gistfile1.txt

#define _GNU_SOURCE
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	char *args[64];
	struct stat st;
	sigset_t set;
	char path[128];
	void *map;
	size_t size;
	int sig, fd_exe, fd_map, fd;
	pid_t pid;
	ssize_t res;

	/*
	 * If called with arguments, we just act as a sleeping process that
	 * waits for any signal to arrive.
	 */
	if (argc > 1) {
		printf("dummy waiter init\n");
		/* dummy waiter; SIGTERM terminates us anyway */
		sigemptyset(&set);
		sigaddset(&set, SIGTERM);
		sigwait(&set, &sig);
		printf("dummy waiter exit\n");
		exit(0);
	}

	fd_exe = open("/proc/self/exe", O_RDONLY);
	if (fd_exe < 0) {
		printf("open(/proc/self/exe) failed: %m\n");
		abort();
	}

	if (fstat(fd_exe, &st) < 0) {
		printf("fstat(fd_exe) failed: %m\n");
		abort();
	}

	/* page-align */
	size = (st.st_size + 4095ULL) & ~4095ULL;

	map = mmap(NULL, size, PROT_READ | PROT_WRITE | PROT_EXEC,
		   MAP_SHARED | MAP_ANONYMOUS, -1, 0);
	if (map == MAP_FAILED) {
		printf("mmap(MAP_ANON) failed: %m\n");
		abort();
	}

	res = read(fd_exe, map, st.st_size);
	if (res != st.st_size) {
		if (res < 0)
			printf("read(fd_exe) failed: %m\n");
		else
			printf("read(fd_exe) was truncated to %zu\n",
			       (size_t)res);
		abort();
	}

	sprintf(path, "/proc/self/map_files/%lx-%lx",
		(unsigned long)map,
		(unsigned long)map + size);
	fd_map = open(path, O_RDONLY);
	if (fd_map < 0) {
		printf("open(%s) failed: %m\n", path);
		abort();
	}

	pid = fork();
	if (pid < 0) {
		printf("fork() failed: %m\n");
		abort();
	} else if (!pid) {
		args[0] = argv[0];
		args[1] = "dummy";
		args[2] = NULL;
		execve(path, args, NULL);
		printf("execve() failed: %m\n");
		abort();
	}

	/* sleep 100ms to make sure the execve() really worked */
	usleep(100 * 1000ULL);

	sprintf(path, "/proc/self/fd/%d", fd_map);
	fd = open(path, O_RDWR);
	if (fd < 0) {
		printf("open(%s) failed: %m\n", path);
		abort();
	}

	munmap(map, size);
	close(fd_exe);

	close(fd);
	close(fd_map);
	printf("exiting\n");

	return 0;
}