Re: [PATCH] rcu: remove unused variable in boot_cpu_state_init

[Ingo Molnar <mingo@kernel.org>]

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

> On Thu, Jun 22, 2017 at 9:59 AM, Ingo Molnar <mingo@kernel.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