mvillard_nest.c

/* $Id: mvillard_nest.c,v 1.2 2017/09/15 14:33:30 kostik Exp kostik $ */

#include <sys/types.h>
#include <sys/syscall.h>
#include <sys/ucontext.h>
#include <machine/atomic.h>
#include <machine/segments.h>
#include <machine/sysarch.h>
#include <err.h>
#include <pthread.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>

static volatile u_int b, s;

static void *
dealloc_ldt(void *arg __unused)
{
	u_int sl;

	for (;;) {
		while (atomic_load_acq_int(&b) == 0)
			;
		sl = s;
		s = 0;
		if (sl != 0)
			i386_set_ldt(sl, NULL, 1);
		atomic_store_rel_int(&b, 0);
	}
	return (NULL);
}

static void
func(void)
{
	union descriptor desc;
	u_int sel, sl;

	bzero(&desc, sizeof(desc));
	desc.sd.sd_type = SDT_MEMRWA;
	desc.sd.sd_dpl = SEL_UPL;
	desc.sd.sd_p = 1;
	desc.sd.sd_def32 = 1;
	desc.sd.sd_gran = 1;
	desc.sd.sd_lolimit = 0xffff;
	desc.sd.sd_hilimit = 0xf;
	sl = i386_set_ldt(LDT_AUTO_ALLOC, &desc, 1);
	if ((int)sl == -1)
		err(1, "i386_set_ldt");
	sel = LSEL(sl, SEL_UPL);
	s = sl;
	__asm volatile("movw\t%w0,%%es" : : "r" (sel));
	atomic_store_rel_int(&b, 1);
	while (atomic_load_acq_int(&b) != 0)
		;
	getpid();
}

static void
sigsegv_handler(int signo __unused, siginfo_t *si __unused, void *rctx)
{
	ucontext_t *uc;

	uc = rctx;
	uc->uc_mcontext.mc_es = uc->uc_mcontext.mc_ds;
}

int
main(void)
{
	pthread_t thr;
	struct sigaction sa;
	int error;

	bzero(&sa, sizeof(sa));
	sa.sa_sigaction = sigsegv_handler;
	sa.sa_flags = SA_SIGINFO;
	error = sigaction(SIGSEGV, &sa, NULL);
	if (error != 0)
		err(1, "sigaction SIGSEGV");
	error = sigaction(SIGBUS, &sa, NULL);
	if (error != 0)
		err(1, "sigaction SIGBUS");

	error = pthread_create(&thr, NULL, dealloc_ldt, NULL);
	if (error != 0)
		errc(1, error, "pthread_create");
	
	for (;;)
	     func();
}