dracut: Add ostree-remount

Linux creates a copy of the soure mount flags when creating a bind
mount; if the source is read-only, then the bind mount is.

The problem is that systemd will remount the rootfs read/write, but
each mount (/home, /var etc.) will still be read-only.  We need to
remount every bind mount except for /usr to read-write too.

This only "worked" with the old ostree-switch-root because it
effectively force mounted the rootfs read-write always, ignoring the
"ro" flag.

src/switchroot/ostree-remount.c

@@ -0,0 +1,79 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2011 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Author: Colin Walters <walters@verbum.org>
+ */
+
+#define _GNU_SOURCE
+
+#include <string.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdint.h>
+#include <sys/param.h>
+#include <sys/mount.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <errno.h>
+
+#include "ostree-mount-util.h"
+
+int
+main(int argc, char *argv[])
+{
+  const char *remounts[] = { "/sysroot", "/etc", "/home", "/root", "/tmp", "/var", NULL };
+  struct stat stbuf;
+  int i;
+
+  if (access ("/", W_OK) == -1)
+    {
+      /* If / isn't writable, don't do any remounts; we don't want
+       * to clear the readonly flag in that case.
+       */
+      exit (0);
+    }
+
+  for (i = 0; remounts[i] != NULL; i++)
+    {
+      const char *target = remounts[i];
+      if (lstat (target, &stbuf) < 0)
+        continue;
+      /* Silently ignore symbolic links; we expect these to point to
+       * /sysroot, and thus there isn't a bind mount there.
+       */
+      if (S_ISLNK (stbuf.st_mode))
+        continue;
+      if (mount (target, target, NULL, MS_REMOUNT | MS_SILENT, NULL) < 0)
+	{
+          /* Also ignore ENINVAL - if the target isn't a mountpoint
+           * already, then assume things are OK.
+           */
+          if (errno != EINVAL)
+            {
+              perrorv ("failed to remount %s", target);
+              exit (1);
+            }
+	}
+    }
+  
+  exit (0);
+}
+