Re: [Xen-devel] [PATCH v3 4/7] sysctl: Add sysctl interface for querying PCI topology

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/10/2015 05:26 AM, Robert Elz wrote:
>      Date:        Mon,  9 Feb 2015 15:04:32 -0500
>      From:        Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>      Message-ID:  <1423512275-6531-5-git-send-email-boris.ostrovsky@xxxxxxxxxx>
>
>
>    | +        num_devs = ti->num_devs - ti->first_dev;
>    | +
>    | +        if ( guest_handle_is_null(ti->devs) ||
>    | +             guest_handle_is_null(ti->nodes) ||
>    | +             (num_devs <= 0) )
>    | +        {
>    | +            ret = -EINVAL;
>    | +            break;
>    | +        }
>
> Does that need a check that ti->first_dev <= ti_num_devs ?
>
> (that is aside from the issue of subtracting one uint from another and hoping
> that you'll get a negative signed int if the one subtracted is bigger).
>
> The (num_devs <= 0) test (even if it works - which it should on most
> rational architectures) isn't good enough, as it is possible to subtract
> a very big uint from a very small unit and end up with an int that is
> positive (and potentially, very big, consider (32bit): 2 - 0x80000001).
>
> So, replace the (num_devs <= 0) test by (ti->first_dev <= ti->num_devs)  ??
>
> Or possibly include both tests, just in case ti->num_devs is very large and
> ti->first_dev is small (like 0) and when the large unsigned is converted
> to signed, it becomes negative - or is all of this just too impossible
> to worry about?

For the latter, I'll just keep num_devs as uint32. The only reason I 
declared it as a signed int was for (num_devs <= 0) test.
-boris

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