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

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Date:	Fri, 6 Sep 2013 12:59:47 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	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>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: simplefb: avoid overflow

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?

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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