On Mon, 22 Nov 2021, Stefano Stabellini wrote:
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.
*/
I sent a new version of the patch with the check below and slightly more concise version of this comment.
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.