linux-kernel - Re: PATCH v6 0/6] livepatch: Atomic replace feature

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Date:   Thu, 1 Feb 2018 16:51:28 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        Petr Mladek <pmladek@...e.com>, jikos@...nel.org,
        Jason Baron <jbaron@...mai.com>, jeyu@...nel.org,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: PATCH v6 0/6] livepatch: Atomic replace feature

On Thu, 1 Feb 2018, Josh Poimboeuf wrote:

> On Thu, Feb 01, 2018 at 04:08:14PM +0100, Miroslav Benes wrote:
> > On Thu, 1 Feb 2018, Joe Lawrence wrote:
> > 
> > > On 02/01/2018 08:49 AM, Miroslav Benes wrote:
> > > > 
> > > > Well, one more thing. I think there is a problem with shadow variables. 
> > > > Similar to callbacks situation. Shadow variables cannot be destroyed the 
> > > > way it is shown in our samples. Cumulative patches want to preserve 
> > > > everything as much as possible. If I'm right, it should be mentioned in 
> > > > the documentation.
> > > 
> > > Are you talking about using klp_shadow_free_all() call in a module_exit
> > > routine?  Yeah, I think in this case, that responsibility would be
> > > passed to the newly loaded cumulative patch, right?
> > 
> > Yes, but we haven't got an option not to call it here (as with callbacks, 
> > where we can omit callbacks completely with atomic replace patches). A 
> > live patch author must be aware of this and use shadow variables 
> > appropriately.
> 
> So maybe we should recommend that shadow variables generally be freed
> from a post-unpatch callback.

Yes, that's a possibility. In other words, if there is a need to call 
klp_shadow_free_all() somewhere, it should be in a post-unpatch callback.

Miroslav