LXR linux/samples/pidfd/pidfd-metadata.c

   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