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

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Date:   Fri, 14 Jul 2017 09:23:29 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Chris J Arges <chris.j.arges@...onical.com>
Subject: Re: [PATCH] livepatch: add (un)patch hooks

On Thu, Jul 13, 2017 at 08:46:40PM -0500, Josh Poimboeuf wrote:
> Date: Thu, 13 Jul 2017 20:46:40 -0500
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> To: Joe Lawrence <joe.lawrence@...hat.com>
> Cc: live-patching@...r.kernel.org, linux-kernel@...r.kernel.org, Jessica Yu
>  <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>, Miroslav Benes
>  <mbenes@...e.cz>, Petr Mladek <pmladek@...e.com>, Chris J Arges
>  <chris.j.arges@...onical.com>
> Subject: Re: [PATCH] livepatch: add (un)patch hooks
> User-Agent: Mutt/1.6.0.1 (2016-04-01)
> 
> 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).
> 
> > 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...)

Hi Josh,

I hesitataed in calling them "enable/disable" hooks as I associated
those terms at the patch level -- a livepatch might be enabled, but
callbacks for a module may not occur until its actually loaded.  (I'm
fine with whatever is most intuitive to the livepatching collective :)

"Callbacks" vs. "hooks" is a good point though, as the latter has
negative connotations, especially when callers of this facility will be
mostly out of tree.

> 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.

"Pre-patch" and "post-unpatch" are a bit wordy, but a good description.
I already felt it was important enough to document the order of
operations in the doc file and commit msg, so I like this idea. 

> For the enable case, I think it would be a nice feature if we checked
> the return code and aborted the patching operation on error.  I think
> that should be easy enough.

Yeah, that should be easy.  To be specific, you're only talking about
the patching operation on the associated klp_object, not the entire
klp_patch right?

> For the unload case, it's too late to do anything, so I'd say a void
> return code would be better.  Otherwise it implies that we actually do
> something about it.  Maybe in that case we can leave it up to the user
> to decide whether to print an error or WARN() or whatever.

Good point.  I can change that in v2.

Thanks,

-- Joe