LXR linux/arch/x86/mm/mmap.c

   1/*
   2 * Flexible mmap layout support
   3 *
   4 * Based on code by Ingo Molnar and Andi Kleen, copyrighted
   5 * as follows:
   6 *
   7 * Copyright 2003-2009 Red Hat Inc.
   8 * All Rights Reserved.
   9 * Copyright 2005 Andi Kleen, SUSE Labs.
  10 * Copyright 2007 Jiri Kosina, SUSE Labs.
  11 *
  12 * This program is free software; you can redistribute it and/or modify
  13 * it under the terms of the GNU General Public License as published by
  14 * the Free Software Foundation; either version 2 of the License, or
  15 * (at your option) any later version.
  16 *
  17 * This program is distributed in the hope that it will be useful,
  18 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20 * GNU General Public License for more details.
  21 *
  22 * You should have received a copy of the GNU General Public License
  23 * along with this program; if not, write to the Free Software
  24 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  25 */
  26
  27#include <linux/personality.h>
  28#include <linux/mm.h>
  29#include <linux/random.h>
  30#include <linux/limits.h>
  31#include <linux/sched.h>
  32#include <asm/elf.h>
  33
  34struct va_alignment __read_mostly va_align = {
  35        .flags = -1,
  36};
  37
  38static unsigned int stack_maxrandom_size(void)
  39{
  40        unsigned int max = 0;
  41        if ((current->flags & PF_RANDOMIZE) &&
  42                !(current->personality & ADDR_NO_RANDOMIZE)) {
  43                max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
  44        }
  45
  46        return max;
  47}
  48
  49/*
  50 * Top of mmap area (just below the process stack).
  51 *
  52 * Leave an at least ~128 MB hole with possible stack randomization.
  53 */
  54#define MIN_GAP (128*1024*1024UL + stack_maxrandom_size())
  55#define MAX_GAP (TASK_SIZE/6*5)
  56
  57static int mmap_is_legacy(void)
  58{
  59        if (current->personality & ADDR_COMPAT_LAYOUT)
  60                return 1;
  61
  62        if (rlimit(RLIMIT_STACK) == RLIM_INFINITY)
  63                return 1;
  64
  65        return sysctl_legacy_va_layout;
  66}
  67
  68static unsigned long mmap_rnd(void)
  69{
  70        unsigned long rnd = 0;
  71
  72        /*
  73        *  8 bits of randomness in 32bit mmaps, 20 address space bits
  74        * 28 bits of randomness in 64bit mmaps, 40 address space bits
  75        */
  76        if (current->flags & PF_RANDOMIZE) {
  77                if (mmap_is_ia32())
  78                        rnd = get_random_int() % (1<<8);
  79                else
  80                        rnd = get_random_int() % (1<<28);
  81        }
  82        return rnd << PAGE_SHIFT;
  83}
  84
  85static unsigned long mmap_base(void)
  86{
  87        unsigned long gap = rlimit(RLIMIT_STACK);
  88
  89        if (gap < MIN_GAP)
  90                gap = MIN_GAP;
  91        else if (gap > MAX_GAP)
  92                gap = MAX_GAP;
  93
  94        return PAGE_ALIGN(TASK_SIZE - gap - mmap_rnd());
  95}
  96
  97/*
  98 * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64
  99 * does, but not when emulating X86_32
 100 */
 101static unsigned long mmap_legacy_base(void)
 102{
 103        if (mmap_is_ia32())
 104                return TASK_UNMAPPED_BASE;
 105        else
 106                return TASK_UNMAPPED_BASE + mmap_rnd();
 107}
 108
 109/*
 110 * This function, called very early during the creation of a new
 111 * process VM image, sets up which VM layout function to use:
 112 */
 113void arch_pick_mmap_layout(struct mm_struct *mm)
 114{
 115        mm->mmap_legacy_base = mmap_legacy_base();
 116        mm->mmap_base = mmap_base();
 117
 118        if (mmap_is_legacy()) {
 119                mm->mmap_base = mm->mmap_legacy_base;
 120                mm->get_unmapped_area = arch_get_unmapped_area;
 121        } else {
 122                mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 123        }
 124}
 125