1// SPDX-License-Identifier: GPL-2.0
2
3#define _GNU_SOURCE
4#include <err.h>
5#include <errno.h>
6#include <fcntl.h>
7#include <inttypes.h>
8#include <limits.h>
9#include <sched.h>
10#include <signal.h>
11#include <stdio.h>
12#include <stdlib.h>
13#include <string.h>
14#include <sys/stat.h>
15#include <sys/syscall.h>
16#include <sys/types.h>
17#include <sys/wait.h>
18#include <unistd.h>
19
20#ifndef CLONE_PIDFD
21#define CLONE_PIDFD 0x00001000
22#endif
23
24#ifndef __NR_pidfd_send_signal
25#define __NR_pidfd_send_signal -1
26#endif
27
28static int do_child(void *args)
29{
30	printf("%d\n", getpid());
31	_exit(EXIT_SUCCESS);
32}
33
34static pid_t pidfd_clone(int flags, int *pidfd)
35{
36	size_t stack_size = 1024;
37	char *stack[1024] = { 0 };
38
39#ifdef __ia64__
40	return __clone2(do_child, stack, stack_size, flags | SIGCHLD, NULL, pidfd);
41#else
42	return clone(do_child, stack + stack_size, flags | SIGCHLD, NULL, pidfd);
43#endif
44}
45
46static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
47					unsigned int flags)
48{
49	return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
50}
51
52static int pidfd_metadata_fd(pid_t pid, int pidfd)
53{
54	int procfd, ret;
55	char path[100];
56
57	snprintf(path, sizeof(path), "/proc/%d", pid);
58	procfd = open(path, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
59	if (procfd < 0) {
60		warn("Failed to open %s\n", path);
61		return -1;
62	}
63
64	/*
65	 * Verify that the pid has not been recycled and our /proc/<pid> handle
66	 * is still valid.
67	 */
68	ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);
69	if (ret < 0) {
70		switch (errno) {
71		case EPERM:
72			/* Process exists, just not allowed to signal it. */
73			break;
74		default:
75			warn("Failed to signal process\n");
76			close(procfd);
77			procfd = -1;
78		}
79	}
80
81	return procfd;
82}
83
84int main(int argc, char *argv[])
85{
86	int pidfd = -1, ret = EXIT_FAILURE;
87	char buf[4096] = { 0 };
88	pid_t pid;
89	int procfd, statusfd;
90	ssize_t bytes;
91
92	pid = pidfd_clone(CLONE_PIDFD, &pidfd);
93	if (pid < 0)
94		err(ret, "CLONE_PIDFD");
95	if (pidfd == -1) {
96		warnx("CLONE_PIDFD is not supported by the kernel");
97		goto out;
98	}
99
100	procfd = pidfd_metadata_fd(pid, pidfd);
101	close(pidfd);
102	if (procfd < 0)
103		goto out;
104
105	statusfd = openat(procfd, "status", O_RDONLY | O_CLOEXEC);
106	close(procfd);
107	if (statusfd < 0)
108		goto out;
109
110	bytes = read(statusfd, buf, sizeof(buf));
111	if (bytes > 0)
112		bytes = write(STDOUT_FILENO, buf, bytes);
113	close(statusfd);
114	ret = EXIT_SUCCESS;
115
116out:
117	(void)wait(NULL);
118
119	exit(ret);
120}
121