linux-kernel - Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format

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Message-ID: <20161123122634.4z2dftmzpbexnhjs@phenom.ffwll.local>
Date:   Wed, 23 Nov 2016 13:26:34 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Liviu Dudau <Liviu.Dudau@....com>
Cc:     Jani Nikula <jani.nikula@...el.com>,
        Eric Engestrom <eric@...estrom.ch>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v2] drm: check for NULL parameter in exported
 drm_get_format_name() function.

On Wed, Nov 23, 2016 at 11:23:23AM +0000, Liviu Dudau wrote:
> On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
> > On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@....com> wrote:
> > > drm_get_format_name() de-references the buf parameter without checking
> > > if the pointer was not NULL. Given that the function is EXPORT-ed, lets
> > > sanitise the parameters before proceeding.
> > >
> > > v2: Use BUG_ON() to annoy users that did not pass valid parameters to function.
> > >
> > > Fixes: b3c11ac267d461d3d5 ("drm: move allocation out of drm_get_format_name())
> > > Cc: Eric Engestrom <eric@...estrom.ch>
> > > Cc: Rob Clark <robdclark@...il.com>
> > > Cc: Jani Nikula <jani.nikula@...el.com>
> > > Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> > >
> > > Signed-off-by: Liviu Dudau <Liviu.Dudau@....com>
> > > ---
> > > I still think sanity checking the parameters of an exported function is worth
> > > doing, even if the way one triggers the NULL pointer crash is priviledged. Not
> > > a big fan of the verbosity of BUG_ON() and would rather silently reject NULL buf
> > > pointer, but that is a matter of taste.
> > 
> > There really is no meaningful difference between doing BUG_ON(!bug)
> > vs. just letting buf->str oops. The kernel is full of functions that
> > expect sensible pointers, and I don't see why this one in particular
> > should be so special to warrant a BUG_ON().
> 
> Agree. That is why I prefer v1 where I return immediately on NULL pointers.

The question for v1 is why did you hit that? "broken driver code" isn't
really a good reason, au contraire it's a reason to not merge your patch:
We do not want to hide driver bugs silently.

There's definitely cases where handling NULL automatically is reasonable,
e.g. kfree(). But a NULL drm_format_name_buf sounds like, at least a quick
grep shows that all callers just put this struct onto the stack.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch