linux-kernel - Re: [PATCH] x86: simplefb: avoid overflow

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Date:	Fri, 6 Sep 2013 13:15:58 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	David Herrmann <dh.herrmann@...il.com>,
	Tom Gundersen <teg@...m.no>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: simplefb: avoid overflow

On Fri, Sep 6, 2013 at 1:08 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 09/06/2013 03:59 AM, Geert Uytterhoeven wrote:
>> On Fri, Sep 6, 2013 at 11:55 AM, David Herrmann <dh.herrmann@...il.com>
>> wrote:
>>> On Fri, Sep 6, 2013 at 11:32 AM, Tom Gundersen <teg@...m.no> wrote:
>>>> lfb_size can easily be say 4M, which would make the bitshit overflow and
>>>> the test fail.
>>>>
>>>> Signed-off-by: Tom Gundersen <teg@...m.no>
>>>> Cc: David Herrmann <dh.herrmann@...il.com>
>>>> Cc: H. Peter Anvin <hpa@...or.com>
>>>> ---
>>>>   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...

Oops, you're right. So that overflows a little bit later.
Still, that doesn't change any reasoning w.r.t implicit casts and overflows.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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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