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

[Rob Clark]

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.

BR,
-R

>>
>> BR,
>> -R
>>
>> >>       snprintf(buf->str, sizeof(buf->str),
>> >>                "%c%c%c%c %s-endian (0x%08x)",
>> >>                printable_char(format & 0xff),
>> >> --
>> >> 2.10.2
>> >>
>> >> _______________________________________________
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>> >
>> > --
>> > Ville SyrjÃlÃ
>> > Intel OTC
>> > _______________________________________________
>> > dri-devel mailing list
>> > dri-devel@xxxxxxxxxxxxxxxxxxxxx
>> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Ville SyrjÃlÃ
> Intel OTC