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

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Date:   Tue, 22 Nov 2016 20:06:19 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Rob Clark <robdclark@...il.com>
Cc:     Liviu Dudau <Liviu.Dudau@....com>,
        Jani Nikula <jani.nikula@...el.com>,
        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] drm: check for NULL parameter in exported
 drm_get_format_name() function.

On Tue, Nov 22, 2016 at 12:35:53PM -0500, Rob Clark wrote:
> On Tue, Nov 22, 2016 at 12:31 PM, Ville Syrjälä
> <ville.syrjala@...ux.intel.com> wrote:
> > On Tue, Nov 22, 2016 at 12:23:59PM -0500, Rob Clark wrote:
> >> On Tue, Nov 22, 2016 at 11:50 AM, Ville Syrjälä
> >> <ville.syrjala@...ux.intel.com> wrote:
> >> > On Tue, Nov 22, 2016 at 04:41:06PM +0000, Liviu Dudau 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.
> >> >>
> >> >> 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>
> >> >> ---
> >> >>  drivers/gpu/drm/drm_fourcc.c | 3 +++
> >> >>  1 file changed, 3 insertions(+)
> >> >>
> >> >> diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> >> >> index 90d2cc8..0a3ff0b 100644
> >> >> --- a/drivers/gpu/drm/drm_fourcc.c
> >> >> +++ b/drivers/gpu/drm/drm_fourcc.c
> >> >> @@ -85,6 +85,9 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
> >> >>   */
> >> >>  const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
> >> >>  {
> >> >> +     if (!buf)
> >> >> +             return NULL;
> >> >> +
> >> >
> >> > Seems rather pointless to me. Why would you ever pass NULL to this guy?
> >>
> >> perhaps BUG_ON(!buf)...
> >
> > And how does that differ from just buf->foo?
> 
> it gets you a file and line # in the error splat.. not that
> drm_get_format_name() is such a big function that it would be
> difficult to decipher the null deref crash, but if we added anything
> it should be BUG_ON() to make it clear that passing null isn't a
> caller error.

Yeah, BUG_ON() at least documents the intent, so it's better than
the null check. But for something like this even BUG_ON() is
just wasted bytes IMO.

BUG_ON() can be useful for those weird bugs where somewhere deep
down you hit a null pointer and you can't figure out where the
bad pointer came from. So you might sprinkle a few BUG_ONs()
further up to catch it sooner. Esp. if you can't reproduce the
bug yourself and have to rely on user(s) to find it for you.

Even WARN_ON() w/ or w/o an early bailout might be a decent idea
sometimes since it might have a slightly higher chance of keeping
the kernel in working condition, but IMO just blindly throwing
it around everywhere is not a good approach.

-- 
Ville Syrjälä
Intel OTC