On Mon, 22 Nov 2021, Jan Beulich wrote:
> On 19.11.2021 21:29, Stefano Stabellini wrote:
> > --- a/drivers/xen/xenbus/xenbus_probe.c
> > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > @@ -951,6 +951,20 @@ static int __init xenbus_init(void)
> > err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
> > if (err)
> > goto out_error;
> > + /* Uninitialized. */
> > + if (v == 0 || v == ULLONG_MAX) {
>
> Didn't you have a comment in v1 here regarding the check against 0? Or was that
> just like now only in the description? IOW I think there ought to be a code
> comment justifying the theoretically wrong check ...
Yeah, I added all the info in the commit message and shortened the
in-code comment this time. I am also happy to keep the details in the
in-code comment, e.g.:
/*
* If the xenstore page hasn't been allocated properly, reading the
* value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually
* return error. Instead, it will succeed and return zero. Instead of
* attempting to xen_remap a bad guest physical address, detect this
* condition and return early.
*
* Note that although a guest physical address of zero for
* HVM_PARAM_STORE_PFN is theoretically possible, it is not a good
* choice and zero has never been validly used in that capacity.
*
* Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
*/
> Also, while I realize there are various other similar assumptions elsewhere, I
> would generally recommend to avoid such: There's no guarantee that now and
> forever unsigned long long and uint64_t are the same thing. And it's easy in
> cases like this one:
>
> if (!v || !(v + 1)) {
I am happy to use this.
