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

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Message-ID: <20170104130628.GQ14217@n2100.armlinux.org.uk>
Date:   Wed, 4 Jan 2017 13:06:28 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        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 Wed, Jan 04, 2017 at 01:23:41PM +0100, Julia Lawall wrote:
> Basically, the strategy of the patch is that one may consider it
> preferable to duplicate the structure for the different alternatives,
> rather than use __ro_after_init.  Perhaps if the structure were larger,
> then __ro_after_init would be a better choice?

It depends on not just the size, but how many members need to be
modified, and obviously whether there are likely to be more than one
user of the structure as well.

So I'd say __ro_after_init rarely makes sense for an operations
structure - the only case I can see is:

- a large structure
- only a small number of elements need to be modified
- it is only single-use

which is probably quite rare - this one falls into two out of those
three.

There's another consideration (imho) too - we may wish, at a later
date, to make .text and .rodata both read-only from the start of the
kernel to harden the kernel against possibly init-time exploitation.
(Think about a buggy built-in driver with emulated hardware - much the
same problem that Kees is trying to address in one of his recent patch
sets but with hotplugged hardware while a screen-saver is active.)
Having function pointers in .rodata rather than the ro-after-init
section would provide better protection.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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