dl-execstack.c

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/* Stack executability handling for GNU dynamic linker.  Linux version.
   Copyright (C) 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <ldsodefs.h>
#include <sys/mman.h>
#include <errno.h>
#include <libintl.h>
#include <stdbool.h>
#include <stackinfo.h>
#include <caller.h>
#include <sysdep.h>

#include <kernel-features.h>


extern int __stack_prot attribute_relro attribute_hidden;


int
internal_function
_dl_make_stack_executable (void **stack_endp)
{
  /* This gives us the highest/lowest page that needs to be changed.  */
  uintptr_t page = ((uintptr_t) *stack_endp
		    & -(intptr_t) GLRO(dl_pagesize));
  int result = 0;

  /* Challenge the caller.  */
  if (__builtin_expect (__check_caller (RETURN_ADDRESS (0),
					allow_ldso|allow_libpthread) != 0, 0)
      || __builtin_expect (*stack_endp != __libc_stack_end, 0))
    return EPERM;

  /* Newer Linux kernels support a flag to make our job easy.  */
#if defined  PROT_GROWSDOWN || defined PROT_GROWSUP
# if __ASSUME_PROT_GROWSUPDOWN == 0
  static bool no_growsupdown;
  if (! no_growsupdown)
# endif
    {
      if (__builtin_expect (__mprotect ((void *) page, GLRO(dl_pagesize),
					__stack_prot) == 0, 1))
	goto return_success;
# if __ASSUME_PROT_GROWSUPDOWN == 0
      if (errno == EINVAL)
	no_growsupdown = true;
      else
# endif
	{
	  result = errno;
	  goto out;
	}
    }
#endif

  /* There is always a hole in the address space below the bottom of the
     stack.  So when we make an mprotect call that starts below the bottom
     of the stack, it will include the hole and fail with ENOMEM.

     We start with a random guess at how deep the stack might have gotten
     so as to have extended the GROWSDOWN mapping to lower pages.  */

#if __ASSUME_PROT_GROWSUPDOWN == 0
  size_t size = GLRO(dl_pagesize) * 8;

# if _STACK_GROWS_DOWN
  page = page + GLRO(dl_pagesize) - size;
  while (1)
    {
      if (__mprotect ((void *) page, size,
		      __stack_prot & ~PROT_GROWSDOWN) == 0)
	/* We got this chunk changed; loop to do another chunk below.  */
	page -= size;
      else
	{
	  if (errno != ENOMEM)	/* Unexpected failure mode.  */
	    {
	      result = errno;
	      goto out;
	    }

	  if (size == GLRO(dl_pagesize))
	    /* We just tried to mprotect the top hole page and failed.
	       We are done.  */
	    break;

	  /* Our mprotect call failed because it started below the lowest
	     stack page.  Try again on just the top half of that region.  */
	  size /= 2;
	  page += size;
	}
    }

# elif _STACK_GROWS_UP
  while (1)
    {
      if (__mprotect ((void *) page, size, __stack_prot & ~PROT_GROWSUP) == 0)
	/* We got this chunk changed; loop to do another chunk below.  */
	page += size;
      else
	{
	  if (errno != ENOMEM)	/* Unexpected failure mode.  */
	    {
	      result = errno;
	      goto out;
	    }

	  if (size == GLRO(dl_pagesize))
	    /* We just tried to mprotect the lowest hole page and failed.
	       We are done.  */
	    break;

	  /* Our mprotect call failed because it extended past the highest
	     stack page.  Try again on just the bottom half of that region.  */
	  size /= 2;
	}
    }

# else
#  error "Define either _STACK_GROWS_DOWN or _STACK_GROWS_UP"
# endif
#endif

 return_success:
  /* Clear the address.  */
  *stack_endp = NULL;

  /* Remember that we changed the permission.  */
  GL(dl_stack_flags) |= PF_X;

 out:
#ifdef check_consistency
  check_consistency ();
#endif

  return result;
}
rtld_hidden_def (_dl_make_stack_executable)