support_enter_mount_namespace: Unshare with mount --make-rprivate

System defaults vary, and a mere unshare (CLONE_NEWNS) (which is part of
support_become_root) is no longer sufficient.

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>

ChangeLog

@@ -1,3 +1,10 @@
+2017-11-18  Florian Weimer  <fweimer@redhat.com>
+
+	* support/namespace.h (support_enter_mount_namespace): Declare.
+	* support/support_enter_mount_namespace.c: New file.
+	* support/Makefile (libsupport-routines): Add
+	support_enter_mount_namespace.
+
 2017-11-18  Florian Weimer  <fweimer@redhat.com>
 
 	* support/temp_file.c (support_create_temp_directory): Use

support/Makefile

@@ -42,6 +42,7 @@ libsupport-routines = \
   support_capture_subprocess \
   support_capture_subprocess_check \
   support_chroot \
+  support_enter_mount_namespace \
   support_enter_network_namespace \
   support_format_address_family \
   support_format_addrinfo \

support/namespace.h

@@ -51,6 +51,11 @@ bool support_can_chroot (void);
    has sufficient privileges.  */
 bool support_enter_network_namespace (void);
 
+/* Enter a mount namespace and mark / as private (not shared).  If
+   this function returns true, mount operations in this process will
+   not affect the host system afterwards.  */
+bool support_enter_mount_namespace (void);
+
 /* Return true if support_enter_network_namespace managed to enter a
    UTS namespace.  */
 bool support_in_uts_namespace (void);

support/support_enter_mount_namespace.c

@@ -0,0 +1,45 @@
+/* Enter a mount namespace.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C 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 2.1 of the License, or (at your option) any later version.
+
+   The GNU C 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 the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/namespace.h>
+
+#include <sched.h>
+#include <stdio.h>
+#include <sys/mount.h>
+
+bool
+support_enter_mount_namespace (void)
+{
+#ifdef CLONE_NEWNS
+  if (unshare (CLONE_NEWNS) == 0)
+    {
+      /* On some systems, / is marked as MS_SHARED, which means that
+         mounts within the namespace leak to the rest of the system,
+         which is not what we want.  */
+      if (mount ("none", "/", NULL, MS_REC | MS_PRIVATE, NULL) != 0)
+        {
+          printf ("warning: making the mount namespace private failed: %m\n");
+          return false;
+        }
+      return true;
+    }
+  else
+    printf ("warning: unshare (CLONE_NEWNS) failed: %m\n");
+#endif /* CLONE_NEWNS */
+  return false;
+}