[PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

[PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
means our clock*1000 will now overflow the 32bit unsigned
integer. Switch to 64bit maths to avoid it.

Cc: stable@vger.kernel.org
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
An interesting question how many other place might suffer from similar
overflows. I think i915 should be mostly OK. The one place I know we use
Hz instead kHz is the hsw DPLL code, which I would prefer we also change
to use kHz. The other concern is whether we have any potential overflows
before we check this against the platform's max dotclock.

I do have this unreviewed igt series 
https://patchwork.freedesktop.org/series/69531/ which extends the
current testing with some other forms of invalid modes. Could probably
extend that with a mode.clock=INT_MAX test to see if anything else might
trip up.

No idea about other drivers.

 drivers/gpu/drm/drm_modes.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
index 501b4fe55a3d..511cde5c7fa6 100644
--- a/drivers/gpu/drm/drm_modes.c
+++ b/drivers/gpu/drm/drm_modes.c
@@ -762,7 +762,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
 	if (mode->htotal == 0 || mode->vtotal == 0)
 		return 0;
 
-	num = mode->clock * 1000;
+	num = mode->clock;
 	den = mode->htotal * mode->vtotal;
 
 	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
@@ -772,7 +772,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
 	if (mode->vscan > 1)
 		den *= mode->vscan;
 
-	return DIV_ROUND_CLOSEST(num, den);
+	return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den);
 }
 EXPORT_SYMBOL(drm_mode_vrefresh);
 
-- 
2.26.2

_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel

Re: [PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

[Randy Dunlap]

On 10/22/20 12:42 PM, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
> means our clock*1000 will now overflow the 32bit unsigned
> integer. Switch to 64bit maths to avoid it.
> 
> Cc: stable@vger.kernel.org
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

This cures the problem that I reported. Thanks.

Tested-by: Randy Dunlap <rdunlap@infradead.org>

> ---
> An interesting question how many other place might suffer from similar
> overflows. I think i915 should be mostly OK. The one place I know we use
> Hz instead kHz is the hsw DPLL code, which I would prefer we also change
> to use kHz. The other concern is whether we have any potential overflows
> before we check this against the platform's max dotclock.
> 
> I do have this unreviewed igt series 
> https://patchwork.freedesktop.org/series/69531/ which extends the
> current testing with some other forms of invalid modes. Could probably
> extend that with a mode.clock=INT_MAX test to see if anything else might
> trip up.
> 
> No idea about other drivers.
> 
>  drivers/gpu/drm/drm_modes.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index 501b4fe55a3d..511cde5c7fa6 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -762,7 +762,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
>  	if (mode->htotal == 0 || mode->vtotal == 0)
>  		return 0;
>  
> -	num = mode->clock * 1000;
> +	num = mode->clock;
>  	den = mode->htotal * mode->vtotal;
>  
>  	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> @@ -772,7 +772,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
>  	if (mode->vscan > 1)
>  		den *= mode->vscan;
>  
> -	return DIV_ROUND_CLOSEST(num, den);
> +	return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den);
>  }
>  EXPORT_SYMBOL(drm_mode_vrefresh);
>  
> 


-- 
~Randy
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel

Re: [PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

[Chris Wilson]

Quoting Ville Syrjala (2020-10-22 20:42:56)
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
> means our clock*1000 will now overflow the 32bit unsigned
> integer. Switch to 64bit maths to avoid it.
> 
> Cc: stable@vger.kernel.org
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
> An interesting question how many other place might suffer from similar
> overflows. I think i915 should be mostly OK. The one place I know we use
> Hz instead kHz is the hsw DPLL code, which I would prefer we also change
> to use kHz. The other concern is whether we have any potential overflows
> before we check this against the platform's max dotclock.
> 
> I do have this unreviewed igt series 
> https://patchwork.freedesktop.org/series/69531/ which extends the
> current testing with some other forms of invalid modes. Could probably
> extend that with a mode.clock=INT_MAX test to see if anything else might
> trip up.
> 
> No idea about other drivers.
> 
>  drivers/gpu/drm/drm_modes.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index 501b4fe55a3d..511cde5c7fa6 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -762,7 +762,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
>         if (mode->htotal == 0 || mode->vtotal == 0)
>                 return 0;
>  
> -       num = mode->clock * 1000;
> +       num = mode->clock;
>         den = mode->htotal * mode->vtotal;

You don't want to promote den to u64 while you are here? We are at
8kx4k, throw in dblscan and some vscan, and we could soon have wacky
refresh rates.
-Chris
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel

Re: [PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

[Ville Syrjälä]

On Fri, Oct 30, 2020 at 02:19:45PM +0000, Chris Wilson wrote:
> Quoting Ville Syrjala (2020-10-22 20:42:56)
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
> > means our clock*1000 will now overflow the 32bit unsigned
> > integer. Switch to 64bit maths to avoid it.
> > 
> > Cc: stable@vger.kernel.org
> > Reported-by: Randy Dunlap <rdunlap@infradead.org>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> > An interesting question how many other place might suffer from similar
> > overflows. I think i915 should be mostly OK. The one place I know we use
> > Hz instead kHz is the hsw DPLL code, which I would prefer we also change
> > to use kHz. The other concern is whether we have any potential overflows
> > before we check this against the platform's max dotclock.
> > 
> > I do have this unreviewed igt series 
> > https://patchwork.freedesktop.org/series/69531/ which extends the
> > current testing with some other forms of invalid modes. Could probably
> > extend that with a mode.clock=INT_MAX test to see if anything else might
> > trip up.
> > 
> > No idea about other drivers.
> > 
> >  drivers/gpu/drm/drm_modes.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> > index 501b4fe55a3d..511cde5c7fa6 100644
> > --- a/drivers/gpu/drm/drm_modes.c
> > +++ b/drivers/gpu/drm/drm_modes.c
> > @@ -762,7 +762,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
> >         if (mode->htotal == 0 || mode->vtotal == 0)
> >                 return 0;
> >  
> > -       num = mode->clock * 1000;
> > +       num = mode->clock;
> >         den = mode->htotal * mode->vtotal;
> 
> You don't want to promote den to u64 while you are here? We are at
> 8kx4k, throw in dblscan and some vscan, and we could soon have wacky
> refresh rates.

i915 has 16kx8k hard limit currently, and we reject vscan>1
(wish we could also reject DBLSCAN). So we should not hit
that, at least not yet. Other drivers might not be so strict
I guess.

I have a nagging feeling that other places are in danger of
overflows if we try to push the current limits significantly.
But I guess no real harm in going full 64bit here, except
maybe making it a bit slower.

-- 
Ville Syrjälä
Intel
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel

Re: [PATCH] drm/modes: Switch to 64bit maths to avoid integer overflow

[Chris Wilson]

Quoting Ville Syrjälä (2020-10-30 14:43:46)
> On Fri, Oct 30, 2020 at 02:19:45PM +0000, Chris Wilson wrote:
> > Quoting Ville Syrjala (2020-10-22 20:42:56)
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > 
> > > The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
> > > means our clock*1000 will now overflow the 32bit unsigned
> > > integer. Switch to 64bit maths to avoid it.
> > > 
> > > Cc: stable@vger.kernel.org
> > > Reported-by: Randy Dunlap <rdunlap@infradead.org>
> > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > ---
> > > An interesting question how many other place might suffer from similar
> > > overflows. I think i915 should be mostly OK. The one place I know we use
> > > Hz instead kHz is the hsw DPLL code, which I would prefer we also change
> > > to use kHz. The other concern is whether we have any potential overflows
> > > before we check this against the platform's max dotclock.
> > > 
> > > I do have this unreviewed igt series 
> > > https://patchwork.freedesktop.org/series/69531/ which extends the
> > > current testing with some other forms of invalid modes. Could probably
> > > extend that with a mode.clock=INT_MAX test to see if anything else might
> > > trip up.
> > > 
> > > No idea about other drivers.
> > > 
> > >  drivers/gpu/drm/drm_modes.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> > > index 501b4fe55a3d..511cde5c7fa6 100644
> > > --- a/drivers/gpu/drm/drm_modes.c
> > > +++ b/drivers/gpu/drm/drm_modes.c
> > > @@ -762,7 +762,7 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode)
> > >         if (mode->htotal == 0 || mode->vtotal == 0)
> > >                 return 0;
> > >  
> > > -       num = mode->clock * 1000;
> > > +       num = mode->clock;
> > >         den = mode->htotal * mode->vtotal;
> > 
> > You don't want to promote den to u64 while you are here? We are at
> > 8kx4k, throw in dblscan and some vscan, and we could soon have wacky
> > refresh rates.
> 
> i915 has 16kx8k hard limit currently, and we reject vscan>1
> (wish we could also reject DBLSCAN). So we should not hit
> that, at least not yet. Other drivers might not be so strict
> I guess.
> 
> I have a nagging feeling that other places are in danger of
> overflows if we try to push the current limits significantly.
> But I guess no real harm in going full 64bit here, except
> maybe making it a bit slower.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel