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 static 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  */
  57 static 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
  69 static 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
  80 static 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 = get_random_int() % (1<<8);
  91                 else
  92                         rnd = get_random_int() % (1<<28);
  93         }
  94         return rnd << PAGE_SHIFT;
  95 }
  96
  97 static 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  */
 113 static 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  */
 125 void 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 }