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 va_alignment __read_mostly va_align = {
  35         .flags = -1,
  36 };
  37
  38 static unsigned long stack_maxrandom_size(void)
  39 {
  40         unsigned long max = 0;
  41         if ((current->flags & PF_RANDOMIZE) &&
  42                 !(current->personality & ADDR_NO_RANDOMIZE)) {
  43                 max = ((-1UL) & 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 unsigned long arch_mmap_rnd(void)
  69 {
  70         unsigned long rnd;
  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 (mmap_is_ia32())
  77                 rnd = (unsigned long)get_random_int() % (1<<8);
  78         else
  79                 rnd = (unsigned long)get_random_int() % (1<<28);
  80
  81         return rnd << PAGE_SHIFT;
  82 }
  83
  84 static unsigned long mmap_base(unsigned long rnd)
  85 {
  86         unsigned long gap = rlimit(RLIMIT_STACK);
  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 - 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  */
 100 static unsigned long mmap_legacy_base(unsigned long rnd)
 101 {
 102         if (mmap_is_ia32())
 103                 return TASK_UNMAPPED_BASE;
 104         else
 105                 return TASK_UNMAPPED_BASE + 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  */
 112 void arch_pick_mmap_layout(struct mm_struct *mm)
 113 {
 114         unsigned long random_factor = 0UL;
 115
 116         if (current->flags & PF_RANDOMIZE)
 117                 random_factor = arch_mmap_rnd();
 118
 119         mm->mmap_legacy_base = mmap_legacy_base(random_factor);
 120
 121         if (mmap_is_legacy()) {
 122                 mm->mmap_base = mm->mmap_legacy_base;
 123                 mm->get_unmapped_area = arch_get_unmapped_area;
 124         } else {
 125                 mm->mmap_base = mmap_base(random_factor);
 126                 mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 127         }
 128 }