linux-kernel - Re: omapdss: Division by zero in kernel

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Message-ID: <20150925113429.GF27119@pali>
Date:	Fri, 25 Sep 2015 13:34:29 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Peter Teoh <htmldeveloper@...il.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Jyri Sarha <jsarha@...com>, omap <linux-omap@...r.kernel.org>,
	linux-fbdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Sebastian Reichel <sre@...nel.org>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	Tony Lindgren <tony@...mide.com>, Nishanth Menon <nm@...com>
Subject: Re: omapdss: Division by zero in kernel

On Thursday 17 September 2015 07:16:44 Pavel Machek wrote:
> 
> > >         if (image->depth == 1) {
> > >                 if (p->fix.visual == FB_VISUAL_TRUECOLOR ||
> > >                     p->fix.visual == FB_VISUAL_DIRECTCOLOR) {
> > >                         fgcolor = ((u32*)(p->pseudo_palette))[image->fg_color];
> > >                         bgcolor = ((u32*)(p->pseudo_palette))[image->bg_color];
> > >                 } else {
> > >                         fgcolor = image->fg_color;
> > >                         bgcolor = image->bg_color;
> > >                 }
> > > 
> > >                 if (32 % bpp == 0 && !start_index && !pitch_index &&
> > >                     ((width & (32/bpp-1)) == 0) &&
> > >                     bpp >= 8 && bpp <= 32)
> > >                         fast_imageblit(image, p, dst1, fgcolor, bgcolor);
> > >                 else
> > >                         slow_imageblit(image, p, dst1, fgcolor, bgcolor,
> > >                                         start_index, pitch_index);
> > >         } else
> > >                 color_imageblit(image, p, dst1, start_index, pitch_i
> > > 
> > > 
> > > Notice that bpp is not checked for zero, and thus bpp==0 is totally
> > > feasible?   resulting in 32/bpp crashing the kernel?
> > > 
> > 
> > Hm... this could really be a problem! But how to patch it? Which branch
> > should be called (fast_ or slow_ function) if bpp is zero?
> > 
> > And is there some way to force kernel to dump backtrace into dmesg when
> > division by zero occur?
> 
> You can do WARN_ON(bpp==1) ... and should probably return in that
> case.
> 
> 								Pavel

Does not make sense to call slow_ function? In that if condition check
also that bpp is nonzero...

-- 
Pali Rohár
pali.rohar@...il.com
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