Re: [PATCH v2] drm: check for NULL parameter in exported drm_get_format_name() function.

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Wed, Nov 23, 2016 at 01:00:07PM +0200, Jani Nikula wrote:
> On Wed, 23 Nov 2016, Liviu Dudau <Liviu.Dudau@arm.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@engestrom.ch>
> > Cc: Rob Clark <robdclark@gmail.com>
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >
> > Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.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.

Best regards,
Liviu

> 
> BR,
> Jani.
> 
> >
> >
> >  drivers/gpu/drm/drm_fourcc.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> > index 90d2cc8..6d80239 100644
> > --- a/drivers/gpu/drm/drm_fourcc.c
> > +++ b/drivers/gpu/drm/drm_fourcc.c
> > @@ -85,6 +85,8 @@ EXPORT_SYMBOL(drm_mode_legacy_fb_format);
> >   */
> >  const char *drm_get_format_name(uint32_t format, struct drm_format_name_buf *buf)
> >  {
> > +	BUG_ON(!buf);
> > +
> >  	snprintf(buf->str, sizeof(buf->str),
> >  		 "%c%c%c%c %s-endian (0x%08x)",
> >  		 printable_char(format & 0xff),
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center
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> dri-devel@lists.freedesktop.org
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