linux-kernel - Re: [PATCH 2/2] livepatch: Allow to unregister or free shadow data using a custom function

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Date:   Wed, 21 Mar 2018 14:05:14 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Nicolai Stange <nstange@...e.de>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] livepatch: Allow to unregister or free shadow data
 using a custom function

On Wed, 14 Mar 2018, Josh Poimboeuf wrote:

> On Tue, Mar 13, 2018 at 04:54:48PM +0100, Petr Mladek wrote:
> > We might need to do some actions before the shadow variable is freed.
> > For example, we might need to remove it from a list or free some data
> > that it points to.
> > 
> > This is already possible now. The user can get the shadow variable
> > by klp_shadow_get(), do the necessary actions, and then call
> > klp_shadow_free().
> > 
> > This patch allow to do this a more elegant way. The user could implement
> > the needed actions in a callback that is passed to klp_shadow_free()
> > as a parameter. The callback usually does reverse operations to
> > the init_func that can be called by klp_shadow_*alloc().
> > 
> > It is especially useful for klp_shadow_free_all(). There we need to do
> > these extra actions for each found shadow variable with the given ID.
> > 
> > Note that the memory used by the shadow variable itself is still released
> > later by rcu callback. It is needed to protect internal structures that
> > keep all shadow variables. But free_func is called immediately. The shadow
> > variable must not be access anyway after klp_shadow_free() is called.
> > The user is responsible to protect this any suitable way.
> > 
> > Be aware that free_func callback is called under klp_shadow_lock. It is
> > the same as for init_func in klp_shadow_alloc().
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> 
> Makes sense, though I'm not sure "free" is the right name:
> 
>   a) "free" isn't the opposite of "init"; and
> 
>   b) it may be used for things other than freeing.
>   
> Shall we call them constructor/destructor?

Agreed. _ctor/_dtor both sound better.

Miroslav