criu-ns: add a helper to hold a pid namespace

The init process can exit if it doesn't have any child processes and its
pidns is destroyed in this case. CRIU dump is running in the target pid
namespace and it kills dumped processes at the end. We need to create a
holder process to be sure that the pid namespace will not be destroy
before criu exits.

Fixes: #1775

Signed-off-by: Andrei Vagin <avagin@gmail.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/scripts/criu-ns b/scripts/criu-ns
index 930d20c80..9fc58b640 100755
--- a/scripts/criu-ns
+++ b/scripts/criu-ns
@@ -82,6 +82,27 @@ def run_criu(args):
     raise OSError(errno.ENOENT, "No such command")
 
 
+# pidns_holder creates a process that is reparented to the init.
+#
+# The init process can exit if it doesn't have any child processes and its
+# pidns is destroyed in this case. CRIU dump is running in the target pid
+# namespace and it kills dumped processes at the end. We need to create a
+# holder process to be sure that the pid namespace will not be destroy before
+# criu exits.
+def pidns_holder():
+    r, w = os.pipe()
+    pid = os.fork()
+    if pid == 0:
+        pid = os.fork()
+        if pid == 0:
+            os.close(w)
+            # The write end is owned by the parent process and it is closed by
+            # kernel when the parent process exits.
+            os.read(r, 1)
+        sys.exit(0)
+    os.waitpid(pid, 0)
+
+
 def wrap_restore():
     restore_args = sys.argv[1:]
     if '--restore-sibling' in restore_args:
@@ -200,6 +221,8 @@ def wrap_dump():
     set_pidns(pid, pid_idx)
     set_mntns(pid)
 
+    pidns_holder()
+
     criu_pid = os.fork()
     if criu_pid == 0:
         run_criu(sys.argv[1:])