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 2003-2004 Red Hat Inc., Durham, North Carolina.
   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
  33/*
  34 * Top of mmap area (just below the process stack).
  35 *
  36 * Leave an at least ~128 MB hole.
  37 */
  38#define MIN_GAP (128*1024*1024)
  39#define MAX_GAP (TASK_SIZE/6*5)
  40
  41/*
  42 * True on X86_32 or when emulating IA32 on X86_64
  43 */
  44static int mmap_is_ia32(void)
  45{
  46#ifdef CONFIG_X86_32
  47        return 1;
  48#endif
  49#ifdef CONFIG_IA32_EMULATION
  50        if (test_thread_flag(TIF_IA32))
  51                return 1;
  52#endif
  53        return 0;
  54}
  55
  56static int mmap_is_legacy(void)
  57{
  58        if (current->personality & ADDR_COMPAT_LAYOUT)
  59                return 1;
  60
  61        if (current->signal->rlim[RLIMIT_STACK].rlim_cur == RLIM_INFINITY)
  62                return 1;
  63
  64        return sysctl_legacy_va_layout;
  65}
  66
  67static unsigned long mmap_rnd(void)
  68{
  69        unsigned long rnd = 0;
  70
  71        /*
  72        *  8 bits of randomness in 32bit mmaps, 20 address space bits
  73        * 28 bits of randomness in 64bit mmaps, 40 address space bits
  74        */
  75        if (current->flags & PF_RANDOMIZE) {
  76                if (mmap_is_ia32())
  77                        rnd = (long)get_random_int() % (1<<8);
  78                else
  79                        rnd = (long)(get_random_int() % (1<<28));
  80        }
  81        return rnd << PAGE_SHIFT;
  82}
  83
  84static unsigned long mmap_base(void)
  85{
  86        unsigned long gap = current->signal->rlim[RLIMIT_STACK].rlim_cur;
  87
  88        if (gap < MIN_GAP)
  89                gap = MIN_GAP;
  90        else if (gap > MAX_GAP)
  91                gap = MAX_GAP;
  92
  93        return PAGE_ALIGN(TASK_SIZE - gap - mmap_rnd());
  94}
  95
  96/*
  97 * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64
  98 * does, but not when emulating X86_32
  99 */
 100static unsigned long mmap_legacy_base(void)
 101{
 102        if (mmap_is_ia32())
 103                return TASK_UNMAPPED_BASE;
 104        else
 105                return TASK_UNMAPPED_BASE + mmap_rnd();
 106}
 107
 108/*
 109 * This function, called very early during the creation of a new
 110 * process VM image, sets up which VM layout function to use:
 111 */
 112void arch_pick_mmap_layout(struct mm_struct *mm)
 113{
 114        if (mmap_is_legacy()) {
 115                mm->mmap_base = mmap_legacy_base();
 116                mm->get_unmapped_area = arch_get_unmapped_area;
 117                mm->unmap_area = arch_unmap_area;
 118        } else {
 119                mm->mmap_base = mmap_base();
 120                mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 121                mm->unmap_area = arch_unmap_area_topdown;
 122        }
 123}
 124