Re: [PATCH] drm: check for NULL parameter in exported drm_get_format_name() function.

[Sean Paul]

On Tue, Nov 22, 2016 at 1:06 PM, Ville SyrjÃlÃ
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> On Tue, Nov 22, 2016 at 12:35:53PM -0500, Rob Clark wrote:
>> On Tue, Nov 22, 2016 at 12:31 PM, Ville SyrjÃlÃ
>> <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
>> > On Tue, Nov 22, 2016 at 12:23:59PM -0500, Rob Clark wrote:
>> >> On Tue, Nov 22, 2016 at 11:50 AM, Ville SyrjÃlÃ
>> >> <ville.syrjala@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> >> >> Cc: Rob Clark <robdclark@xxxxxxxxx>
>> >> >> Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
>> >> >> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
>> >> >>
>> >> >> Signed-off-by: Liviu Dudau <Liviu.Dudau@xxxxxxx>
>> >> >> ---
>> >> >>  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.

+1

The patch has a Fixes line, but AFAICT, the referenced patch didn't
introduce any NULL opportunities that weren't previously there.

Sean


>
> 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 SyrjÃlÃ
> Intel OTC
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