| Message ID | 20210111125702.360745-1-geert+renesas@glider.be (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit | expand |
Hi Geert, On Mon, Jan 11, 2021 at 10:02 AM Geert Uytterhoeven <geert+renesas@glider.be> wrote: > > As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 1000000000L, 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 <festevam@gmail.com>
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 1000000000L, 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 <geert+renesas@glider.be> > --- > 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 <laurent.pinchart@ideasonboard.com> How did you come across this by the way ? > } > > /*
Hi Laurent, On Tue, Jan 12, 2021 at 5:38 AM Laurent Pinchart <laurent.pinchart@ideasonboard.com> 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 1000000000L, 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 <geert+renesas@glider.be> > > --- 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 <laurent.pinchart@ideasonboard.com> Thanks! > How did you come across this by the way ? https://lore.kernel.org/linux-renesas-soc/CAMuHMdXQvPY_mYicjPKjDSCwdO_rP-9PJOvqD0J6=S3Opr1ycg@mail.gmail.com/ and of course I grepped for similar use patterns... Gr{oetje,eeting}s, Geert
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); } /*
As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 1000000000L, 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 <geert+renesas@glider.be> --- Compile-tested only. --- drivers/gpu/drm/bridge/nwl-dsi.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)