add support hints to test-container V2

diff --git a/support/test-container.c b/support/test-container.c
index c56b53ed81..8445c0e76d 100644
--- a/support/test-container.c
+++ b/support/test-container.c
@@ -609,6 +609,47 @@  rsync (char *src, char *dest, int and_delete)
 }
 
 
+
+/* See if we can detect what the user needs to do to get unshare
+   support working for us.  */
+void
+check_for_unshare_hints (void)
+{
+  FILE *f;
+  int i;
+
+  /* Default Debian Linux disables user namespaces, but allows a way
+     to enable them.  */
+  f = fopen ("/proc/sys/kernel/unprivileged_userns_clone", "r");
+  if (f != NULL)
+    {
+      i = 99; /* Sentinel.  */
+      fscanf (f, "%d", &i);
+      if (i == 0)
+	{
+	  printf ("To enable test-container, please run this as root:\n");
+	  printf ("  echo 1 > /proc/sys/kernel/unprivileged_userns_clone\n");
+	}
+      fclose (f);
+      return;
+    }
+
+  /* ALT Linux has an alternate way of doing the same.  */
+  f = fopen ("/proc/sys/kernel/userns_restrict", "r");
+  if (f != NULL)
+    {
+      i = 99; /* Sentinel.  */
+      fscanf (f, "%d", &i);
+      if (i == 1)
+	{
+	  printf ("To enable test-container, please run this as root:\n");
+	  printf ("  echo 0 > /proc/sys/kernel/userns_restrict\n");
+	}
+      fclose (f);
+      return;
+    }
+}
+
 int
 main (int argc, char **argv)
 {
@@ -873,7 +914,12 @@  main (int argc, char **argv)
       /* Older kernels may not support all the options, or security
 	 policy may block this call.  */
       if (errno == EINVAL || errno == EPERM)
-	FAIL_UNSUPPORTED ("unable to unshare user/fs: %s", strerror (errno));
+	{
+	  int saved_errno = errno;
+	  if (errno == EPERM)
+	    check_for_unshare_hints ();
+	  FAIL_UNSUPPORTED ("unable to unshare user/fs: %s", strerror (saved_errno));
+	}
       else
 	FAIL_EXIT1 ("unable to unshare user/fs: %s", strerror (errno));
     }