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Linux/arch/x86/mm/mmap.c

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  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 }
125 

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