selftests/pidfd: add pidfd setns tests

This is basically a test-suite for setns() and as of now contains:
- test that we can't pass garbage flags
- test that we can't attach to the namespaces of  task that has already exited
- test that we can incrementally setns into all namespaces of a target task
  using a pidfd
- test that we can setns atomically into all namespaces of a target task
- test that we can't cross setns into a user namespace outside of our user
  namespace hierarchy
- test that we can't setns into namespaces owned by user namespaces over which
  we are not privileged

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200505140432.181565-4-christian.brauner@ubuntu.com

tools/testing/selftests/pidfd/.gitignore

@@ -5,3 +5,4 @@ pidfd_test
 pidfd_wait
 pidfd_fdinfo_test
 pidfd_getfd_test
+pidfd_setns_test

tools/testing/selftests/pidfd/Makefile

@@ -1,7 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -g -I../../../../usr/include/ -pthread
 
-TEST_GEN_PROGS := pidfd_test pidfd_fdinfo_test pidfd_open_test pidfd_poll_test pidfd_wait pidfd_getfd_test
+TEST_GEN_PROGS := pidfd_test pidfd_fdinfo_test pidfd_open_test \
+	pidfd_poll_test pidfd_wait pidfd_getfd_test pidfd_setns_test
 
 include ../lib.mk
 

tools/testing/selftests/pidfd/config

@@ -0,0 +1,6 @@
+CONFIG_UTS_NS=y
+CONFIG_IPC_NS=y
+CONFIG_USER_NS=y
+CONFIG_PID_NS=y
+CONFIG_NET_NS=y
+CONFIG_CGROUPS=y