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Date:   Mon, 22 Oct 2018 14:53:10 +0200 (CEST)
From:   Miroslav Benes <mbenes@suse.cz>
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
cc:     Jessica Yu <jeyu@kernel.org>, Torsten Duwe <duwe@lst.de>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        AKASHI Takahiro <takahiro.akashi@linaro.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        live-patching@vger.kernel.org
Subject: Re: [PATCH v3 3/4] arm64: implement live patching
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On Sat, 20 Oct 2018, Ard Biesheuvel wrote:

> On 19 October 2018 at 23:21, Miroslav Benes <mbenes@suse.cz> wrote:
> >
> >> >> 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.
> >
> 
> So I suppose this could get interesting in cases where modules are far
> away from the kernel (i.e., more than -/+ 128 MB). Fortunately, the
> modules themselves are always placed in a 128 MB window, but this
> window could be out of reach for branches into the kernel proper. If
> we find ourselves in the situation where we need to patch calls into
> the kernel proper to point into this module, this may get interesting,
> since the PLT entries *must* be allocated along with the module that
> contains the branch instruction, or the PLT entry itself may be out of
> range, defeating the purpose.

Hm... Torsten, didn't you have to solve something similar in powerpc case? 

> >> 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.