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
do_child(void * args)28 static int do_child(void *args)
29 {
30 printf("%d\n", getpid());
31 _exit(EXIT_SUCCESS);
32 }
33
pidfd_clone(int flags,int * pidfd)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
sys_pidfd_send_signal(int pidfd,int sig,siginfo_t * info,unsigned int flags)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
pidfd_metadata_fd(pid_t pid,int pidfd)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
main(int argc,char * argv[])84 int 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
116 out:
117 (void)wait(NULL);
118
119 exit(ret);
120 }
121