xen: add sysfs node for hypervisor build id

For support of Xen hypervisor live patching the hypervisor build id is
needed. Add a node /sys/hypervisor/properties/buildid containing the
information.

Signed-off-by: Juergen Gross <[e-mail]>
Reviewed-by: Boris Ostrovsky <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

Documentation/ABI/testing/sysfs-hypervisor-xen

···
 What:		/sys/hypervisor/guest_type
-Date:		May 2017
+Date:		June 2017
 KernelVersion:	4.13
 Contact:	[e-mail]
 Description:	If running under Xen:
···
 		Describes Xen PMU features (as an integer). A set bit indicates
 		that the corresponding feature is enabled. See
 		include/xen/interface/xenpmu.h for available features
+
+What:		/sys/hypervisor/properties/buildid
+Date:		June 2017
+KernelVersion:	4.13
+Contact:	[e-mail]
+Description:	If running under Xen:
+		Build id of the hypervisor, needed for hypervisor live patching.
+		Might return "<denied>" in case of special security settings
+		in the hypervisor.

drivers/xen/sys-hypervisor.c

···
 
 HYPERVISOR_ATTR_RO(features);
 
+static ssize_t buildid_show(struct hyp_sysfs_attr *attr, char *buffer)
+{
+	ssize_t ret;
+	struct xen_build_id *buildid;
+
+	ret = HYPERVISOR_xen_version(XENVER_build_id, NULL);
+	if (ret < 0) {
+		if (ret == -EPERM)
+			ret = sprintf(buffer, "<denied>");
+		return ret;
+	}
+
+	buildid = kmalloc(sizeof(*buildid) + ret, GFP_KERNEL);
+	if (!buildid)
+		return -ENOMEM;
+
+	buildid->len = ret;
+	ret = HYPERVISOR_xen_version(XENVER_build_id, buildid);
+	if (ret > 0)
+		ret = sprintf(buffer, "%s", buildid->buf);
+	kfree(buildid);
+
+	return ret;
+}
+
+HYPERVISOR_ATTR_RO(buildid);
+
 static struct attribute *xen_properties_attrs[] = {
 	&capabilities_attr.attr,
 	&changeset_attr.attr,
 	&virtual_start_attr.attr,
 	&pagesize_attr.attr,
 	&features_attr.attr,
+	&buildid_attr.attr,
 	NULL
 };