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>

3 files changed, 388 insertions, 0 deletions

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 1a2bd15c5b6e..cfbc3528e9ef 100644
--- a/tools/testing/selftests/Makefile
+++ b/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
diff --git a/tools/testing/selftests/pidfd/Makefile b/tools/testing/selftests/pidfd/Makefile
new file mode 100644
index 000000000000..deaf8073bc06
--- /dev/null
+++ b/tools/testing/selftests/pidfd/Makefile
@@ -0,0 +1,6 @@
+CFLAGS += -g -I../../../../usr/include/
+
+TEST_GEN_PROGS := pidfd_test
+
+include ../lib.mk
+
diff --git a/tools/testing/selftests/pidfd/pidfd_test.c b/tools/testing/selftests/pidfd/pidfd_test.c
new file mode 100644
index 000000000000..d59378a93782
--- /dev/null
+++ b/tools/testing/selftests/pidfd/pidfd_test.c
@@ -0,0 +1,381 @@
+/* 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();
+}