Messages in this thread | | | Date | Thu, 7 Jul 2016 19:51:52 -0400 | From | Jessica Yu <> | Subject | Re: Fix issue with alternatives/paravirt patches |
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+++ Petr Mladek [07/07/16 17:56 +0200]: >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.${objname}" so that the module loader ignores them >> and livepatch can apply them manually. > >The solution looks correct to me. The fun will be how to generate >the sections. If I get this correctly, it is not enough to rename >the existing ones. Instead, we need to split .parainstructions >and .altinstructions sections into per-object ones.
Hi Petr,
That is correct, this means .parainstructions and .altinstructions will be split per-object and follow a similar naming scheme to the existing .klp.rela.${objname} sections. Then, we can apply these sections per-object with apply_alternatives() and apply_paravirt().
>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.
I currently have a working branch of the kpatch-build tools that generates these sections. I'm hoping to get the code up in some shape or form soon; I'll post an update when it's up.
Jessica
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