linux-kernel - Re: [PATCH v2 1/8] drm/mipi-dsi: Fix theoretical int overflow in mipi_dsi_dcs_write

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Message-ID: <CAD=FV=XLoz1S+iQG6qowEGUV_oSF7tcHuXhsw3_0sn30byC0ZA@mail.gmail.com>
Date: Mon, 29 Apr 2024 14:42:38 -0700
From: Doug Anderson <dianders@...omium.org>
To: Sam Ravnborg <sam@...nborg.org>
Cc: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, dri-devel@...ts.freedesktop.org, 
	Linus Walleij <linus.walleij@...aro.org>, lvzhaoxiong@...qin.corp-partner.google.com, 
	Jani Nikula <jani.nikula@...ux.intel.com>, Hsin-Yi Wang <hsinyi@...gle.com>, 
	Javier Martinez Canillas <javierm@...hat.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Joel Selvaraj <jo@...amily.in>, Cong Yang <yangcong5@...qin.corp-partner.google.com>, 
	Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/8] drm/mipi-dsi: Fix theoretical int overflow in mipi_dsi_dcs_write_seq()

Hi,

On Fri, Apr 26, 2024 at 11:22 PM Sam Ravnborg <sam@...nborg.org> wrote:
>
> On Sat, Apr 27, 2024 at 04:44:33AM +0300, Dmitry Baryshkov wrote:
> > On Sat, 27 Apr 2024 at 02:59, Douglas Anderson <dianders@...omium.org> wrote:
> > >
> > > The mipi_dsi_dcs_write_seq() macro makes a call to
> > > mipi_dsi_dcs_write_buffer() which returns a type ssize_t. The macro
> > > then stores it in an int and checks to see if it's negative. This
> > > could theoretically be a problem if "ssize_t" is larger than "int".
> > >
> > > To see the issue, imagine that "ssize_t" is 32-bits and "int" is
> > > 16-bits, you could see a problem if there was some code out there that
> > > looked like:
> > >
> > >   mipi_dsi_dcs_write_seq(dsi, cmd, <32767 bytes as arguments>);
> > >
> > > ...since we'd get back that 32768 bytes were transferred and 32768
> > > stored in a 16-bit int would look negative.
> > >
> > > Though there are no callsites where we'd actually hit this (even if
> > > "int" was only 16-bit), it's cleaner to make the types match so let's
> > > fix it.
> > >
> > > Fixes: 2a9e9daf7523 ("drm/mipi-dsi: Introduce mipi_dsi_dcs_write_seq macro")
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > > ---
> > >
> > > Changes in v2:
> > > - New
> > >
> > >  include/drm/drm_mipi_dsi.h | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
> > > index 82b1cc434ea3..b3576be22bfa 100644
> > > --- a/include/drm/drm_mipi_dsi.h
> > > +++ b/include/drm/drm_mipi_dsi.h
> > > @@ -337,12 +337,12 @@ int mipi_dsi_dcs_get_display_brightness_large(struct mipi_dsi_device *dsi,
> > >         do {                                                               \
> > >                 static const u8 d[] = { cmd, seq };                        \
> > >                 struct device *dev = &dsi->dev;                            \
> > > -               int ret;                                                   \
> > > +               ssize_t ret;                                               \
> > >                 ret = mipi_dsi_dcs_write_buffer(dsi, d, ARRAY_SIZE(d));    \
> > >                 if (ret < 0) {                                             \
> > >                         dev_err_ratelimited(                               \
> > >                                 dev, "sending command %#02x failed: %d\n", \
> > > -                               cmd, ret);                                 \
> > > +                               cmd, (int)ret);                            \
> >
> > Please consider using %zd instead
>
> Hi Douglas,
> please consider the above for all the pathces, there are more places
> where a cast can be dropped.

Sure, I'll change in the next version. I personally prefer the %d with
an "int" type because technically we're printing an error code and
errors are int-sized. ...but I don't feel strongly and I guess there's
a tiny chance some bug in the code could lead to a negative value
that's more useful as 64-bits than 32-bits. ;-)

I will note that I will still need a cast in some of the later patches
for "%*ph" since, I believe, the size passed for the "*" in a printf
format string is defined to be an int, not a size_t or ssize_t.

-Doug