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 
28 static int do_child(void *args)
29 {
30 	printf("%d\n", getpid());
31 	_exit(EXIT_SUCCESS);
32 }
33 
34 static 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 
46 static 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 
52 static 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 
84 int main(int argc, char *argv[])
85 {
86 	int pidfd = 0, 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 		exit(ret);
95 
96 	procfd = pidfd_metadata_fd(pid, pidfd);
97 	close(pidfd);
98 	if (procfd < 0)
99 		goto out;
100 
101 	statusfd = openat(procfd, "status", O_RDONLY | O_CLOEXEC);
102 	close(procfd);
103 	if (statusfd < 0)
104 		goto out;
105 
106 	bytes = read(statusfd, buf, sizeof(buf));
107 	if (bytes > 0)
108 		bytes = write(STDOUT_FILENO, buf, bytes);
109 	close(statusfd);
110 	ret = EXIT_SUCCESS;
111 
112 out:
113 	(void)wait(NULL);
114 
115 	exit(ret);
116 }
117