Re: [PATCH] x86: simplefb: avoid overflow

[Geert Uytterhoeven]

On Fri, Sep 6, 2013 at 1:08 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> 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...

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@xxxxxxxxxxxxxx

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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