linux-kernel - Re: [PATCH] rtc: armada38x: add __ro_after_init to armada38x_rtc

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Message-ID: <20170104110750.dtu54t74qkuuvkvq@piout.net>
Date:   Wed, 4 Jan 2017 12:07:50 +0100
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Kees Cook <keescook@...omium.org>, andrew@...n.ch,
        Jason Cooper <jason@...edaemon.net>,
        rtc-linux@...glegroups.com, a.zummo@...ertech.it,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        gregory.clement@...e-electrons.com,
        Bhumika Goyal <bhumirks@...il.com>,
        sebastian.hesselbarth@...il.com
Subject: Re: [PATCH] rtc: armada38x: add __ro_after_init to armada38x_rtc_ops

On 04/01/2017 at 11:57:00 +0100, Julia Lawall wrote :
> 
> 
> On Tue, 3 Jan 2017, Russell King - ARM Linux wrote:
> 
> > On Tue, Jan 03, 2017 at 09:31:18PM +0000, Russell King - ARM Linux wrote:
> > > On Tue, Jan 03, 2017 at 01:18:29PM -0800, Kees Cook wrote:
> > > > On Mon, Jan 2, 2017 at 6:06 AM, Russell King - ARM Linux
> > > > <linux@...linux.org.uk> wrote:
> > > > > On Mon, Dec 26, 2016 at 05:01:02PM +0530, Bhumika Goyal wrote:
> > > > >> The object armada38x_rtc_ops of type rtc_class_ops structure is not
> > > > >> modified after getting initialized by armada38x_rtc_probe. Apart from
> > > > >> getting referenced in init it is also passed as an argument to the function
> > > > >> devm_rtc_device_register but this argument is of type const struct
> > > > >> rtc_class_ops *. Therefore add __ro_after_init to its declaration.
> > > > >
> > > > > What I'd prefer here is for the structure to be duplicated, with one
> > > > > copy having the alarm methods and one which does not.  Both can then
> > > > > be made "const" (so placed into the read-only section at link time)
> > > > > and the probe function select between the two.
> > > > >
> > > > > I think that's a cleaner and better solution, even though it's
> > > > > slightly larger.
> > > > >
> > > > > I'm not a fan of __ro_after_init being used where other solutions are
> > > > > possible.
> > > >
> > > > Can the pointer that points to the struct rtc_class_ops be made ro_after_init?
> > >
> > > It's passed into the RTC core code, and probably stored in some dynamically
> > > allocated object, so probably no.  It's the same class of problem as every
> > > file_operations pointer in the kernel, or the thousand other operations
> > > structure pointers that a running kernel has.
> 
> I'm not sure to understand the question and the response.  A quick check
> with grep suggests that most rtc_class_ops pointers are already const.
> There seem to be just some instances in specific drivers that are not.
> 

The question was whether the point to the rtc_class_ops could be made
__ro_after_init. And Russell is right, it is pointed to by the ops
pointer in a struct rtc_device and that struct is dynamically allocated
in rtc_device_register().


-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com