Re: [PATCH] PM / clock_ops: Fix clock error check in __pm_clk_add()

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Wed, May 13, 2015 at 02:22:50AM +0200, Rafael J. Wysocki wrote:
> On Tuesday, May 12, 2015 11:07:33 AM Dmitry Torokhov wrote:
> > On Tue, May 12, 2015 at 08:59:03PM +0300, Grygorii.Strashko@linaro.org wrote:
> > > On 05/12/2015 07:42 PM, Dmitry Torokhov wrote:
> > > > On Tue, May 12, 2015 at 04:55:39PM +0300, Grygorii.Strashko@linaro.org wrote:
> > > >> On 05/09/2015 12:05 AM, Dmitry Torokhov wrote:
> > > >>> On Fri, May 08, 2015 at 10:59:04PM +0200, Geert Uytterhoeven wrote:
> > > >>>> On Fri, May 8, 2015 at 7:19 PM, Dmitry Torokhov
> > > >>>> <dmitry.torokhov@gmail.com> wrote:
> > > >>>>> On Fri, May 08, 2015 at 10:47:43AM +0200, Geert Uytterhoeven wrote:
> > > >>>>>> In the final iteration of commit 245bd6f6af8a62a2 ("PM / clock_ops: Add
> > > >>>>>> pm_clk_add_clk()"), a refcount increment was added by Grygorii Strashko.
> > > >>>>>> However, the accompanying IS_ERR() check operates on the wrong clock
> > > >>>>>> pointer, which is always zero at this point, i.e. not an error.
> > > >>>>>> This may lead to a NULL pointer dereference later, when __clk_get()
> > > >>>>>> tries to dereference an error pointer.
> > > >>>>>>
> > > >>>>>> Check the passed clock pointer instead to fix this.
> > > >>>>>
> > > >>>>> Frankly I would remove the check altogether. Why do we only check for
> > > >>>>> IS_ERR and not NULL or otherwise validate the pointer? The clk is passed
> > > >>>>
> > > >>>> __clk_get() does the NULL check.
> > > >>>
> > > >>> No, not really. It _handles_ clk being NULL and returns "everything is
> > > >>> fine". In any case it is __clk_get's decision what to do.
> > > >>>
> > > >>> I dislike gratuitous checks of arguments passed in. Instead of relying
> > > >>> on APIs refusing grabage we better not pass garbage to these APIs in the
> > > >>> first place. So I'd change it to trust that we are given a usable
> > > >>> pointer and simply do:
> > > >>>
> > > >>> 	if (!__clk_get(clk)) {
> > > >>> 		kfree(ce);
> > > >>> 		return -ENOENTl
> > > >>> 	}
> > > >>
> > > >> Not sure this is right thing to do, because this API initially
> > > >> was intended to be used as below [1]:
> > > >> 	clk = of_clk_get(dev->of_node, i));
> > > >>   	ret = pm_clk_add_clk(dev, clk);
> > > >>   	clk_put(clk);
> > > >>
> > > >> and of_clk_get may return ERR_PTR().
> > > > 
> > > > Jeez, that sequence was not meant to be taken literally, it does miss
> > > > error handling completely. If you notice the majority of users of this
> > > > API do something like below:
> > > > 
> > > > 	i = 0;
> > > > 	while ((clk = of_clk_get(dev->of_node, i++)) && !IS_ERR(clk)) {
> > > > 		dev_dbg(dev, "adding clock '%s' to list of PM clocks\n",
> > > > 			__clk_get_name(clk));
> > > > 		error = pm_clk_add_clk(dev, clk);
> > > > 		clk_put(clk);
> > > > 		if (error) {
> > > > 			dev_err(dev, "pm_clk_add_clk failed %d\n", error);
> > > > 			pm_clk_destroy(dev);
> > > > 			return error;
> > > > 		}
> > > > 	}
> > > > 
> > > > i.e. it already validates clk pointer before passing it on since it
> > > > needs to know when to stop iterating.
> > > 
> > > np. It's just my opinion - if you agree that code will just crash
> > > in case of passing invalid @clk argument (in worst case:)
> > > 
> > > int __clk_get(struct clk *clk)
> > > {
> > > 	struct clk_core *core = !clk ? NULL : clk->core;
> > > 						^^^ here
> > 
> > Yes, it will crash if you pass invalid pointer here, be it
> > ERR_PTR-encoded value, or, for example, 0x1, or maybe (void
> > *)random_32(). The latter will probably not crash right away, but cause
> > some random damage that will manifest later.
> 
> Oh well.  Shouldn't we actually do:
> 
> int __clk_get(struct clk *clk)
> {
>  	struct clk_core *core = IS_ERR_OR_NULL(clk) ? NULL : clk->core;
> 
> and remove the check from __pm_clk_add() at the same time?
> 
> Knowingly crashing on an error encoded as a pointer is kind of disgusting to me
> and the difference between that and a random invalid pointer is that poeple who
> pass error values encoded as pointers up the stack usually expect them to be
> handled cleanly.

I think the operative work here is "up". Returning ERR_PTR-encoded
pointer is fine, checking it fine as well, blindly passing it *down*
into a random API is not fine and we should not try to accommodate this.

Thanks.

-- 
Dmitry