libxl: when using pygrub, do not segfault if no blktap

Running xl create configfile where configfile includes the lines
   bootloader = "/usr/bin/pygrub"
   disk = [ 'file:/dev/mapper/vg0-partname,xvda1,w' ]
then xl segfaults at the line
     ret = strdup(dev);
of libxl_device_disk_local_attach() in tools/libxl/libxl.c . The
problem is that dev is not set if libxl__blktap_enabled(&gc) is false
or if phystype isn't recognized.  In the latter case we want to skip
that line and return NULL, but if libxl__blktap_enabled(&gc) is false
we should be returning something, at least in the cases where the
device has a name in the host which we can just refer to.

Also improve the error message when QCOW or QCOW2 are specified, and
avoid using an uninitialised value of "ret".

Signed-off-by: M A Young <m.a.young@durham.ac.uk>
Signed-off-by: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index ccbc84218b..9658d6714e 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1021,7 +1021,7 @@ char * libxl_device_disk_local_attach(libxl_ctx *ctx, libxl_device_disk *disk)
 {
     libxl__gc gc = LIBXL_INIT_GC(ctx);
     const char *dev = NULL;
-    char *ret;
+    char *ret = NULL;
     int phystype = disk->phystype;
     switch (phystype) {
         case PHYSTYPE_PHY: {
@@ -1033,18 +1033,27 @@ char * libxl_device_disk_local_attach(libxl_ctx *ctx, libxl_device_disk *disk)
             /* let's pretend is tap:aio for the moment */
             phystype = PHYSTYPE_AIO;
         case PHYSTYPE_AIO:
-        case PHYSTYPE_QCOW:
-        case PHYSTYPE_QCOW2:
+            if (!libxl__blktap_enabled(&gc)) {
+                dev = disk->physpath;
+                break;
+            }
         case PHYSTYPE_VHD:
             if (libxl__blktap_enabled(&gc))
                 dev = libxl__blktap_devpath(&gc, disk->physpath, phystype);
+            else
+                LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "tapdisk2 is required to open a vhd disk\n");
+            break;
+        case PHYSTYPE_QCOW:
+        case PHYSTYPE_QCOW2:
+            LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "cannot locally attach a qcow or qcow2 disk image\n");
             break;
 
         default:
             LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "unrecognized disk physical type: %d\n", phystype);
             break;
     }
-    ret = strdup(dev);
+    if (dev != NULL)
+        ret = strdup(dev);
     libxl__free_all(&gc);
     return ret;
 }