[Libguestfs] [PATCH] launch: libvirt: Autodetect backing format for drive overlays (RHBZ#1354335).

[Richard W.M. Jones]

If the user doesn't specify a format (ie. they want autodetection)
then we must do the file format detection ourselves since libvirt
disabled this and now doesn't work at all on qcow2 overlays that have
no backing_fmt field set.
---
 src/launch-libvirt.c | 90 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 56 insertions(+), 34 deletions(-)

diff --git a/src/launch-libvirt.c b/src/launch-libvirt.c
index e8b6e27..3b53a3a 100644
--- a/src/launch-libvirt.c
+++ b/src/launch-libvirt.c
@@ -162,6 +162,50 @@ static int check_bridge_exists (guestfs_h *g, const char *brname);
 static void selinux_warning (guestfs_h *g, const char *func, const char *selinux_op, const char *data);
 #endif
 
+/* Return drv->src.format, but if it is NULL, autodetect the format.
+ *
+ * libvirt has disabled the feature of detecting the disk format,
+ * unless the administrator sets allow_disk_format_probing=1 in
+ * qemu.conf.  There is no way to detect if this option is set, so we
+ * have to do format detection here using qemu-img and pass that to
+ * libvirt.
+ *
+ * This can still be a security issue, so in most cases it is
+ * recommended the users pass the format to libguestfs which will
+ * faithfully pass that to libvirt and this function won't be used.
+ *
+ * Caller must free the returned string.
+ */
+static char *
+get_source_format_or_autodetect (guestfs_h *g, struct drive *drv)
+{
+  if (drv->src.format)
+    return safe_strdup (g, drv->src.format);
+
+  if (drv->src.protocol == drive_protocol_file) {
+    char *format;
+
+    format = guestfs_disk_format (g, drv->src.u.path);
+    if (!format)
+      return NULL;
+
+    if (STREQ (format, "unknown")) {
+      error (g, _("could not auto-detect the format.\n"
+                  "If the format is known, pass the format to libguestfs, eg. using the\n"
+                  "'--format' option, or via the optional 'format' argument to 'add-drive'."));
+      return NULL;
+    }
+
+    return format;
+  }
+
+  /* Non-file protocol. */
+  error (g, _("could not auto-detect the format when using a non-file protocol.\n"
+              "If the format is known, pass the format to libguestfs, eg. using the\n"
+              "'--format' option, or via the optional 'format' argument to 'add-drive'."));
+  return NULL;
+}
+
 static char *
 make_qcow2_overlay (guestfs_h *g, const char *backing_drive,
                     const char *format)
@@ -169,6 +213,8 @@ make_qcow2_overlay (guestfs_h *g, const char *backing_drive,
   char *overlay;
   struct guestfs_disk_create_argv optargs;
 
+  assert (format != NULL);
+
   if (guestfs_int_lazy_make_tmpdir (g) == -1)
     return NULL;
 
@@ -176,10 +222,8 @@ make_qcow2_overlay (guestfs_h *g, const char *backing_drive,
 
   optargs.bitmask = GUESTFS_DISK_CREATE_BACKINGFILE_BITMASK;
   optargs.backingfile = backing_drive;
-  if (format) {
-    optargs.bitmask |= GUESTFS_DISK_CREATE_BACKINGFORMAT_BITMASK;
-    optargs.backingformat = format;
-  }
+  optargs.bitmask |= GUESTFS_DISK_CREATE_BACKINGFORMAT_BITMASK;
+  optargs.backingformat = format;
 
   if (guestfs_disk_create_argv (g, overlay, "qcow2", -1, &optargs) == -1) {
     free (overlay);
@@ -196,13 +240,18 @@ create_cow_overlay_libvirt (guestfs_h *g, void *datav, struct drive *drv)
   struct backend_libvirt_data *data = datav;
 #endif
   CLEANUP_FREE char *backing_drive = NULL;
+  CLEANUP_FREE char *format = NULL;
   char *overlay;
 
   backing_drive = guestfs_int_drive_source_qemu_param (g, &drv->src);
   if (!backing_drive)
     return NULL;
 
-  overlay = make_qcow2_overlay (g, backing_drive, drv->src.format);
+  format = get_source_format_or_autodetect (g, drv);
+  if (!format)
+    return NULL;
+
+  overlay = make_qcow2_overlay (g, backing_drive, format);
   if (!overlay)
     return NULL;
 
@@ -1480,36 +1529,9 @@ construct_libvirt_xml_disk (guestfs_h *g,
       if (construct_libvirt_xml_disk_target (g, xo, drv_index) == -1)
         return -1;
 
-      if (drv->src.format)
-        format = safe_strdup (g, drv->src.format);
-      else if (drv->src.protocol == drive_protocol_file) {
-        /* libvirt has disabled the feature of detecting the disk format,
-         * unless the administrator sets allow_disk_format_probing=1 in
-         * qemu.conf.  There is no way to detect if this option is set, so we
-         * have to do format detection here using qemu-img and pass that to
-         * libvirt.
-         *
-         * This is still a security issue, so in most cases it is recommended
-         * the users pass the format to libguestfs which will faithfully pass
-         * that to libvirt and this function won't be used.
-         */
-        format = guestfs_disk_format (g, drv->src.u.path);
-        if (!format)
-          return -1;
-
-        if (STREQ (format, "unknown")) {
-          error (g, _("could not auto-detect the format.\n"
-                      "If the format is known, pass the format to libguestfs, eg. using the\n"
-                      "'--format' option, or via the optional 'format' argument to 'add-drive'."));
-          return -1;
-        }
-      }
-      else {
-        error (g, _("could not auto-detect the format when using a non-file protocol.\n"
-                    "If the format is known, pass the format to libguestfs, eg. using the\n"
-                    "'--format' option, or via the optional 'format' argument to 'add-drive'."));
+      format = get_source_format_or_autodetect (g, drv);
+      if (!format)
         return -1;
-      }
 
       if (construct_libvirt_xml_disk_driver_qemu (g, data, drv, xo, format,
                                                   drv->cachemode ? : "writeback",
-- 
2.7.4