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

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Message-ID: <alpine.LSU.2.20.1707181310550.24826@pobox.suse.cz>
Date:   Tue, 18 Jul 2017 13:15:16 +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 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.
 
> > The patch-hook executes right before patching objects and the
> > unpatch-hook executes right after unpatching objects.
> > 
> > Signed-off-by: Joe Lawrence <joe.lawrence@...hat.com>
> 
> Thanks for posting it.  We found this to be a useful feature in the
> past, not quite as useful as shadow data, but still good to have for
> certain cases.
> 
> Instead of "load hooks" I think it would be more accurate to call them
> "enable/disable hooks".  (Maybe "callbacks" would be better than
> "hooks"?  Not sure...)
> 
> Even better, we might want to be specific: "pre enable hooks" and "post
> disable hooks".  (Or "pre patch hooks" and "post unpatch hooks"?)
> Because we might eventually decide we need the corresponding "post
> enable hooks" and "pre disable hooks" as well.

And this is what I'm worried about. I think we don't want to have hooks 
sprinkled here and there in the code.

Thanks,
Miroslav