linux-kernel - Re: [PATCH drm-next] drm/msm/dp: Fix potential division by zero issue

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Message-ID: <Zz3N7IvdN4L8N62p@HOME-PC>
Date: Wed, 20 Nov 2024 17:24:20 +0530
From: Dheeraj Reddy Jonnalagadda <dheeraj.linuxdev@...il.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: robdclark@...il.com, quic_abhinavk@...cinc.com,
	linux-arm-msm@...r.kernel.org, sean@...rly.run,
	marijn.suijten@...ainline.org, airlied@...il.com, simona@...ll.ch,
	quic_bjorande@...cinc.com, quic_parellan@...cinc.com,
	quic_khsieh@...cinc.com, dri-devel@...ts.freedesktop.org,
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dianders@...omium.org
Subject: Re: [PATCH drm-next] drm/msm/dp: Fix potential division by zero issue

On Wed, Nov 20, 2024 at 01:02:32PM +0200, Dmitry Baryshkov wrote:
> On Wed, Nov 20, 2024 at 10:34:51AM +0530, Dheeraj Reddy Jonnalagadda wrote:
> > The variable pixel_div can remain zero due to an invalid rate input,
> 
> No, it can not. Rate is set by the driver, which knowns which rates are
> supported. 
> 
> > leading to a potential division by zero issue. This patch fixes it and
> > the function now logs an error and returns early.
> 
> See Documentation/process/submitting-patches.rst, look for "This patch"
> string.
> 
> > 
> > Additionally, this patch resolves trailing whitespace issues detected
> > by checkpatch.pl in the same file.
> 
> Additionally perform ... => separate commits.
> 
> > 
> > Signed-off-by: Dheeraj Reddy Jonnalagadda <dheeraj.linuxdev@...il.com>
> > ---
> >  drivers/gpu/drm/msm/dp/dp_catalog.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/msm/dp/dp_catalog.c b/drivers/gpu/drm/msm/dp/dp_catalog.c
> > index b4c8856fb25d..e170f70f1d42 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_catalog.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_catalog.c
> > @@ -225,7 +225,7 @@ int msm_dp_catalog_aux_clear_hw_interrupts(struct msm_dp_catalog *msm_dp_catalog
> >   * This function reset AUX controller
> >   *
> >   * NOTE: reset AUX controller will also clear any pending HPD related interrupts
> > - * 
> > + *
> >   */
> >  void msm_dp_catalog_aux_reset(struct msm_dp_catalog *msm_dp_catalog)
> >  {
> > @@ -487,8 +487,10 @@ void msm_dp_catalog_ctrl_config_msa(struct msm_dp_catalog *msm_dp_catalog,
> >  		pixel_div = 2;
> >  	else if (rate == link_rate_hbr2)
> >  		pixel_div = 4;
> > -	else
> > +	else {
> >  		DRM_ERROR("Invalid pixel mux divider\n");
> > +		return;
> > +	}
> >  
> >  	dispcc_input_rate = (rate * 10) / pixel_div;
> >  
> > @@ -579,7 +581,7 @@ u32 msm_dp_catalog_hw_revision(const struct msm_dp_catalog *msm_dp_catalog)
> >   * This function reset the DP controller
> >   *
> >   * NOTE: reset DP controller will also clear any pending HPD related interrupts
> > - * 
> > + *
> >   */
> >  void msm_dp_catalog_ctrl_reset(struct msm_dp_catalog *msm_dp_catalog)
> >  {
> > -- 
> > 2.34.1
> > 
> 
> -- 
> With best wishes
> Dmitry

Hello Dmitry,

Thank you for the valuable feedback. Will update my commit messages
accordingly. I wanted to seek clarification on the the divide by zero
issue. Would pixel_dev not be zero upon hitting the else case as
indicated below casuing a div by zero?

	u32 mvid, nvid, pixel_div = 0, dispcc_input_rate;
	u32 const nvid_fixed = DP_LINK_CONSTANT_N_VALUE;
	
	[..]
	
	if (rate == link_rate_hbr3)
		pixel_div = 6;
	else if (rate == 162000 || rate == 270000)
		pixel_div = 2;
	else if (rate == link_rate_hbr2)
		pixel_div = 4;
	else
		DRM_ERROR("Invalid pixel mux divider\n"); <<-- here

	dispcc_input_rate = (rate * 10) / pixel_div;

-Dheeraj