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

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Message-ID: <20161123133835.GY1005@e106497-lin.cambridge.arm.com>
Date:   Wed, 23 Nov 2016 13:38:36 +0000
From:   Liviu Dudau <Liviu.Dudau@....com>
To:     Jani Nikula <jani.nikula@...el.com>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Eric Engestrom <eric@...estrom.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 02:47:53PM +0200, Jani Nikula wrote:
> On Wed, 23 Nov 2016, Daniel Vetter <daniel@...ll.ch> wrote:
> > 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.

I was updating a stashed series and discovered that signature of the function has changed.
When I looked at how it changed and I got past the "you pass as a parameter a pointer 
to a struct that is used as a buffer and then that buffer is returned by function" weirdness,
I thought that at least checking for bad parameters should be done.

> 
> Moreover, v1 puts the burden back on the *caller* of the function to
> check for NULL return, while it previously could not even return NULL.
> 
> The function is fine. It isn't broken. Don't try to fix it.

OK. I just like defensive programming, that's all. :)

Best regards,
Liviu

> 
> BR,
> Jani.
> 
> 
> 
> >
> > 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
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center

-- 
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