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 = -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