Re: [PATCH v3 1/7] xen/gnttab: Rework resource acquisition

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.01.2021 17:26, Jan Beulich wrote:
> On 15.01.2021 17:03, Andrew Cooper wrote:
>> On 15/01/2021 11:43, Jan Beulich wrote:
>>>> +    mfn_t tmp;
>>>> +    void **vaddrs;
>>>> +    int rc;
>>>> +
>>>> +    /* Overflow checks */
>>>> +    if ( frame + nr_frames < frame )
>>>> +        return -EINVAL;
>>>> +
>>>> +    tot_frames = frame + nr_frames;
>>>> +    if ( tot_frames != frame + nr_frames )
>>>> +        return -EINVAL;
>>> Can't these two be folded into
>>>
>>>     unsigned int tot_frames = frame + nr_frames;
>>>
>>>     if ( tot_frames < frame )
>>>         return -EINVAL;
>>>
>>> ? Both truncation and wrapping look to be taken care of this
>>> way.
>>
>> Not when frame is a multiple of 4G (or fractionally over, I think).
> 
> How that? In this case any unsigned int value will be less than
> frame.

And in the 32-bit case the above becomes a simple overflow
check.

Jan