linux-kernel - Re: [PATCH] rcu: remove unused variable in boot_cpu_state

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Message-ID: <20170622090241.3cwwfesg7c43lgbq@gmail.com>
Date:   Thu, 22 Jun 2017 11:02:41 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: remove unused variable in boot_cpu_state_init


* Arnd Bergmann <arnd@...db.de> wrote:

> On Thu, Jun 22, 2017 at 9:59 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > So, to continue this side thought about uninitialized_var(), it is dangerous
> > because the following buggy pattern does not generate a compiler warning:
> >
> >         long uninitialized_var(error);
> >
> >         ...
> >
> >         if (error)
> >                 return error;
> >
> >
> > ... and still there are over 290 uses of uninitialized_var() in the kernel - and
> > any of them could turn into a silent but real uninitialized variable bugs due to
> > subsequent changes.
> 
> Right, absolutely agreed on that. A related problem however is blindly
> initializing variables to NULL to get rid of uninitialized variable warnings,
> such as
> 
>       struct subsystem_specific *obj = NULL;
>       if (function_argument > 10)
>               goto err;
>      obj = create_obj();
> ...
> err:
>       clean_up(obj->member);
> 
> 
> I've seen a couple of variations of that problem, so simply outlawing
> uninitialized_var() will only solve a subset of these issues, and ideally
> we should also make sure that initializations at declaration time are
> used properly, and not just to shut up compiler warnings.

Well, a deterministic crash on a NULL dereference is still (much) better than a 
non-deterministic 'use random value from stack and corrupt memory or crash' bug 
pattern, right?

Also, static analysis tools ought to be pretty good about finding control flows 
where a NULL gets dereferenced.

Thanks,

	Ingo