linux-kernel - Re: [PATCH v5 1/3] livepatch: add (un)patch callbacks

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Date:   Wed, 13 Sep 2017 09:29:41 +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 v5 1/3] livepatch: add (un)patch callbacks

On Tue, 12 Sep 2017, Josh Poimboeuf wrote:

> On Tue, Sep 12, 2017 at 06:05:44PM -0400, Joe Lawrence wrote:
> > On Tue, Sep 12, 2017 at 11:48:48AM -0400, Joe Lawrence wrote:
> > > On 09/12/2017 04:53 AM, Miroslav Benes wrote:
> > > >> @@ -871,13 +882,27 @@ int klp_module_coming(struct module *mod)
> > > >>  			pr_notice("applying patch '%s' to loading module '%s'\n",
> > > >>  				  patch->mod->name, obj->mod->name);
> > > >>  
> > > >> +			ret = klp_pre_patch_callback(obj);
> > > >> +			if (ret) {
> > > >> +				pr_warn("pre-patch callback failed for object '%s'\n",
> > > >> +					obj->name);
> > > >> +				goto err;
> > > >> +			}
> > > > 
> > > > There is a problem here. We cycle through all enabled patches (or 
> > > > klp_transition_patch) and call klp_pre_patch_callback() everytime an 
> > > > enabled patch contains a patch for a coming module. Now, it can easily 
> > > > happen that klp_pre_patch_callback() fails. And not the first one from the 
> > > > first relevant patch, but the next one. In that case we need to call 
> > > > klp_post_unpatch_callback() for all already processed relevant patches in 
> > > > the error path.
> > > 
> > > Good test case, if I understand you correctly:
> > > 
> > >  - Load target modules mod1 and mod2
> > >  - Load a livepatch that targets mod1 and mod2
> > >    - pre-patch succeeds for mod1
> > >    - pre-patch fails for mod2
> > > 
> > > and then we should:
> > > 
> > >  - NOT run post-patch or pre/post-unpatch handlers for mod2
> > >  - NOT run post-patch or pre-unpatch handlers for mod1
> > >  - do run post-unpatch handler for mod1
> > >  - Refuse to load the livepatch
> > > 
> > > Does that sound right?
> > 
> > Erm, probably not...
> > 
> > > > Unfortunately, we need to do the same for klp_patch_object() below, 
> > > > because there is the same problem and we missed it.
> > > > 
> > > >> +
> > > >>  			ret = klp_patch_object(obj);
> > > >>  			if (ret) {
> > > >>  				pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
> > > >>  					patch->mod->name, obj->mod->name, ret);
> > > >> +
> > > >> +				if (patch != klp_transition_patch)
> > > >> +					klp_post_unpatch_callback(obj);
> > > >> +
> > > >>  				goto err;
> > > > 
> > > > Here.
> > > > 
> > > > Could you do it as a part of the patch set (or send it separately), 
> > > > please?
> > 
> > I've re-read this a few times, and I think I might have been originally
> > off-base with what I thought you were concerned about.  But I think I
> > grok it now: the problem you pointed out arises because
> > klp_module_coming() iterates like so:
> > 
> >   for each klp_patch
> >     for each kobj in klp_patch
> > 
> > which means that we may have made pre-patch callbacks and patched a
> > given kobj for an earlier klp_patch that now fails for a later
> > klp_patch.

Yes, that's the scenario.
 
> > What should be the defined behavior in this case?  I would expect that
> > we need to unpatch all similar kobjs across klp_patches which have
> > already been successfully patched.  In turn, their post-unpatch
> > callbacks should be invoked.
> > 
> > If that's true, maybe this would make a better follow-on patch.

Yes, you'd need to loop back, unpatch everything and call post-unpatch 
callbacks too. Probably too much for this patch set, so we can deal with 
the problem later.

> The rabbit hole seems to be getting deeper, is it really worth it?  I'd
> rather we just make the pre-patch handler return void and be done with
> it, as Joe originally proposed.
> 
> So far, allowing the pre-patch handler to halt patching is a purely
> theoretical feature, nobody even knows if we need it yet, and whether
> it's worth the pain.  So I'd vote to just simplify this mess and let
> whoever wants the feature try to implement it :-)

Unfortunately, the problem is there even without Joe's callbacks. If it 
was only a problem of callbacks, I'd go along with you. I see two options.

1. we'll fix this for klp_patch_object(). Then callbacks' problem would be 
simple to solve, because the infrastructure would be already there.

2. we'll remove any error handling from klp_coming_module and we'll allow 
target modules to load even with a patching failure. This doesn't seem to 
be the right approach...

Miroslav