diff --git a/sys/kern/subr_bus.c b/sys/kern/subr_bus.c
--- a/sys/kern/subr_bus.c
+++ b/sys/kern/subr_bus.c
@@ -2715,6 +2715,37 @@
 	args->memattr = VM_MEMATTR_DEVICE;
 }
 
+int
+resource_validate_map_request(struct resource *r,
+    struct resource_map_request *in, struct resource_map_request *out,
+    rman_res_t *startp, rman_res_t *lengthp)
+{
+	rman_res_t end, length, start;
+
+	/*
+	 * This assumes that any callers of this function are compiled
+	 * into the kernel and use the same version of the structure
+	 * as this file.
+	 */
+	MPASS(out->size == sizeof(struct resource_map_request));
+
+	if (in != NULL)
+		bcopy(in, out, imin(in->size, out->size));
+	start = rman_get_start(r) + out->offset;
+	if (out->length == 0)
+		length = rman_get_size(r);
+	else
+		length = out->length;
+	end = start + length - 1;
+	if (start > rman_get_end(r) || start < rman_get_start(r))
+		return (EINVAL);
+	if (end > rman_get_end(r) || end < start)
+		return (EINVAL);
+	*lengthp = length;
+	*startp = start;
+	return (0);
+}
+
 /**
  * @brief Initialise a resource list.
  *
diff --git a/sys/sys/bus.h b/sys/sys/bus.h
--- a/sys/sys/bus.h
+++ b/sys/sys/bus.h
@@ -317,6 +317,8 @@
 	KOBJ_CLASS_FIELDS;
 };
 
+struct resource;
+
 /**
  * @brief A resource mapping.
  */
@@ -341,12 +343,14 @@
 	    size_t _sz);
 #define	resource_init_map_request(rmr) 					\
 	resource_init_map_request_impl((rmr), sizeof(*(rmr)))
+int	resource_validate_map_request(struct resource *r,
+	    struct resource_map_request *in, struct resource_map_request *out,
+	    rman_res_t *startp, rman_res_t *lengthp);
 
 /*
  * Definitions for drivers which need to keep simple lists of resources
  * for their child devices.
  */
-struct	resource;
 
 /**
  * @brief An entry for a single resource in a resource list.