/*
 * Copyright (c) 2014-2015 Andy Lutomirski
 * GPL v2
 */

#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <err.h>
#include <asm/ldt.h>
#include <sys/syscall.h>

static unsigned short GDT3(int idx)
{
	return (idx << 3) | 3;
}

static int create_tls(int idx, unsigned int base)
{
	struct user_desc desc = {
		.entry_number    = idx,
		.base_addr       = base,
		.limit           = 0xfffff,
		.seg_32bit       = 1,
		.contents        = 0, /* Data, grow-up */
		.read_exec_only  = 0,
		.limit_in_pages  = 1,
		.seg_not_present = 0,
		.useable         = 0,
	};

	if (syscall(SYS_set_thread_area, &desc) != 0)
		err(1, "set_thread_area");

	return desc.entry_number;
}

int main()
{
	int idx = create_tls(-1, 0);
	printf("Allocated GDT index %d\n", idx);

	unsigned short orig_es;
	asm volatile ("mov %%es,%0" : "=rm" (orig_es));

	int errors = 0;
	int total = 1000;
	for (int i = 0; i < total; i++) {
		struct timespec req = {
			.tv_sec = 0,
			.tv_nsec = 100000,
		};
		int ret;

		asm volatile ("mov %0,%%es" : : "rm" (GDT3(idx)));

		/*
		 * Force rescheduling.  On 32-bit kernels, fast syscalls
		 * destroy DS and ES, so force int 80.
		 */
		asm volatile ("int $0x80"
			      : "=a" (ret)
			      : "a" (SYS_nanosleep), "b" (&req),
				"c" (0));

		unsigned short es;
		asm volatile ("mov %%es,%0" : "=rm" (es));
		asm volatile ("mov %0,%%es" : : "rm" (orig_es));
		if (es != GDT3(idx)) {
			if (errors == 0)
				printf("[FAIL]\tES changed from 0x%hx to 0x%hx\n",
				       GDT3(idx), es);
			errors++;
		}
	}

	if (errors) {
		printf("[FAIL]\tES was corrupted %d/%d times\n", errors, total);
		return 1;
	} else {
		printf("[OK]\tES was preserved\n");
		return 0;
	}
}