#define _GNU_SOURCE
#include <stdio.h>
#include <sys/syscall.h>
#include <asm/prctl.h>
#include <asm/ldt.h>
#include <sys/types.h>
#include <stdint.h>
#include <stdlib.h>

int var = 9;
int status = 0;

void print_base(char *who) {
	unsigned long base;
	int val;
	syscall(SYS_arch_prctl, ARCH_GET_GS, &base);
	asm("movl %%gs:0,%0":"=r"(val));
	printf("%s:\tbase %16lx val %d var %p\n", who, base, val, &var);
	if (val != var)
		status = 1;
}

int main(int argc, char *argv[]) {
	unsigned short entry = 1;
	unsigned short selector = (entry*8) | 0x4;
	struct user_desc desc = {
		.entry_number = entry,
		.base_addr = (unsigned) (uintptr_t) &var,
		.limit = 0xfffffffful,
		.contents = MODIFY_LDT_CONTENTS_DATA,
		.read_exec_only = 0,
		.limit_in_pages = 1,
		.seg_not_present = 0,
		.useable = 1,
	};
	pid_t pid;
	int i;

	if (syscall(SYS_modify_ldt, 0x11, &desc, sizeof(desc)))
		perror("modify_ldt");

#if 1
	syscall(SYS_arch_prctl, ARCH_SET_GS, &argc);
#else
	syscall(SYS_arch_prctl, ARCH_SET_GS, &status);
#endif
	asm volatile("movw %w0,%%gs"::"q"(selector));
	print_base("parent");

#if 0
	{
		struct timespec ts = {0, 1000000};
		nanosleep(&ts, NULL);
	}
#endif
	pid = syscall(SYS_fork);
	print_base(pid ? "parent" : "child");
	asm volatile("movw %w0,%%gs"::"q"(selector));
	print_base(pid ? "parent" : "child");
	if (pid)
		waitpid(pid, &status, 0);
	return status != 0;
}