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
  34 struct __read_mostly va_alignment va_align = {
  35         .flags = -1,
  36 };
  37
  38 static 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
  57 static 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
  68 static 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
  85 static 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  */
 101 static 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  */
 113 void 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 }