linux-kernel - Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

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Date:   Thu, 20 Jul 2017 16:45:24 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Joe Lawrence <joe.lawrence@...hat.com>
cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v2 1/2] livepatch: introduce shadow variable API


> > > > + *
> > > > + * Note: allocates @new_size space for shadow variable data and copies
> > > > + * @new_size bytes from @new_data into the shadow varaible's own @new_data
> > > > + * space.  If @new_data is NULL, @new_size is still allocated, but no
> > > > + * copy is performed.
> > > 
> > > I must say I'm not entirely happy with this. I don't know if this is what 
> > > Petr had in mind (I'm sure he'll get to the patch set soon). Calling 
> > > memcpy instead of a simple assignment in v1 seems worse. 
> > 
> > This change was a bit of a experiment on my part in reaction to
> > adding klp_shadow_get_or_attach().
> > 
> > I like the simplicity of v1's pointer assignment -- in fact, moving all
> > allocation responsiblity (klp_shadow meta-data and data[] area) out to
> > the caller is doable, though implementing klp_shadow_get_or_attach() and
> > and klp_shadow_detach_all() complicates matters, for example, adding an
> > alloc/release callback.  I originally attempted this for v2, but turned
> > back when the API and implementation grew complicated.  If the memcpy
> > and gfp_flag restrictions are too ugly, I can try revisting that
> > approach.  Ideas welcome :)
> 
> Well, I didn't like callbacks either :). And no, I do not have a better 
> idea. I still need to think about it.

Done and I agree that memcpy approach is not so bad after all :). So I'm 
fine with it.

Miroslav