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

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Date:   Tue, 18 Jul 2017 21:08:57 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
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, 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.

-- 
Josh