linux-kernel - Re: [PATCH v3 0/9] klp-convert livepatch build tooling

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Message-ID: <57510765-be85-4f91-493b-f7213a7580f3@redhat.com>
Date:   Tue, 16 Apr 2019 09:37:36 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        linux-kbuild@...r.kernel.org, Jessica Yu <jeyu@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Joao Moreira <jmoreira@...e.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michael Matz <matz@...e.de>, Miroslav Benes <mbenes@...e.cz>,
        Nicolai Stange <nstange@...e.de>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v3 0/9] klp-convert livepatch build tooling

On 4/16/19 1:24 AM, Balbir Singh wrote:
> 
> Could we get some details with examples or a sample, sorry I might be dense
> and missing simple information. The problem being solved is not clear to
> me from the changelog.
> 

Hi Balbir,

I see that Miroslav has already pointed to documentation, samples and 
self-tests for the patchset, but here is a summary in my own words:

kpatch-build constructs livepatch kernel modules by comparing a 
reference build with a patched build, combing through ELF object 
sections and extracting new and changed sections that include patched code.

An alternative approach is "source-based", in which a livepatch kernel 
module is built (mostly entirely) using the kernel's build 
infrastructure.  Sources for a livepatch are gathered ahead of time and 
then built like an ordinary kernel module.

In either approach, there lies the problem of symbol visibility: how can 
a livepatch resolve symbols that a kernel module ordinarily can't.  For 
example, file local or simply unexported symbols in across the kernel 
and other modules.  Enter the concept of "livepatch symbols" described 
in module-elf-format.txt.

kpatch-build already creates such "livepatch symbols" (see its 
create_klp_relasecs_and_syms()) and the livepatching core kernel code 
already knows how resolve such symbols at klp_object patch time (see 
klp_write_object_relocations()).

The klp-convert tool and this supporting patchset would empower 
source-based-constructed livepatch modules to do the same.

-- Joe