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  */
  44 static 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
  56 static 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
  67 static 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
  84 static 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  */
 100 static 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  */
 112 void 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 }