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).

2 files changed, 21 insertions, 5 deletions

diff --git a/src/generator.ml b/src/generator.ml
index 4f569ba5..83f307b1 100755
--- a/src/generator.ml
+++ b/src/generator.ml
@@ -544,13 +544,15 @@ handle is closed.  We don't currently have any method to enable
 changes to be committed, although qemu can support this.
 
 This is equivalent to the qemu parameter
-C<-drive file=filename,snapshot=on,if=...>.
+C<-drive file=filename,snapshot=on,readonly=on,if=...>.
 
 C<if=...> is set at compile time by the configuration option
 C<./configure --with-drive-if=...>.  In the rare case where you
 might need to change this at run time, use C<guestfs_add_drive_with_if>
 or C<guestfs_add_drive_ro_with_if>.
 
+C<readonly=on> is only added where qemu supports this option.
+
 Note that this call checks for the existence of C<filename>.  This
 stops you from specifying other types of drive which are supported
 by qemu such as C<nbd:> and C<http:> URLs.  To specify those, use
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);
 }