Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit

2021-01-12 Thread Geert Uytterhoeven
Hi Laurent,

On Tue, Jan 12, 2021 at 5:38 AM Laurent Pinchart
 wrote:
> On Mon, Jan 11, 2021 at 01:57:02PM +0100, Geert Uytterhoeven wrote:
> > As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 10L, the second
> > multiplication in
> >
> > dsi->lanes * 8 * NSEC_PER_SEC
> >
> > will overflow on a 32-bit platform.  Fix this by making the constant
> > unsigned long long, forcing 64-bit arithmetic.
> >
> > While iMX8 is arm64, this driver is currently used on 64-bit platforms
> > only, where long is 64-bit, so this cannot happen.  But the issue may
> > start to happen when the driver is reused for a 32-bit SoC, or when code
> > is copied for a new driver.
> >
> > Signed-off-by: Geert Uytterhoeven 

> > --- a/drivers/gpu/drm/bridge/nwl-dsi.c
> > +++ b/drivers/gpu/drm/bridge/nwl-dsi.c
> > @@ -195,7 +195,7 @@ static u32 ps2bc(struct nwl_dsi *dsi, unsigned long 
> > long ps)
> >   u32 bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
> >
> >   return DIV64_U64_ROUND_UP(ps * dsi->mode.clock * bpp,
> > -   dsi->lanes * 8 * NSEC_PER_SEC);
> > +   dsi->lanes * 8ULL * NSEC_PER_SEC);
>
> I wonder if we could get rid of a whole class of bugs by turning
> NSEC_PER_SEC into a ULL, but I suppose there are valid cases where a
> 32-bit integer is enough.

Indeed, and 64-bit arithmetic is more expensive on 32-bit platforms.
I considered that change, but doing so would require updates all over
the place (e.g. printing a value derived from NSEC_PER_SEC, divisions
 that need to be changed to do_div or div_u64(), ...)

Note that the selftests already use such a definition.

> Reviewed-by: Laurent Pinchart 

Thanks!

> How did you come across this by the way ?

https://lore.kernel.org/linux-renesas-soc/CAMuHMdXQvPY_mYicjPKjDSCwdO_rP-9PJOvqD0J6=s3opr1...@mail.gmail.com/
and of course I grepped for similar use patterns...

Gr{oetje,eeting}s,

Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit

2021-01-11 Thread Laurent Pinchart
Hi Geert,

Thank you for the patch.

On Mon, Jan 11, 2021 at 01:57:02PM +0100, Geert Uytterhoeven wrote:
> As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 10L, the second
> multiplication in
> 
> dsi->lanes * 8 * NSEC_PER_SEC
> 
> will overflow on a 32-bit platform.  Fix this by making the constant
> unsigned long long, forcing 64-bit arithmetic.
> 
> While iMX8 is arm64, this driver is currently used on 64-bit platforms
> only, where long is 64-bit, so this cannot happen.  But the issue may
> start to happen when the driver is reused for a 32-bit SoC, or when code
> is copied for a new driver.
> 
> Signed-off-by: Geert Uytterhoeven 
> ---
> Compile-tested only.
> ---
>  drivers/gpu/drm/bridge/nwl-dsi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c 
> b/drivers/gpu/drm/bridge/nwl-dsi.c
> index 66b67402f1acd57d..a8da3081efdcc84e 100644
> --- a/drivers/gpu/drm/bridge/nwl-dsi.c
> +++ b/drivers/gpu/drm/bridge/nwl-dsi.c
> @@ -195,7 +195,7 @@ static u32 ps2bc(struct nwl_dsi *dsi, unsigned long long 
> ps)
>   u32 bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
>  
>   return DIV64_U64_ROUND_UP(ps * dsi->mode.clock * bpp,
> -   dsi->lanes * 8 * NSEC_PER_SEC);
> +   dsi->lanes * 8ULL * NSEC_PER_SEC);

I wonder if we could get rid of a whole class of bugs by turning
NSEC_PER_SEC into a ULL, but I suppose there are valid cases where a
32-bit integer is enough.

Reviewed-by: Laurent Pinchart 

How did you come across this by the way ?

>  }
>  
>  /*

-- 
Regards,

Laurent Pinchart
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Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit

2021-01-11 Thread Fabio Estevam
Hi Geert,

On Mon, Jan 11, 2021 at 10:02 AM Geert Uytterhoeven
 wrote:
>
> As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 10L, the second
> multiplication in
>
> dsi->lanes * 8 * NSEC_PER_SEC
>
> will overflow on a 32-bit platform.  Fix this by making the constant
> unsigned long long, forcing 64-bit arithmetic.
>
> While iMX8 is arm64, this driver is currently used on 64-bit platforms
> only, where long is 64-bit, so this cannot happen.  But the issue may
> start to happen when the driver is reused for a 32-bit SoC, or when code
> is copied for a new driver.

This IP is also present on i.MX7ULP, which is 32-bit, but not supported yet.

Thanks for taking care of this.

Reviewed-by: Fabio Estevam 
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[PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit

2021-01-11 Thread Geert Uytterhoeven
As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 10L, the second
multiplication in

dsi->lanes * 8 * NSEC_PER_SEC

will overflow on a 32-bit platform.  Fix this by making the constant
unsigned long long, forcing 64-bit arithmetic.

While iMX8 is arm64, this driver is currently used on 64-bit platforms
only, where long is 64-bit, so this cannot happen.  But the issue may
start to happen when the driver is reused for a 32-bit SoC, or when code
is copied for a new driver.

Signed-off-by: Geert Uytterhoeven 
---
Compile-tested only.
---
 drivers/gpu/drm/bridge/nwl-dsi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
index 66b67402f1acd57d..a8da3081efdcc84e 100644
--- a/drivers/gpu/drm/bridge/nwl-dsi.c
+++ b/drivers/gpu/drm/bridge/nwl-dsi.c
@@ -195,7 +195,7 @@ static u32 ps2bc(struct nwl_dsi *dsi, unsigned long long ps)
u32 bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
 
return DIV64_U64_ROUND_UP(ps * dsi->mode.clock * bpp,
- dsi->lanes * 8 * NSEC_PER_SEC);
+ dsi->lanes * 8ULL * NSEC_PER_SEC);
 }
 
 /*
-- 
2.25.1

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