selftests: add tests for pidfd_send_signal()

As suggested by Andrew Morton in [1] add selftests for the new
sys_pidfd_send_signal() syscall:

/* test_pidfd_send_signal_syscall_support */
Test whether the pidfd_send_signal() syscall is supported and the tests can
be run or need to be skipped.

/* test_pidfd_send_signal_simple_success */
Test whether sending a signal via a pidfd works.

/* test_pidfd_send_signal_exited_fail */
Verify that sending a signal to an already exited process fails with ESRCH.

/* test_pidfd_send_signal_recycled_pid_fail */
Verify that a recycled pid cannot be signaled via a pidfd referring to an
already exited process that had the same pid (cf. [2], [3]).

[1]: https://lore.kernel.org/lkml/20181228152012.dbf0508c2508138efc5f2bbe@linux-foundation.org/
[2]: https://lore.kernel.org/lkml/20181230210245.GA30252@mail.hallyn.com/
[3]: https://lore.kernel.org/lkml/20181230232711.7aayb7vnhogbv4co@brauner.io/

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Jann Horn <jannh@google.com>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Christian Brauner <christian@brauner.io>
Reviewed-by: Tycho Andersen <tycho@tycho.ws>
Acked-by: Serge Hallyn <serge@hallyn.com>

tools/testing/selftests/Makefile

@@ -30,6 +30,7 @@ TARGETS += net
 TARGETS += netfilter
 TARGETS += networking/timestamping
 TARGETS += nsfs
+TARGETS += pidfd
 TARGETS += powerpc
 TARGETS += proc
 TARGETS += pstore

tools/testing/selftests/pidfd/Makefile

+CFLAGS += -g -I../../../../usr/include/
+
+TEST_GEN_PROGS := pidfd_test
+
+include ../lib.mk
+

tools/testing/selftests/pidfd/pidfd_test.c

+/* SPDX-License-Identifier: GPL-2.0 */
+
+#define _GNU_SOURCE
+#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/mount.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "../kselftest.h"
+
+static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
+					unsigned int flags)
+{
+	return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
+}
+
+static int signal_received;
+
+static void set_signal_received_on_sigusr1(int sig)
+{
+	if (sig == SIGUSR1)
+		signal_received = 1;
+}
+
+/*
+ * Straightforward test to see whether pidfd_send_signal() works is to send
+ * a signal to ourself.
+ */
+static int test_pidfd_send_signal_simple_success(void)
+{
+	int pidfd, ret;
+	const char *test_name = "pidfd_send_signal send SIGUSR1";
+
+	pidfd = open("/proc/self", O_DIRECTORY | O_CLOEXEC);
+	if (pidfd < 0)
+		ksft_exit_fail_msg(
+			"%s test: Failed to open process file descriptor\n",
+			test_name);
+
+	signal(SIGUSR1, set_signal_received_on_sigusr1);
+
+	ret = sys_pidfd_send_signal(pidfd, SIGUSR1, NULL, 0);
+	close(pidfd);
+	if (ret < 0)
+		ksft_exit_fail_msg("%s test: Failed to send signal\n",
+				   test_name);
+
+	if (signal_received != 1)
+		ksft_exit_fail_msg("%s test: Failed to receive signal\n",
+				   test_name);
+
+	signal_received = 0;
+	ksft_test_result_pass("%s test: Sent signal\n", test_name);
+	return 0;
+}
+
+static int wait_for_pid(pid_t pid)
+{
+	int status, ret;
+
+again:
+	ret = waitpid(pid, &status, 0);
+	if (ret == -1) {
+		if (errno == EINTR)
+			goto again;
+
+		return -1;
+	}
+
+	if (ret != pid)
+		goto again;
+
+	if (!WIFEXITED(status))
+		return -1;
+
+	return WEXITSTATUS(status);
+}
+
+static int test_pidfd_send_signal_exited_fail(void)
+{
+	int pidfd, ret, saved_errno;
+	char buf[256];
+	pid_t pid;
+	const char *test_name = "pidfd_send_signal signal exited process";
+
+	pid = fork();
+	if (pid < 0)
+		ksft_exit_fail_msg("%s test: Failed to create new process\n",
+				   test_name);
+
+	if (pid == 0)
+		_exit(EXIT_SUCCESS);
+
+	snprintf(buf, sizeof(buf), "/proc/%d", pid);
+
+	pidfd = open(buf, O_DIRECTORY | O_CLOEXEC);
+
+	(void)wait_for_pid(pid);
+
+	if (pidfd < 0)
+		ksft_exit_fail_msg(
+			"%s test: Failed to open process file descriptor\n",
+			test_name);
+
+	ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
+	saved_errno = errno;
+	close(pidfd);
+	if (ret == 0)
+		ksft_exit_fail_msg(
+			"%s test: Managed to send signal to process even though it should have failed\n",
+			test_name);
+
+	if (saved_errno != ESRCH)
+		ksft_exit_fail_msg(
+			"%s test: Expected to receive ESRCH as errno value but received %d instead\n",
+			test_name, saved_errno);
+
+	ksft_test_result_pass("%s test: Failed to send signal as expected\n",
+			      test_name);
+	return 0;
+}
+
+/*
+ * The kernel reserves 300 pids via RESERVED_PIDS in kernel/pid.c
+ * That means, when it wraps around any pid < 300 will be skipped.
+ * So we need to use a pid > 300 in order to test recycling.
+ */
+#define PID_RECYCLE 1000
+
+/*
+ * Maximum number of cycles we allow. This is equivalent to PID_MAX_DEFAULT.
+ * If users set a higher limit or we have cycled PIDFD_MAX_DEFAULT number of
+ * times then we skip the test to not go into an infinite loop or block for a
+ * long time.
+ */
+#define PIDFD_MAX_DEFAULT 0x8000
+
+/*
+ * Define a few custom error codes for the child process to clearly indicate
+ * what is happening. This way we can tell the difference between a system
+ * error, a test error, etc.
+ */
+#define PIDFD_PASS 0
+#define PIDFD_FAIL 1
+#define PIDFD_ERROR 2
+#define PIDFD_SKIP 3
+#define PIDFD_XFAIL 4
+
+static int test_pidfd_send_signal_recycled_pid_fail(void)
+{
+	int i, ret;
+	pid_t pid1;
+	const char *test_name = "pidfd_send_signal signal recycled pid";
+
+	ret = unshare(CLONE_NEWPID);
+	if (ret < 0)
+		ksft_exit_fail_msg("%s test: Failed to unshare pid namespace\n",
+				   test_name);
+
+	ret = unshare(CLONE_NEWNS);
+	if (ret < 0)
+		ksft_exit_fail_msg(
+			"%s test: Failed to unshare mount namespace\n",
+			test_name);
+
+	ret = mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, 0);
+	if (ret < 0)
+		ksft_exit_fail_msg("%s test: Failed to remount / private\n",
+				   test_name);
+
+	/* pid 1 in new pid namespace */
+	pid1 = fork();
+	if (pid1 < 0)
+		ksft_exit_fail_msg("%s test: Failed to create new process\n",
+				   test_name);
+
+	if (pid1 == 0) {
+		char buf[256];
+		pid_t pid2;
+		int pidfd = -1;
+
+		(void)umount2("/proc", MNT_DETACH);
+		ret = mount("proc", "/proc", "proc", 0, NULL);
+		if (ret < 0)
+			_exit(PIDFD_ERROR);
+
+		/* grab pid PID_RECYCLE */
+		for (i = 0; i <= PIDFD_MAX_DEFAULT; i++) {
+			pid2 = fork();
+			if (pid2 < 0)
+				_exit(PIDFD_ERROR);
+
+			if (pid2 == 0)
+				_exit(PIDFD_PASS);
+
+			if (pid2 == PID_RECYCLE) {
+				snprintf(buf, sizeof(buf), "/proc/%d", pid2);
+				ksft_print_msg("pid to recycle is %d\n", pid2);
+				pidfd = open(buf, O_DIRECTORY | O_CLOEXEC);
+			}
+
+			if (wait_for_pid(pid2))
+				_exit(PIDFD_ERROR);
+
+			if (pid2 >= PID_RECYCLE)
+				break;
+		}
+
+		/*
+		 * We want to be as predictable as we can so if we haven't been
+		 * able to grab pid PID_RECYCLE skip the test.
+		 */
+		if (pid2 != PID_RECYCLE) {
+			/* skip test */
+			close(pidfd);
+			_exit(PIDFD_SKIP);
+		}
+
+		if (pidfd < 0)
+			_exit(PIDFD_ERROR);
+
+		for (i = 0; i <= PIDFD_MAX_DEFAULT; i++) {
+			char c;
+			int pipe_fds[2];
+			pid_t recycled_pid;
+			int child_ret = PIDFD_PASS;
+
+			ret = pipe2(pipe_fds, O_CLOEXEC);
+			if (ret < 0)
+				_exit(PIDFD_ERROR);
+
+			recycled_pid = fork();
+			if (recycled_pid < 0)
+				_exit(PIDFD_ERROR);
+
+			if (recycled_pid == 0) {
+				close(pipe_fds[1]);
+				(void)read(pipe_fds[0], &c, 1);
+				close(pipe_fds[0]);
+
+				_exit(PIDFD_PASS);
+			}
+
+			/*
+			 * Stop the child so we can inspect whether we have
+			 * recycled pid PID_RECYCLE.
+			 */
+			close(pipe_fds[0]);
+			ret = kill(recycled_pid, SIGSTOP);
+			close(pipe_fds[1]);
+			if (ret) {
+				(void)wait_for_pid(recycled_pid);
+				_exit(PIDFD_ERROR);
+			}
+
+			/*
+			 * We have recycled the pid. Try to signal it. This
+			 * needs to fail since this is a different process than
+			 * the one the pidfd refers to.
+			 */
+			if (recycled_pid == PID_RECYCLE) {
+				ret = sys_pidfd_send_signal(pidfd, SIGCONT,
+							    NULL, 0);
+				if (ret && errno == ESRCH)
+					child_ret = PIDFD_XFAIL;
+				else
+					child_ret = PIDFD_FAIL;
+			}
+
+			/* let the process move on */
+			ret = kill(recycled_pid, SIGCONT);
+			if (ret)
+				(void)kill(recycled_pid, SIGKILL);
+
+			if (wait_for_pid(recycled_pid))
+				_exit(PIDFD_ERROR);
+
+			switch (child_ret) {
+			case PIDFD_FAIL:
+				/* fallthrough */
+			case PIDFD_XFAIL:
+				_exit(child_ret);
+			case PIDFD_PASS:
+				break;
+			default:
+				/* not reached */
+				_exit(PIDFD_ERROR);
+			}
+
+			/*
+			 * If the user set a custom pid_max limit we could be
+			 * in the millions.
+			 * Skip the test in this case.
+			 */
+			if (recycled_pid > PIDFD_MAX_DEFAULT)
+				_exit(PIDFD_SKIP);
+		}
+
+		/* failed to recycle pid */
+		_exit(PIDFD_SKIP);
+	}
+
+	ret = wait_for_pid(pid1);
+	switch (ret) {
+	case PIDFD_FAIL:
+		ksft_exit_fail_msg(
+			"%s test: Managed to signal recycled pid %d\n",
+			test_name, PID_RECYCLE);
+	case PIDFD_PASS:
+		ksft_exit_fail_msg("%s test: Failed to recycle pid %d\n",
+				   test_name, PID_RECYCLE);
+	case PIDFD_SKIP:
+		ksft_print_msg("%s test: Skipping test\n", test_name);
+		ret = 0;
+		break;
+	case PIDFD_XFAIL:
+		ksft_test_result_pass(
+			"%s test: Failed to signal recycled pid as expected\n",
+			test_name);
+		ret = 0;
+		break;
+	default /* PIDFD_ERROR */:
+		ksft_exit_fail_msg("%s test: Error while running tests\n",
+				   test_name);
+	}
+
+	return ret;
+}
+
+static int test_pidfd_send_signal_syscall_support(void)
+{
+	int pidfd, ret;
+	const char *test_name = "pidfd_send_signal check for support";
+
+	pidfd = open("/proc/self", O_DIRECTORY | O_CLOEXEC);
+	if (pidfd < 0)
+		ksft_exit_fail_msg(
+			"%s test: Failed to open process file descriptor\n",
+			test_name);
+
+	ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
+	if (ret < 0) {
+		/*
+		 * pidfd_send_signal() will currently return ENOSYS when
+		 * CONFIG_PROC_FS is not set.
+		 */
+		if (errno == ENOSYS)
+			ksft_exit_skip(
+				"%s test: pidfd_send_signal() syscall not supported (Ensure that CONFIG_PROC_FS=y is set)\n",
+				test_name);
+
+		ksft_exit_fail_msg("%s test: Failed to send signal\n",
+				   test_name);
+	}
+
+	close(pidfd);
+	ksft_test_result_pass(
+		"%s test: pidfd_send_signal() syscall is supported. Tests can be executed\n",
+		test_name);
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	ksft_print_header();
+
+	test_pidfd_send_signal_syscall_support();
+	test_pidfd_send_signal_simple_success();
+	test_pidfd_send_signal_exited_fail();
+	test_pidfd_send_signal_recycled_pid_fail();
+
+	return ksft_exit_pass();
+}