linux-kernel - Re: [PATCH v13 02/12] livepatch: Helper macros to define livepatch structures

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Date:   Wed, 21 Nov 2018 13:17:51 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 02/12] livepatch: Helper macros to define livepatch
 structures

On Wed, 24 Oct 2018, Petr Mladek wrote:

> On Thu 2018-10-18 07:58:24, Josh Poimboeuf wrote:
> > On Thu, Oct 18, 2018 at 01:11:53PM +0200, Petr Mladek wrote:
> > > On Wed 2018-10-17 13:17:56, Josh Poimboeuf wrote:
> > > > On Mon, Oct 15, 2018 at 02:37:03PM +0200, Petr Mladek wrote:
> > > > > The definition of struct klp_func might be a bit confusing.
> > > > > The original function is defined by name as a string.
> > > > > The new function is defined by name as a function pointer
> > > > > casted to unsigned long.
> > > > > 
> > > > > This patch adds helper macros that hide the different types.
> > > > > The functions are defined just by the name. For example:
> > > > 
> > > > On one hand, these macros are kind of nice, because they do the function
> > > > pointer casting for the user.
> > > > 
> > > > On the other hand, they hide the field names, which hurts readability a
> > > > bit.  For example, it would be easy to accidentally assign the wrong
> > > > callback function.
> > > 
> > > I am not a big fan of the macros either.
> > > 
> > > 
> > > > Also, it's unfortunate that these macros are needed in the first place.
> > > > 
> > > > What if we just change new_addr (and old_addr) to be 'void *'?  Then the
> > > > macros wouldn't be as useful, and we could just get rid of them.
> > > 
> > > Hmm, I wonder if any change make sense then. The above proposal might
> > > just exchange one confusion with another one:
> > > 
> > >    + I would expect that a variable called addr is of the type
> > >      unsigned long
> > > 
> > >    + we would need casting when calling ftrace API
> > > 
> > >    + .new_addr = function_xxx looks a bit weird and it will
> > >      be used many times in all livepatch sources.
> > 
> > Fair points.  Instead of changing new_func to new_addr, how about we
> > leave it alone, and instead change 'unsigned long old_addr' to 'void
> > *old_func'?
> > 
> > That would give us consistent naming internally, while making the
> > external interface more sensible and cast-free.
> > 
> > We'd still have to cast when passing to the ftrace API, but IMO it would
> > be worth it.
> 
> I am fine with this. I will do this in v14 unless anyone complains ;-)

I agree. I don't like the change from 1/12 much as it exposes casting to 
user. Macros introduced here do not make any better and I go along with 
Josh.

Miroslav