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

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Date:   Wed, 19 Jul 2017 21:01:55 +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


> > > +/**
> > > + * _klp_shadow_attach() - allocate and add a new shadow variable
> > > + * @obj:	pointer to original data
> > > + * @num:	numerical description of new data
> > > + * @new_data:	pointer to new data
> > > + * @new_size:	size of new data
> > > + * @gfp_flags:	GFP mask for allocation
> > > + * @lock:	take klp_shadow_lock during klp_shadow_hash operations
> > 
> > I am not sure about lock argument. Do we need it? Common practice is to 
> > have function foo() which takes a lock, and function __foo() which does 
> > not.
> > 
> > In klp_shadow_get_or_attach(), you use it as I'd expect. You take the 
> > spinlock, call this function and release the spinlock. Is it possible 
> > to do the same in klp_shadow_attach() and have __klp_shadow_attach() 
> > without lock argument?
> 
> Yes, this would be possible, though it would restrict
> klp_shadow_attach() from accepting gfp_flags that might allow for
> sleeping.  More on that below ...

Ok, that is a good remark. The problem is that it applies to 
klp_shadow_get_or_attach() too. There you acquire a spin_lock and call 
_klp_shadow_attach() with gfp_flags, which are then used for kzalloc.

I might misread the code. It is getting late here.

> > > + *
> > > + * 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.

Miroslav