Re: [PATCH] x86: simplefb: avoid overflow

[H. Peter Anvin]

On 09/06/2013 03:59 AM, Geert Uytterhoeven wrote:
> On Fri, Sep 6, 2013 at 11:55 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> > On Fri, Sep 6, 2013 at 11:32 AM, Tom Gundersen <teg@xxxxxxx> wrote:
> > > lfb_size can easily be say 4M, which would make the bitshit overflow and
> > > the test fail.
> > >
> > > Signed-off-by: Tom Gundersen <teg@xxxxxxx>
> > > Cc: David Herrmann <dh.herrmann@xxxxxxxxx>
> > > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > > ---
> > >   arch/x86/kernel/sysfb_simplefb.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kernel/sysfb_simplefb.c b/arch/x86/kernel/sysfb_simplefb.c
> > > index 22513e9..fff44a5 100644
> > > --- a/arch/x86/kernel/sysfb_simplefb.c
> > > +++ b/arch/x86/kernel/sysfb_simplefb.c
> > > @@ -72,7 +72,7 @@ __init int create_simplefb(const struct screen_info *si,
> > >           * the part that is occupied by the framebuffer */
> > >          len = mode->height * mode->stride;
> > >          len = PAGE_ALIGN(len);
> > > -       if (len > si->lfb_size << 16) {
> > > +       if (len > ((unsigned long) si->lfb_size) << 16) {
>
> On 32-bit, "unsigned long" is the same size as __u32, so this doesn't
> make any difference.
>
> > Nice catch. vesafb uses "lfb_size * 65535" which causes an implicit
> > cast. I thought <<16 looks nicer but that doesn't do any implicit
> > cast..
>
> "lfb_size * 65535" is the same. "lfb_size" is __u32, "65535" is int.
> So there's no implicit cast. Or am I missing something?

<< 16 is * 65536 not 65535...

	-hpa


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