1/*
  2 *  flexible mmap layout support
  3 *
  4 * Copyright 2003-2004 Red Hat Inc., Durham, North Carolina.
  5 * All Rights Reserved.
  6 *
  7 * This program is free software; you can redistribute it and/or modify
  8 * it under the terms of the GNU General Public License as published by
  9 * the Free Software Foundation; either version 2 of the License, or
 10 * (at your option) any later version.
 11 *
 12 * This program is distributed in the hope that it will be useful,
 13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 15 * GNU General Public License for more details.
 16 *
 17 * You should have received a copy of the GNU General Public License
 18 * along with this program; if not, write to the Free Software
 19 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 20 *
 21 *
 22 * Started by Ingo Molnar <mingo@elte.hu>
 23 */
 24
 25#include <linux/personality.h>
 26#include <linux/mm.h>
 27#include <linux/random.h>
 28#include <linux/sched.h>
 29
 30/*
 31 * Top of mmap area (just below the process stack).
 32 *
 33 * Leave at least a ~128 MB hole on 32bit applications.
 34 *
 35 * On 64bit applications we randomise the stack by 1GB so we need to
 36 * space our mmap start address by a further 1GB, otherwise there is a
 37 * chance the mmap area will end up closer to the stack than our ulimit
 38 * requires.
 39 */
 40#define MIN_GAP32 (128*1024*1024)
 41#define MIN_GAP64 ((128 + 1024)*1024*1024UL)
 42#define MIN_GAP ((is_32bit_task()) ? MIN_GAP32 : MIN_GAP64)
 43#define MAX_GAP (TASK_SIZE/6*5)
 44
 45static inline int mmap_is_legacy(void)
 46{
 47	if (current->personality & ADDR_COMPAT_LAYOUT)
 48		return 1;
 49
 50	if (rlimit(RLIMIT_STACK) == RLIM_INFINITY)
 51		return 1;
 52
 53	return sysctl_legacy_va_layout;
 54}
 55
 56unsigned long arch_mmap_rnd(void)
 57{
 58	unsigned long rnd;
 59
 60	/* 8MB for 32bit, 1GB for 64bit */
 61	if (is_32bit_task())
 62		rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
 63	else
 64		rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
 65
 66	return rnd << PAGE_SHIFT;
 67}
 68
 69static inline unsigned long mmap_base(unsigned long rnd)
 70{
 71	unsigned long gap = rlimit(RLIMIT_STACK);
 72
 73	if (gap < MIN_GAP)
 74		gap = MIN_GAP;
 75	else if (gap > MAX_GAP)
 76		gap = MAX_GAP;
 77
 78	return PAGE_ALIGN(TASK_SIZE - gap - rnd);
 79}
 80
 81/*
 82 * This function, called very early during the creation of a new
 83 * process VM image, sets up which VM layout function to use:
 84 */
 85void arch_pick_mmap_layout(struct mm_struct *mm)
 86{
 87	unsigned long random_factor = 0UL;
 88
 89	if (current->flags & PF_RANDOMIZE)
 90		random_factor = arch_mmap_rnd();
 91
 92	/*
 93	 * Fall back to the standard layout if the personality
 94	 * bit is set, or if the expected stack growth is unlimited:
 95	 */
 96	if (mmap_is_legacy()) {
 97		mm->mmap_base = TASK_UNMAPPED_BASE;
 98		mm->get_unmapped_area = arch_get_unmapped_area;
 99	} else {
100		mm->mmap_base = mmap_base(random_factor);
101		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
102	}
103}