libmudflap/mf-heuristics.c

/* Mudflap: narrow-pointer bounds-checking by tree rewriting.
   Copyright (C) 2002-2013 Free Software Foundation, Inc.
   Contributed by Frank Ch. Eigler <fche@redhat.com>
   and Graydon Hoare <graydon@redhat.com>

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC 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 General Public License
for more details.

Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.

You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
<http://www.gnu.org/licenses/>.  */

#include "config.h"

#include <stdio.h>

#include "mf-runtime.h"
#include "mf-impl.h"

#ifdef _MUDFLAP
#error "Do not compile this file with -fmudflap!"
#endif


extern char _end[];
extern char ENTRY_POINT[];


/* Run some quick validation of the given region.
   Return -1 / 0 / 1 if the access known-invalid, possibly-valid, or known-valid.
*/
int
__mf_heuristic_check (uintptr_t ptr, uintptr_t ptr_high)
{
  VERBOSE_TRACE ("mf: heuristic check\n");

  /* XXX: Disable the stack bounding check for libmudflapth.  We do
     actually have enough information to track stack bounds (see
     __mf_pthread_info in mf-hooks.c), so with a bit of future work,
     this heuristic can be turned on.  */
#ifndef LIBMUDFLAPTH

  /* The first heuristic is to check stack bounds.  This is a
     transient condition and quick to check. */
  if (__mf_opts.heur_stack_bound)
    {
      uintptr_t stack_top_guess = (uintptr_t)__builtin_frame_address(0);
#if defined(__i386__) && defined (__linux__)
      uintptr_t stack_segment_base = 0xC0000000; /* XXX: Bad assumption. */
#else
      /* Cause tests to fail. */
      uintptr_t stack_segment_base = 0;
#endif

      VERBOSE_TRACE ("mf: stack estimated as %p-%p\n",
		     (void *) stack_top_guess, (void *) stack_segment_base);

      if (ptr_high <= stack_segment_base &&
	  ptr >= stack_top_guess &&
	  ptr_high >= ptr)
	{
	  return 1;
	}
    }
#endif


  /* The second heuristic is to scan the range of memory regions
     listed in /proc/self/maps, a special file provided by the Linux
     kernel.  Its results may be cached, and in fact, a GUESS object
     may as well be recorded for interesting matching sections.  */
  if (__mf_opts.heur_proc_map)
    {
      /* Keep a record of seen records from /proc/self/map.  */
      enum { max_entries = 500 };
      struct proc_self_map_entry
      {
	uintptr_t low;
	uintptr_t high;
      };
      static struct proc_self_map_entry entry [max_entries];
      static unsigned entry_used [max_entries];

      /* Look for a known proc_self_map entry that may cover this
	 region.  If one exists, then this heuristic has already run,
	 and should not be run again.  The check should be allowed to
	 fail.  */
      unsigned i;
      unsigned deja_vu = 0;
      for (i=0; i<max_entries; i++)
	{
	  if (entry_used[i] &&
	      (entry[i].low <= ptr) &&
	      (entry[i].high >= ptr_high))
	    deja_vu = 1;
	}

      if (! deja_vu)
	{
	  /* Time to run the heuristic.  Rescan /proc/self/maps; update the
	     entry[] array; XXX: remove expired entries, add new ones.
	     XXX: Consider entries that have grown (e.g., stack).  */
	  char buf[512];
	  char flags[4];
	  void *low, *high;
	  FILE *fp;

	  fp = fopen ("/proc/self/maps", "r");
	  if (fp)
	    {
	      while (fgets (buf, sizeof(buf), fp))
		{
		  if (sscanf (buf, "%p-%p %4c", &low, &high, flags) == 3)
		    {
		      if ((uintptr_t) low <= ptr &&
			  (uintptr_t) high >= ptr_high)
			{
			  for (i=0; i<max_entries; i++)
			    {
			      if (! entry_used[i])
				{
				  entry[i].low = (uintptr_t) low;
				  entry[i].high = (uintptr_t) high;
				  entry_used[i] = 1;
				  break;
				}
			    }

			  VERBOSE_TRACE ("mf: registering region #%d "
					 "%p-%p given %s",
					 i, (void *) low, (void *) high, buf);

			  __mfu_register ((void *) low, (size_t) (high-low),
					  __MF_TYPE_GUESS,
					  "/proc/self/maps segment");

			  return 0; /* undecided (tending to cachable) */
			}
		    }
		}
	      fclose (fp);
	    }
	}
    }


  /* The third heuristic is to approve all accesses between _start (or its
     equivalent for the given target) and _end, which should include all
     text and initialized data.  */
  if (__mf_opts.heur_start_end)
    if (ptr >= (uintptr_t) & ENTRY_POINT && ptr_high <= (uintptr_t) & _end)
      return 1; /* uncacheable */

  return 0; /* unknown */
}