libxl: adjust point of backend name resolution

Resolution of a backend name to a domid needs to happen a little earlier
in some cases.

For example, if a domU is specified as a backend for a
disk and, as previously written, libxl__device_disk_setdefault() calls
libxl__resolve_domid() last, then disk->backend_domid still equals
LIBXL_TOOLSTACK_DOMID when libxl__device_disk_set_backend() is called.
This results in libxl__device_disk_set_backend() making an incorrect
attempt to validate the target by calling stat() on a file on dom0,
resulting in ERROR_INVAL (see libxl_device.c lines 239-248), which
prevents creation of the frontend domain.

Likewise, libxl__device_nic_setdefault() previously made use of
nic->backend_domid before it was set.

Signed-off-by: Eric Shelton <eshelton@pobox.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>

1 files changed, 7 insertions, 4 deletions

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index ea8281d35b..87bda724bf 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1978,10 +1978,10 @@ int libxl__device_disk_setdefault(libxl__gc *gc, libxl_device_disk *disk)
 {
     int rc;
 
-    rc = libxl__device_disk_set_backend(gc, disk);
-    if (rc) return rc;
-
     rc = libxl__resolve_domid(gc, disk->backend_domname, &disk->backend_domid);
+    if (rc < 0) return rc;
+
+    rc = libxl__device_disk_set_backend(gc, disk);
     return rc;
 }
 
@@ -2812,6 +2812,10 @@ int libxl__device_nic_setdefault(libxl__gc *gc, libxl_device_nic *nic,
         LOG(ERROR, "unable to get current hotplug scripts execution setting");
         return run_hotplug_scripts;
     }
+
+    rc = libxl__resolve_domid(gc, nic->backend_domname, &nic->backend_domid);
+    if (rc < 0) return rc;
+
     if (nic->backend_domid != LIBXL_TOOLSTACK_DOMID && run_hotplug_scripts) {
         LOG(ERROR, "cannot use a backend domain different than %d if"
                    "hotplug scripts are executed from libxl",
@@ -2837,7 +2841,6 @@ int libxl__device_nic_setdefault(libxl__gc *gc, libxl_device_nic *nic,
         abort();
     }
 
-    rc = libxl__resolve_domid(gc, nic->backend_domname, &nic->backend_domid);
     return rc;
 }