selftests/namespaces: add active reference count regression test

Add a regression test for setns() with pidfd.

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-7-ae8a4ad5a3b3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/namespaces/.gitignore

@@ -8,3 +8,4 @@ siocgskns_test
 cred_change_test
 stress_test
 listns_pagination_bug
+regression_pidfd_setns_test

tools/testing/selftests/namespaces/Makefile

@@ -11,7 +11,8 @@ TEST_GEN_PROGS := nsid_test \
 		  siocgskns_test \
 		  cred_change_test \
 		  stress_test \
-		  listns_pagination_bug
+		  listns_pagination_bug \
+		  regression_pidfd_setns_test
 
 include ../lib.mk
 
@@ -22,4 +23,5 @@ $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 $(OUTPUT)/cred_change_test: ../filesystems/utils.c
 $(OUTPUT)/stress_test: ../filesystems/utils.c
 $(OUTPUT)/listns_pagination_bug: ../filesystems/utils.c
+$(OUTPUT)/regression_pidfd_setns_test: ../filesystems/utils.c
 

tools/testing/selftests/namespaces/regression_pidfd_setns_test.c

@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include "../pidfd/pidfd.h"
+#include "../kselftest_harness.h"
+
+/*
+ * Regression tests for the setns(pidfd) active reference counting bug.
+ *
+ * These tests are based on the reproducers that triggered the race condition
+ * fixed by commit 1c465d0518dc ("ns: handle setns(pidfd, ...) cleanly").
+ *
+ * The bug: When using setns() with a pidfd, if the target task exits between
+ * prepare_nsset() and commit_nsset(), the namespaces would become inactive.
+ * Then ns_ref_active_get() would increment from 0 without properly resurrecting
+ * the owner chain, causing active reference count underflows.
+ */
+
+/*
+ * Simple pidfd setns test using create_child()+unshare().
+ *
+ * Without the fix, this would trigger active refcount warnings when the
+ * parent exits after doing setns(pidfd) on a child that has already exited.
+ */
+TEST(simple_pidfd_setns)
+{
+	pid_t child_pid;
+	int pidfd = -1;
+	int ret;
+	int sv[2];
+	char c;
+
+	/* Ignore SIGCHLD for autoreap */
+	ASSERT_NE(signal(SIGCHLD, SIG_IGN), SIG_ERR);
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create a child process without namespaces initially */
+	child_pid = create_child(&pidfd, 0);
+	ASSERT_GE(child_pid, 0);
+
+	if (child_pid == 0) {
+		close(sv[0]);
+
+		if (unshare(CLONE_NEWUTS | CLONE_NEWIPC | CLONE_NEWNET | CLONE_NEWUSER) < 0) {
+			close(sv[1]);
+			_exit(1);
+		}
+
+		/* Signal parent that namespaces are ready */
+		if (write_nointr(sv[1], "1", 1) < 0) {
+			close(sv[1]);
+			_exit(1);
+		}
+
+		close(sv[1]);
+		_exit(0);
+	}
+	ASSERT_GE(pidfd, 0);
+	EXPECT_EQ(close(sv[1]), 0);
+
+	ret = read_nointr(sv[0], &c, 1);
+	ASSERT_EQ(ret, 1);
+	EXPECT_EQ(close(sv[0]), 0);
+
+	/* Set to child's namespaces via pidfd */
+	ret = setns(pidfd, CLONE_NEWUTS | CLONE_NEWIPC);
+	TH_LOG("setns() returned %d", ret);
+	close(pidfd);
+}
+
+/*
+ * Simple pidfd setns test using create_child().
+ *
+ * This variation uses create_child() with namespace flags directly.
+ * Namespaces are created immediately at clone time.
+ */
+TEST(simple_pidfd_setns_clone)
+{
+	pid_t child_pid;
+	int pidfd = -1;
+	int ret;
+
+	/* Ignore SIGCHLD for autoreap */
+	ASSERT_NE(signal(SIGCHLD, SIG_IGN), SIG_ERR);
+
+	/* Create a child process with new namespaces using create_child() */
+	child_pid = create_child(&pidfd, CLONE_NEWUSER | CLONE_NEWUTS | CLONE_NEWIPC | CLONE_NEWNET);
+	ASSERT_GE(child_pid, 0);
+
+	if (child_pid == 0) {
+		/* Child: sleep for a while so parent can setns to us */
+		sleep(2);
+		_exit(0);
+	}
+
+	/* Parent: pidfd was already created by create_child() */
+	ASSERT_GE(pidfd, 0);
+
+	/* Set to child's namespaces via pidfd */
+	ret = setns(pidfd, CLONE_NEWUTS | CLONE_NEWIPC);
+	close(pidfd);
+	TH_LOG("setns() returned %d", ret);
+}
+
+TEST_HARNESS_MAIN