chroot-pivot-root-prep.patch

diff --git a/bind-mount.c b/bind-mount.c
index 7d3543f..d069c96 100644
--- a/bind-mount.c
+++ b/bind-mount.c
@@ -438,3 +438,27 @@ bind_mount (int           proc_fd,
 
   return 0;
 }
+
+bool
+mount_exists (int         proc_fd,
+              const char *path)
+{
+  cleanup_free char *resolved_path = NULL;
+  cleanup_mount_tab MountTab mount_tab = NULL;
+
+  /* The mount operation will resolve any symlinks in the destination
+     path, so to find it in the mount table we need to do that too. */
+  resolved_path = realpath (path, NULL);
+  if (resolved_path == NULL)
+    return FALSE;
+
+  mount_tab = parse_mountinfo (proc_fd, resolved_path);
+  if (mount_tab[0].mountpoint == NULL)
+    {
+      /* No mountpoint at dest */
+      errno = ENOENT;
+      return FALSE;
+    }
+
+  return TRUE;
+}
diff --git a/bind-mount.h b/bind-mount.h
index c763763..b0f468a 100644
--- a/bind-mount.h
+++ b/bind-mount.h
@@ -28,3 +28,6 @@ int bind_mount (int           proc_fd,
                 const char   *src,
                 const char   *dest,
                 bind_option_t options);
+
+bool mount_exists (int         proc_fd,
+                   const char *path);
diff --git a/bubblewrap.c b/bubblewrap.c
index 6e04459..8371fb9 100644
--- a/bubblewrap.c
+++ b/bubblewrap.c
@@ -1853,12 +1853,32 @@ main (int    argc,
   /* Need to do this before the chroot, but after we're the real uid */
   resolve_symlinks_in_ops ();
 
-  /* Mark everything as slave, so that we still
-   * receive mounts from the real root, but don't
-   * propagate mounts to the real root. */
+  if (!mount_exists (proc_fd, "/"))
+    {
+      printf("No / mount\n");
+      if (errno != ENOENT)
+        die_with_error ("Failed to get mount entry for /");
+
+      /* No / mount, likely in a chroot. Bind mount / to itself so that
+       * the subsequent slave mark succeeds. */
+      if (mount ("/", "/", NULL, MS_BIND| MS_REC, NULL) < 0)
+        die_with_error ("Failed to mount /");
+    }
+
+  /* Mark everything as slave, so that we still receive mounts from the
+   * real root, but don't propagate mounts to the real root. This also
+   * satisfies the pivot_root requirement that the old root mount is not
+   * shared. */
   if (mount (NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) < 0)
     die_with_error ("Failed to make / slave");
 
+  /* pivot_root requires that both the old root and its parent are not
+   * shared mounts. To ensure control of the parent mount, bind mount /
+   * to itself. Since / was marked as slave above, both mounts will be
+   * non-shared. */
+  if (mount ("/", "/", NULL, MS_BIND| MS_REC, NULL) < 0)
+    die_with_error ("Failed to bind mount / to itself");
+
   /* Create a tmpfs which we will use as / in the namespace */
   if (mount ("", base_path, "tmpfs", MS_NODEV | MS_NOSUID, NULL) != 0)
     die_with_error ("Failed to mount tmpfs");