mmap.c

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/*
 * Based on arch/arm/mm/mmap.c
 *
 * Copyright (C) 2012 ARM Ltd.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * 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, see <http://www.gnu.org/licenses/>.
 */

#include <linux/elf.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/export.h>
#include <linux/shm.h>
#include <linux/sched.h>
#include <linux/io.h>
#include <linux/personality.h>
#include <linux/random.h>

#include <asm/cputype.h>

/*
 * Leave enough space between the mmap area and the stack to honour ulimit in
 * the face of randomisation.
 */
#define MIN_GAP (SZ_128M + ((STACK_RND_MASK << PAGE_SHIFT) + 1))
#define MAX_GAP (STACK_TOP/6*5)

static 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;
}

/*
 * Since get_random_int() returns the same value within a 1 jiffy window, we
 * will almost always get the same randomisation for the stack and mmap
 * region. This will mean the relative distance between stack and mmap will be
 * the same.
 *
 * To avoid this we can shift the randomness by 1 bit.
 */
static unsigned long mmap_rnd(void)
{
    unsigned long rnd = 0;

    if (current->flags & PF_RANDOMIZE)
        rnd = (long)get_random_int() & (STACK_RND_MASK >> 1);

    return rnd << (PAGE_SHIFT + 1);
}

static 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(STACK_TOP - 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;
    }
}
EXPORT_SYMBOL_GPL(arch_pick_mmap_layout);


/*
 * You really shouldn't be using read() or write() on /dev/mem.  This might go
 * away in the future.
 */
int valid_phys_addr_range(unsigned long addr, size_t size)
{
    if (addr < PHYS_OFFSET)
        return 0;
    if (addr + size > __pa(high_memory - 1) + 1)
        return 0;

    return 1;
}

/*
 * Do not allow /dev/mem mappings beyond the supported physical range.
 */
int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
{
    return !(((pfn << PAGE_SHIFT) + size) & ~PHYS_MASK);
}

#ifdef CONFIG_STRICT_DEVMEM

#include <linux/ioport.h>

/*
 * devmem_is_allowed() checks to see if /dev/mem access to a certain address
 * is valid. The argument is a physical page number.  We mimic x86 here by
 * disallowing access to system RAM as well as device-exclusive MMIO regions.
 * This effectively disable read()/write() on /dev/mem.
 */
int devmem_is_allowed(unsigned long pfn)
{
    if (iomem_is_exclusive(pfn << PAGE_SHIFT))
        return 0;
    if (!page_is_ram(pfn))
        return 1;
    return 0;
}

#endif