Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit
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
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 ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit
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 ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
[PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit
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 ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
