch-specific code to address this problem.
> This patchset presents a possible fix for the bug by adding a new
> arch-specific arch_klp_init_object_loaded() function that by default
> does nothing but can be overridden by different arches.
> 
> To fix this issue for x86, since we can access a patch module's Elf
> sections through mod->klp_info, we can simply delay the calls to
> apply_paravirt() and apply_alternatives() to arch_klp_init_object_loaded(),
> which is called after relocations have been written for an object.
> In addition, for patch modules, .parainstructions and .altinstructions are
> prefixed by ".klp.arch.${objname}" so that the module loader ignores them
> and livepatch can apply them manually.

The solution looks correct to me. The fun will be how to generate
the sections. If I get this correctly, it is not enough to rename
the existing ones. Instead, we need to split .parainstructions
and .altinstructions sections into per-object ones.

I wonder if there is a plan for this. Especially I am interested
into the patches created from sources ;-) I wonder if we could add
a tag somewhere and improve the build infrastructure.

Best Regards,
Petr