linux-kernel - Re: [PATCH v12 06/12] livepatch: Simplify API by removing registration step

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Date:   Fri, 19 Oct 2018 14:16:19 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 06/12] livepatch: Simplify API by removing registration
 step

On Thu, 18 Oct 2018, Josh Poimboeuf wrote:

> On Thu, Oct 18, 2018 at 04:54:56PM +0200, Petr Mladek wrote:
> > On Mon 2018-10-15 18:01:43, Miroslav Benes wrote:
> > > On Fri, 12 Oct 2018, Petr Mladek wrote:
> > > 
> > > > On Wed 2018-09-05 11:34:06, Miroslav Benes wrote:
> > > > > On Tue, 28 Aug 2018, Petr Mladek wrote:
> > > > > > Also the API and logic is much easier. It is enough to call
> > > > > > klp_enable_patch() in module_init() call. The patch patch can be disabled
> > > > > > by writing '0' into /sys/kernel/livepatch/<patch>/enabled. Then the module
> > > > > > can be removed once the transition finishes and sysfs interface is freed.
> > > > > 
> > > > > I think it would be good to discuss our sysfs interface here as well.
> > > > > 
> > > > > Writing '1' to enabled attribute now makes sense only when you need to 
> > > > > reverse an unpatching transition. Writing '0' means "disable" or a 
> > > > > reversion again.
> > > > > 
> > > > > Wouldn't be better to split it to two different attributes? Something like 
> > > > > "disable" and "reverse"? It could be more intuitive.
> > > > > 
> > > > > Maybe we'd also find out that even patch->enabled member is not useful 
> > > > > anymore in such case.
> > > > 
> > > > I though about this as well. I kept "enabled" because:
> > > > 
> > > >   + It keeps the public interface the same as before. Most people
> > > >     would not notice any change in the behavior except maybe that
> > > >     the interface disappears when the patch gets disabled.
> > > 
> > > Well our sysfs interface is still in a testing phase as far as ABI is 
> > > involved. Moreover, each live patch is bound to its base kernel by 
> > > definition anyway. So we can change this without remorse, I think.
> 
> But it would break tooling, which is not kernel specific.  I'm not sure
> whether it would be worth the headache.  After all I think the livepatch
> sysfs interface is designed for tools, not humans.

You're right. It's probably not worth it. Oh well.
 
> > > >   + The reverse operation makes most sense when the transition
> > > >     cannot get finished. In theory, it might be problem to
> > > >     finish even the reversed one. People might want to
> > > >     reverse once again and force it. Then "reverse" file
> > > >     might be confusing. They might not know in which direction
> > > >     they do the reverse.
> > > 
> > > I still think it would be better to have a less confusing interface and it 
> > > would outweigh the second remark.
> > 
> > OK, what about having just "disable" in sysfs. I agree that it makes
> > much more sense than "enable" now.
> > 
> > It might be used also for the reverse operation the same way as
> > "enable" was used before. I think that standalone "reverse" might
> > be confusing when we allow to reverse the operation in both
> > directions.
> 
> As long as we're talking about radical changes... how about we just
> don't allow disabling patches at all?  Instead a patch can be replaced
> with a 'revert' patch, or an empty 'nop' patch.  That would make our
> code simpler and also ensure there's an audit trail.
> 
> (Apologies if we've already talked about this.  My brain is still mushy
> thanks to Spectre and friends.)

I think we talked about it last year in Prague and I think we convinced 
you that it was not a good idea (...not to allow disabling patches at 
all).

BUT! Empty 'nop' patch is a new idea and we may certainly discuss it.

> The amount of flexibility we allow is kind of crazy, considering how
> delicate of an operation live patching is.  That reminds me that I
> should bring up my other favorite idea at LPC: require modules to be
> loaded before we "patch" them.

We talked about this as well and if I remember correctly we came to a 
conclusion that it is all about a distribution and maintenance. We cannot 
ask customers to load modules they do not need just because we need to 
patch them. One cumulative patch is not that great in this case. I 
remember you had a crazy idea how to solve it, but I don't remember 
details. My notes from the event say...

	- livepatch code complexity
		- make it synchronous with respect to modules loading
		- Josh's crazy idea

That's not much :D

So yes, we can talk about it and hopefully make proper notes this time.

Miroslav