selftests: add pid namespace ENOMEM regression test

We recently regressed (cf. [1] and its corresponding fix in [2]) returning
ENOMEM when trying to create a process in a pid namespace whose init
process/child subreaper has already died. This has caused confusion at
least once before that (cf. [3]). Let's add a simple regression test to
catch this in the future.

[1]: 49cb2fc4 ("fork: extend clone3() to support setting a PID")
[2]: b26ebfe1 ("pid: Fix error return value in some cases")
[3]: 35f71bc0 ("fork: report pid reservation failure properly")
Cc: Corey Minyard <cminyard@mvista.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Adrian Reber <areber@redhat.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

MAINTAINERS

@@ -13158,6 +13158,7 @@ S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git
 F:	samples/pidfd/
 F:	tools/testing/selftests/pidfd/
+F:	tools/testing/selftests/pid_namespace/
 F:	tools/testing/selftests/clone3/
 K:	(?i)pidfd
 K:	(?i)clone3

tools/testing/selftests/Makefile

@@ -38,6 +38,7 @@ TARGETS += netfilter
 TARGETS += networking/timestamping
 TARGETS += nsfs
 TARGETS += pidfd
+TARGETS += pid_namespace
 TARGETS += powerpc
 TARGETS += proc
 TARGETS += pstore

tools/testing/selftests/pid_namespace/.gitignore

+regression_enomem

tools/testing/selftests/pid_namespace/Makefile

+# SPDX-License-Identifier: GPL-2.0
+CFLAGS += -g -I../../../../usr/include/
+
+TEST_GEN_PROGS := regression_enomem
+
+include ../lib.mk
+
+$(OUTPUT)/regression_enomem: regression_enomem.c ../pidfd/pidfd.h

tools/testing/selftests/pid_namespace/config

+CONFIG_PID_NS=y
+CONFIG_USER_NS=y

tools/testing/selftests/pid_namespace/regression_enomem.c

+#define _GNU_SOURCE
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/types.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/wait.h>
+
+#include "../kselftest.h"
+#include "../kselftest_harness.h"
+#include "../pidfd/pidfd.h"
+
+/*
+ * Regression test for:
+ * 35f71bc0a09a ("fork: report pid reservation failure properly")
+ * b26ebfe12f34 ("pid: Fix error return value in some cases")
+ */
+TEST(regression_enomem)
+{
+	pid_t pid;
+
+	if (geteuid())
+		EXPECT_EQ(0, unshare(CLONE_NEWUSER));
+
+	EXPECT_EQ(0, unshare(CLONE_NEWPID));
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0)
+		exit(EXIT_SUCCESS);
+
+	EXPECT_EQ(0, wait_for_pid(pid));
+
+	pid = fork();
+	ASSERT_LT(pid, 0);
+	ASSERT_EQ(errno, ENOMEM);
+}
+
+TEST_HARNESS_MAIN

tools/testing/selftests/pidfd/pidfd.h

@@ -13,6 +13,8 @@
 #include <string.h>
 #include <syscall.h>
 #include <sys/mount.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 
 #include "../kselftest.h"