linux-kernel - Re: [PATCH] livepatch: add (un)patch hooks

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Message-ID: <alpine.LSU.2.20.1707192109390.15946@pobox.suse.cz>
Date:   Wed, 19 Jul 2017 21:11:06 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Chris J Arges <chris.j.arges@...onical.com>
Subject: Re: [PATCH] livepatch: add (un)patch hooks

On Tue, 18 Jul 2017, Josh Poimboeuf wrote:

> On Tue, Jul 18, 2017 at 01:15:16PM +0200, Miroslav Benes wrote:
> > On Thu, 13 Jul 2017, Josh Poimboeuf wrote:
> > 
> > > On Wed, Jul 12, 2017 at 10:10:00AM -0400, Joe Lawrence wrote:
> > > > When the livepatch core executes klp_(un)patch_object, call out to a
> > > > livepatch-module specified array of callback hooks.  These hooks provide
> > > > a notification mechanism for livepatch modules when klp_objects are
> > > > (un)patching. This may be most interesting when another kernel module
> > > > is a klp_object target and the livepatch module needs to execute code
> > > > after the target is loaded, but before its module_init code is run.
> > > 
> > > And it's also useful for vmlinux.  Patch module load/unload is separate
> > > from enable/disable, so the module init/exit functions can't be used for
> > > patch-specific changes (e.g., global data changes).
> > 
> > I admit that I don't understand this, which is probably the reason for my 
> > question. Why do we need it when we have module notifiers and module 
> > init/exit functions in the kernel? Petr described different possible 
> > scenarios and they can be solved either in init/exit function of a patch 
> > module or in a module notifier which the patch module can register.
> > 
> > If there is a difference, it should be mentioned in the documentation and 
> > in the changelog.
> 
> Some differences:
> 
> - The patch module init/exit code doesn't run when disabling and
>   re-enabling a patch.
> 
> - The module notifier can't stop the to-be-patched module from loading.

Ah, right. Thanks.

Joe, could you add both points to the changelog and the documentation, 
please?

Miroslav