libgo/runtime/go-trampoline.c

   1 /* go-trampoline.c -- allocate a trampoline for a nested function.
   2
   3    Copyright 2009 The Go Authors. All rights reserved.
   4    Use of this source code is governed by a BSD-style
   5    license that can be found in the LICENSE file.  */
   6
   7 #include "config.h"
   8
   9 #include <stddef.h>
  10 #include <stdint.h>
  11 #include <unistd.h>
  12
  13 #ifdef HAVE_SYS_MMAN_H
  14 #include <sys/mman.h>
  15 #endif
  16
  17 #include "go-alloc.h"
  18 #include "go-assert.h"
  19
  20 /* In order to build a trampoline we need space which is both writable
  21    and executable.  We currently just allocate a whole page.  This
  22    needs to be more system dependent.  */
  23
  24 void *
  25 __go_allocate_trampoline (uintptr_t size, void *closure)
  26 {
  27   unsigned int page_size;
  28   void *ret;
  29   size_t off;
  30
  31   page_size = getpagesize ();
  32   __go_assert (page_size >= size);
  33   ret = __go_alloc (2 * page_size - 1);
  34   ret = (void *) (((uintptr_t) ret + page_size - 1)
  35                   & ~ ((uintptr_t) page_size - 1));
  36
  37   /* Because the garbage collector only looks at correct address
  38      offsets, we need to ensure that it will see the closure
  39      address.  */
  40   off = ((size + sizeof (void *) - 1) / sizeof (void *)) * sizeof (void *);
  41   __go_assert (size + off + sizeof (void *) <= page_size);
  42   __builtin_memcpy (ret + off, &closure, sizeof (void *));
  43
  44 #ifdef HAVE_SYS_MMAN_H
  45   {
  46     int i;
  47     i = mprotect (ret, size, PROT_READ | PROT_WRITE | PROT_EXEC);
  48     __go_assert (i == 0);
  49   }
  50 #endif
  51
  52   return ret;
  53 }