run.py: Add --overlay-rwdir flag

README.md

@@ -35,6 +35,8 @@ For script support, you need CONFIG_VIRTIO_CONSOLE.
 
 For disk support, you need CONFIG_SCSI_VIRTIO.
 
+For --overlay-rwdir support, you need CONFIG_OVERLAY_FS.
+
 That kernel needs to be sane.  Your kernel is probably sane, but allmodconfig and allyesconfig generate insane kernels.  Sanity includes:
 
     CONFIG_CMDLINE_OVERRIDE=n

virtme/commands/run.py

@@ -127,6 +127,9 @@ def make_parser() -> argparse.ArgumentParser:
     g.add_argument('--rodir', action='append', default=[],
                    help="Supply a read-only directory to the guest.  Use --rodir=path or --rodir=guestpath=hostpath.")
 
+    g.add_argument('--overlay-rwdir', action='append', default=[],
+                   help="Supply a directory that is r/w to the guest but read-only in the host.  Use --overlay-rwdir=path.")
+
     return parser
 
 _ARGPARSER = make_parser()
@@ -292,6 +295,9 @@ def do_it() -> int:
 
     config = mkinitramfs.Config()
 
+    if len(args.overlay_rwdir) > 0:
+        virtmods.MODALIASES.append('overlay')
+
     kernel = find_kernel_and_mods(arch, args)
     config.modfiles = kernel.modfiles
     if config.modfiles:
@@ -357,6 +363,9 @@ def do_it() -> int:
         export_virtfs(qemu, arch, qemuargs, hostpath, tag, readonly=(dirtype != 'rwdir'))
         kernelargs.append('virtme_initmount%d=%s' % (idx, guestpath))
 
+    for i, d in enumerate(args.overlay_rwdir):
+        kernelargs.append('virtme_rw_overlay%d=%s' % (i, d))
+
     # Turn on KVM if available
     if is_native:
         qemuargs.extend(['-machine', 'accel=kvm:tcg'])

virtme/guest/virtme-init

@@ -86,6 +86,16 @@ if ! findmnt --kernel --mountpoint /dev &>/dev/null; then
     fi
 fi
 
+for tag in "${!virtme_rw_overlay@}"; do
+    td=`mktemp -d`
+    dir="${!tag}"
+    mount -t tmpfs none "$td"
+    mkdir "$td/cow" "$td/work"
+    mount -t overlay -o "lowerdir=$dir,upperdir=$td/cow,workdir=$td/work" cow "$dir"
+    umount "$td"
+    rmdir "$td"
+done
+
 for tag in "${!virtme_initmount@}"; do
     if [[ ! -d "${!tag}" ]]; then
         mkdir -p "${!tag}"