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