#include <stdio.h>

#ifdef __i386__
# include <linux/unistd.h>
# include <asm/ldt.h>
# include <sys/mman.h>
# include <unistd.h>
# include <sys/syscall.h>
# include <errno.h>
#else
# include <asm/prctl.h>
# include <sys/prctl.h>
#endif

int arch_prctl(int code, unsigned long addr);

int test_func()
{
  return (42);
}

int main()
{
#ifdef __i386__
  void *seg = mmap(NULL, getpagesize(), PROT_WRITE | PROT_READ,
                   MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
  struct user_desc u_info;
  int val;

  ((void **)seg)[0x10 / sizeof(void *)] = &test_func;
  u_info.entry_number = 6;
  u_info.base_addr = (unsigned long)seg;
  u_info.limit = getpagesize();
  u_info.seg_32bit = 1;
  u_info.contents = MODIFY_LDT_CONTENTS_DATA;
  u_info.read_exec_only = 0;
  u_info.limit_in_pages = 0;
  u_info.seg_not_present = 0;
  u_info.useable = 1;
  val = (6 << 3 | 0 << 2 | 3);

  /* if a 32 bit program run on a 64 bit system, the first free gdt slot is 12
   * instead of 6 for 32 bit system.
   */
  if (syscall(SYS_set_thread_area, &u_info) < 0 && errno == EINVAL) {
      u_info.entry_number = 12;
      val = (12 << 3 | 0 << 2 | 3);
      if (syscall(SYS_set_thread_area, &u_info) < 0)
          puts("Fail");
  }

  __asm__ volatile ("mov %0, %%eax; \n\
                     mov %0, %%fs; \n\
                     call *%%fs:0x10; \n\
                     mov $0x10, %%eax; \n\
                     call *%%fs:(%%eax);" \
                    : : "m" (val) : "eax");
#else
  void (*funcs[10])();

  funcs[0x10 / sizeof(void *)] = (void *)&test_func;

  arch_prctl(ARCH_SET_GS, (unsigned long)funcs);

  __asm__ volatile ("call *%gs:0x10; \n\
                     mov $0x10, %rax; \n\
                     call *%gs:(%rax);");
#endif
  return 0;
}