linux-kernel - Re: refactoring timers to avoid init

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Message-ID: <CAGXu5jKku1ZNYSFuan9tug4yuyHKixUzUm3mvBmMoq5THKOBrw@mail.gmail.com>
Date:   Thu, 17 Aug 2017 07:39:26 -0700
From:   Kees Cook <keescook@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: refactoring timers to avoid init_timer*()

On Thu, Aug 17, 2017 at 7:30 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 16 Aug 2017, Kees Cook wrote:
>> In the process I noticed that we already have
>> scripts/coccinelle/api/setup_timer.cocci to detect existing cases of:
>>
>> init_timer(t);
>> t->function = func;
>> t->data = data;
>>
>> And replace it with: setup_timer(t, func, data);
>>
>> Another pattern was:
>>
>> t->expires = when;
>> add_timer(t);
>>
>> Which can be replaced with mod_timer(t, when);
>>
>> So, I've created scripts/coccinelle/api/mod_timer.cocci for the
>> latter, and done a few passes with manual review. The current result
>> doesn't fully eliminate init_timer() yet, but it gets much closer. I
>> just wanted to be sure that this whole clean-up would actually be
>> welcome before I try to nail down the last many cases.
>
> I think it's worth the trouble, but rather than having a gazillion of
> commits with the same changelog, we should do that based on a cocci script
> right before the next rc1 in one go and be done with it.
>
> That will cover most of the init_timer() cases and we can fixup the
> remaining few oddballs manually after that.

Okay, that sounds good. I'll work on improving the cocci scripts. For
example, I noticed that it doesn't notice having assignments _before_
the init_timer() call:

timer->function = func;
timer->data = data;
init_timer(timer);

Whee. :)

BTW, I may back off on doing ->expires = when; add_timer(timer) -->
mod_timer(timer, when) changes since that actually removes a sanity
check in add_timer().

My plans currently are collapsing all the open-coded init_timer*()s
into setup_*timer() and then adding something like aim_timer(timer,
func, data) for post-setup function/data changes.

-Kees

>
> I just noticed that we have the same pattern with hrtimer_init(). I had a
> stab on adding hrtimer_setup() and friends which takes a function argument
> and converted the bulk with coccinelle.
>
>     82 files changed, 186 insertions(+), 210 deletions(-)
>
> We can do that in the same sweep as the init_timer() one and then you can
> do the canary magic on hrtimers as well.
>
> Thanks,
>
>         tglx
>
>



-- 
Kees Cook
Pixel Security