Re: [PATCH 4/4] drm/i915/display: initialize string params to empty strings

[Gui-Dong Han]

On Wed, Mar 18, 2026 at 4:12 AM Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
>
> On Wed, 18 Mar 2026, Gui-Dong Han <hanguidong02@xxxxxxxxx> wrote:
> > Passing NULL to debugfs_create_str() causes a NULL pointer dereference
> > upon reading, and is no longer permitted. Change the default values of
> > dmc_firmware_path and vbt_firmware to empty strings ("").
> >
> > Existing code that consumes these parameters already verifies both
> > pointer validity and string length, so empty strings are handled
> > correctly. Furthermore, heap allocation is not required here: these
> > debugfs parameters are created with strictly read-only permissions
> > (0400). As a result, the debugfs write operation is never invoked,
> > meaning the static empty string will not be erroneously freed by
> > kfree().
> >
> > Fixes: e9913f0bd2e1 ("drm/i915/display: move dmc_firmware_path to display params")
> > Fixes: 29292bc6cc37 ("drm/i915/display: Move vbt_firmware module parameter under display")
> > Signed-off-by: Gui-Dong Han <hanguidong02@xxxxxxxxx>
> > ---
> >  drivers/gpu/drm/i915/display/intel_display_params.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_display_params.h b/drivers/gpu/drm/i915/display/intel_display_params.h
> > index b95ecf728daa..0a8cad98d480 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display_params.h
> > +++ b/drivers/gpu/drm/i915/display/intel_display_params.h
> > @@ -23,8 +23,8 @@ struct drm_printer;
> >   *       debugfs file
> >   */
> >  #define INTEL_DISPLAY_PARAMS_FOR_EACH(param) \
> > -     param(char *, dmc_firmware_path, NULL, 0400) \
> > -     param(char *, vbt_firmware, NULL, 0400) \
> > +     param(char *, dmc_firmware_path, "", 0400) \
> > +     param(char *, vbt_firmware, "", 0400) \
>
> Admittedly this is all very convoluted, but these NULL pointers (or
> pointers to them) are never passed to debugfs_create_str().

Hi Jani,

Thanks for your review.

Could you elaborate on why they are never passed? Looking at
intel_display_debugfs_params.c, the intel_display_debugfs_params()
function iterates over INTEL_DISPLAY_PARAMS_FOR_EACH using the
REGISTER macro. This eventually calls
_intel_display_param_create_file(), which uses _Generic to dispatch
char ** types to debugfs_create_str().

Thanks.