add_drive_ro adds readonly=on option if available.

Change the add_drive_ro call so it adds the readonly=on option
if qemu supports that.

This just means that qemu will not try to open the drive with
O_RDWR, and should not otherwise change the behaviour of qemu or
libguestfs.  (In particular, writes to the read-only drive are
still permitted, and are just discarded when the handle is closed).

However it should alleviate RHBZ#571714 where udev was deciding
to incorrectly relabel a device because we had opened the device
for writing (even though we didn't actually write to it).

1 files changed, 18 insertions, 4 deletions

diff --git a/src/guestfs.c b/src/guestfs.c
index af60699d..02d5fdbf 100644
--- a/src/guestfs.c
+++ b/src/guestfs.c
@@ -792,9 +792,6 @@ int
 guestfs__add_drive_ro_with_if (guestfs_h *g, const char *filename,
                                const char *drive_if)
 {
-  size_t len = strlen (filename) + 64;
-  char buf[len];
-
   if (strchr (filename, ',') != NULL) {
     error (g, _("filename cannot contain ',' (comma) character"));
     return -1;
@@ -805,7 +802,24 @@ guestfs__add_drive_ro_with_if (guestfs_h *g, const char *filename,
     return -1;
   }
 
-  snprintf (buf, len, "file=%s,snapshot=on,if=%s", filename, drive_if);
+  if (qemu_supports (g, NULL) == -1)
+    return -1;
+
+  /* Only SCSI and virtio drivers support readonly mode.
+   * This is only supported as a QEMU feature since 2010/01.
+   */
+  int supports_ro = 0;
+  if ((STREQ (drive_if, "scsi") || STREQ (drive_if, "virtio")) &&
+      qemu_supports (g, "readonly=on"))
+    supports_ro = 1;
+
+  size_t len = strlen (filename) + 100;
+  char buf[len];
+
+  snprintf (buf, len, "file=%s,snapshot=on,%sif=%s",
+            filename,
+            supports_ro ? "readonly=on," : "",
+            drive_if);
 
   return guestfs__config (g, "-drive", buf);
 }