inspect: improve UsrMove detection (RHBZ#1186935)

In case /usr is a symlink to /usr/bin, then we cannot rely on /usr/bin
to exist, since /usr might be in a different partition.  Thus, in case
/bin is a symlink, check it points to "usr/bin".

src/inspect-fs.c

@@ -43,6 +43,7 @@ static int check_filesystem (guestfs_h *g, const char *mountable,
                              int whole_device);
 static void extend_fses (guestfs_h *g);
 static int get_partition_context (guestfs_h *g, const char *partition, int *partnum_ret, int *nr_partitions_ret);
+static int is_symlink_to (guestfs_h *g, const char *file, const char *wanted_target);
 
 /* Find out if 'device' contains a filesystem.  If it does, add
  * another entry in g->fses.
@@ -215,8 +216,7 @@ check_filesystem (guestfs_h *g, const char *mountable,
   /* Linux root? */
   else if (is_dir_etc &&
            (is_dir_bin ||
-            (guestfs_is_symlink (g, "/bin") > 0 &&
-             guestfs_is_dir (g, "/usr/bin") > 0)) &&
+            is_symlink_to (g, "/bin", "usr/bin") > 0) &&
            guestfs_is_file (g, "/etc/fstab") > 0) {
     fs->is_root = 1;
     fs->format = OS_FORMAT_INSTALLED;
@@ -366,6 +366,22 @@ get_partition_context (guestfs_h *g, const char *partition,
   return 0;
 }
 
+static int
+is_symlink_to (guestfs_h *g, const char *file, const char *wanted_target)
+{
+  CLEANUP_FREE char *target = NULL;
+
+  if (guestfs_is_symlink (g, file) == 0)
+    return 0;
+
+  target = guestfs_readlink (g, file);
+  /* This should not fail, but play safe. */
+  if (target == NULL)
+    return 0;
+
+  return STREQ (target, wanted_target);
+}
+
 int
 guestfs_int_is_file_nocase (guestfs_h *g, const char *path)
 {