mmap_64.c

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/*
 *  flexible mmap layout support
 *
 * Copyright 2003-2004 Red Hat Inc., Durham, North Carolina.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 *
 * Started by Ingo Molnar <[email protected]>
 */

#include <linux/personality.h>
#include <linux/mm.h>
#include <linux/random.h>
#include <linux/sched.h>

/*
 * Top of mmap area (just below the process stack).
 *
 * Leave at least a ~128 MB hole on 32bit applications.
 *
 * On 64bit applications we randomise the stack by 1GB so we need to
 * space our mmap start address by a further 1GB, otherwise there is a
 * chance the mmap area will end up closer to the stack than our ulimit
 * requires.
 */
#define MIN_GAP32 (128*1024*1024)
#define MIN_GAP64 ((128 + 1024)*1024*1024UL)
#define MIN_GAP ((is_32bit_task()) ? MIN_GAP32 : MIN_GAP64)
#define MAX_GAP (TASK_SIZE/6*5)

static inline int mmap_is_legacy(void)
{
    if (current->personality & ADDR_COMPAT_LAYOUT)
        return 1;

    if (rlimit(RLIMIT_STACK) == RLIM_INFINITY)
        return 1;

    return sysctl_legacy_va_layout;
}

static unsigned long mmap_rnd(void)
{
    unsigned long rnd = 0;

    if (current->flags & PF_RANDOMIZE) {
        /* 8MB for 32bit, 1GB for 64bit */
        if (is_32bit_task())
            rnd = (long)(get_random_int() % (1<<(23-PAGE_SHIFT)));
        else
            rnd = (long)(get_random_int() % (1<<(30-PAGE_SHIFT)));
    }
    return rnd << PAGE_SHIFT;
}

static inline unsigned long mmap_base(void)
{
    unsigned long gap = rlimit(RLIMIT_STACK);

    if (gap < MIN_GAP)
        gap = MIN_GAP;
    else if (gap > MAX_GAP)
        gap = MAX_GAP;

    return PAGE_ALIGN(TASK_SIZE - gap - mmap_rnd());
}

/*
 * This function, called very early during the creation of a new
 * process VM image, sets up which VM layout function to use:
 */
void arch_pick_mmap_layout(struct mm_struct *mm)
{
    /*
     * Fall back to the standard layout if the personality
     * bit is set, or if the expected stack growth is unlimited:
     */
    if (mmap_is_legacy()) {
        mm->mmap_base = TASK_UNMAPPED_BASE;
        mm->get_unmapped_area = arch_get_unmapped_area;
        mm->unmap_area = arch_unmap_area;
    } else {
        mm->mmap_base = mmap_base();
        mm->get_unmapped_area = arch_get_unmapped_area_topdown;
        mm->unmap_area = arch_unmap_area_topdown;
    }
}