#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/syscall.h>
#include <sys/user.h>
#include <unistd.h>
#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <err.h>

static siginfo_t wait_trap(pid_t chld)
{
	siginfo_t si;
	if (waitid(P_PID, chld, &si, WEXITED|WSTOPPED) != 0)
		err(1, "waitid");
	if (si.si_pid != chld)
		errx(1, "got unexpected pid in event\n");
	if (si.si_code != CLD_TRAPPED)
		errx(1, "got unexpected event type %d\n", si.si_code);
	return si;
}

int main()
{
	unsigned long tmp;

	pid_t chld = fork();
	if (chld < 0)
		err(1, "fork");

	if (chld == 0) {
		if (ptrace(PTRACE_TRACEME, 0, 0, 0) != 0)
			err(1, "PTRACE_TRACEME");
		raise(SIGSTOP);

		fork();
		return 0;
	}

	int status;

	/* Wait for SIGSTOP and enable seccomp tracing. */
	if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
		err(1, "waitpid");

	if (ptrace(PTRACE_SETOPTIONS, chld, 0, PTRACE_O_TRACEFORK) != 0)
		err(1, "PTRACE_O_TRACEFORK");
	if (ptrace(PTRACE_CONT, chld, 0, 0) != 0)
		err(1, "PTRACE_CONT");

	wait_trap(chld);

	errno = 0;
	tmp = ptrace(PTRACE_PEEKUSER, chld,
		     offsetof(struct user_regs_struct, rip), NULL);
	if (errno)
		err(1, "PTRACE_PEEKUSER");
	printf("child RIP = 0x%lx\n", tmp);

	if (ptrace(PTRACE_POKEUSER, chld,
		   offsetof(struct user_regs_struct, rip), (void *)(1ULL << 48)) != 0)
		err(1, "PTRACE_POKEUSER");

	errno = 0;
	tmp = ptrace(PTRACE_PEEKUSER, chld, offsetof(struct user_regs_struct, rip), NULL);
	if (errno)
		err(1, "PTRACE_PEEKUSER");
	printf("child RIP = 0x%lx\n", tmp);

	if (ptrace(PTRACE_CONT, chld, 0, 0) != 0)
		err(1, "PTRACE_CONT");
	ptrace(PTRACE_DETACH, chld, 0, 0);

	return 0;
}