linux/arch/x86/mm/mmap.c
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   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
  34static unsigned int stack_maxrandom_size(void)
  35{
  36        unsigned int max = 0;
  37        if ((current->flags & PF_RANDOMIZE) &&
  38                !(current->personality & ADDR_NO_RANDOMIZE)) {
  39                max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
  40        }
  41
  42        return max;
  43}
  44
  45
  46/*
  47 * Top of mmap area (just below the process stack).
  48 *
  49 * Leave an at least ~128 MB hole with possible stack randomization.
  50 */
  51#define MIN_GAP (128*1024*1024UL + stack_maxrandom_size())
  52#define MAX_GAP (TASK_SIZE/6*5)
  53
  54/*
  55 * True on X86_32 or when emulating IA32 on X86_64
  56 */
  57static int mmap_is_ia32(void)
  58{
  59#ifdef CONFIG_X86_32
  60        return 1;
  61#endif
  62#ifdef CONFIG_IA32_EMULATION
  63        if (test_thread_flag(TIF_IA32))
  64                return 1;
  65#endif
  66        return 0;
  67}
  68
  69static int mmap_is_legacy(void)
  70{
  71        if (current->personality & ADDR_COMPAT_LAYOUT)
  72                return 1;
  73
  74        if (rlimit(RLIMIT_STACK) == RLIM_INFINITY)
  75                return 1;
  76
  77        return sysctl_legacy_va_layout;
  78}
  79
  80static unsigned long mmap_rnd(void)
  81{
  82        unsigned long rnd = 0;
  83
  84        /*
  85        *  8 bits of randomness in 32bit mmaps, 20 address space bits
  86        * 28 bits of randomness in 64bit mmaps, 40 address space bits
  87        */
  88        if (current->flags & PF_RANDOMIZE) {
  89                if (mmap_is_ia32())
  90                        rnd = (long)get_random_int() % (1<<8);
  91                else
  92                        rnd = (long)(get_random_int() % (1<<28));
  93        }
  94        return rnd << PAGE_SHIFT;
  95}
  96
  97static unsigned long mmap_base(void)
  98{
  99        unsigned long gap = rlimit(RLIMIT_STACK);
 100
 101        if (gap < MIN_GAP)
 102                gap = MIN_GAP;
 103        else if (gap > MAX_GAP)
 104                gap = MAX_GAP;
 105
 106        return PAGE_ALIGN(TASK_SIZE - gap - mmap_rnd());
 107}
 108
 109/*
 110 * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64
 111 * does, but not when emulating X86_32
 112 */
 113static unsigned long mmap_legacy_base(void)
 114{
 115        if (mmap_is_ia32())
 116                return TASK_UNMAPPED_BASE;
 117        else
 118                return TASK_UNMAPPED_BASE + mmap_rnd();
 119}
 120
 121/*
 122 * This function, called very early during the creation of a new
 123 * process VM image, sets up which VM layout function to use:
 124 */
 125void arch_pick_mmap_layout(struct mm_struct *mm)
 126{
 127        if (mmap_is_legacy()) {
 128                mm->mmap_base = mmap_legacy_base();
 129                mm->get_unmapped_area = arch_get_unmapped_area;
 130                mm->unmap_area = arch_unmap_area;
 131        } else {
 132                mm->mmap_base = mmap_base();
 133                mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 134                mm->unmap_area = arch_unmap_area_topdown;
 135        }
 136}
 137
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