linux-kernel - Re: [PATCH v3 3/4] arm64: implement live patching

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Date:   Fri, 19 Oct 2018 17:21:15 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc:     Jessica Yu <jeyu@...nel.org>, Torsten Duwe <duwe@....de>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v3 3/4] arm64: implement live patching


> >> Ad relocations. I checked that everything in struct mod_arch_specific
> >> stays after the module is load. Both core and init get SHF_ALLOC set
> >> (mod->arch.core.plt->sh_flags in module_frob_arch_sections(). It is
> >> important because apply_relocate_add() may use those sections
> >> through module_emit_plt_entry() call.
> >
> >
> > Yes, it looks like the needed .plt sections will remain in module
> > memory. However, I think I found a slight issue... :/
> >
> > In module_frob_arch_sections(), mod->arch.core.plt is set to an offset
> > within info->sechdrs:
> >
> >             if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
> >                     mod->arch.core.plt = sechdrs + i;
> >
> > sechdrs is from info->sechdrs, which is freed at the end of
> > load_module() via free_copy(). So although the relevant plt section(s)
> > are in module memory, mod->arch.core.plt will point to invalid memory
> > after info is freed. In other words, the section (.plt) *is* in memory
> > but the section header (struct elf64_shdr) containing section metadata
> > like sh_addr, sh_size etc., is not.
> >
> 
> Just for my understanding: this only matters for live patching, right?

Yes. Live patching can do deferred relocations. When a module is loaded 
and livepatched, the relevant symbols in the patching module are resolved. 
We call apply_relocate_add(), which is arch-specific, and we must be sure 
that everything used by apply_relocate_add() is preserved in the patching 
module after it is loaded.
 
> The original PLT support was implemented to support loading modules
> outside of the -/+ 128 MB range of an arm64 relative branch/jump
> instruction, and was later enhanced [for the same reason] to support
> emitting a trampoline for the ftrace entrypoint.

Regards,
Miroslav