ashmem.c

Go to the documentation of this file.

/* mm/ashmem.c
 *
 * Anonymous Shared Memory Subsystem, ashmem
 *
 * Copyright (C) 2008 Google, Inc.
 *
 * Robert Love <[email protected]>
 *
 * This software is licensed under the terms of the GNU General Public
 * License version 2, as published by the Free Software Foundation, and
 * may be copied, distributed, and modified under those terms.
 *
 * 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.
 */

#define pr_fmt(fmt) "ashmem: " fmt

#include <linux/module.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/falloc.h>
#include <linux/miscdevice.h>
#include <linux/security.h>
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/uaccess.h>
#include <linux/personality.h>
#include <linux/bitops.h>
#include <linux/mutex.h>
#include <linux/shmem_fs.h>
#include "ashmem.h"

#define ASHMEM_NAME_PREFIX "dev/ashmem/"
#define ASHMEM_NAME_PREFIX_LEN (sizeof(ASHMEM_NAME_PREFIX) - 1)
#define ASHMEM_FULL_NAME_LEN (ASHMEM_NAME_LEN + ASHMEM_NAME_PREFIX_LEN)

/*
 * ashmem_area - anonymous shared memory area
 * Lifecycle: From our parent file's open() until its release()
 * Locking: Protected by `ashmem_mutex'
 * Big Note: Mappings do NOT pin this structure; it dies on close()
 */
struct ashmem_area {
    char name[ASHMEM_FULL_NAME_LEN]; /* optional name in /proc/pid/maps */
    struct list_head unpinned_list;  /* list of all ashmem areas */
    struct file *file;       /* the shmem-based backing file */
    size_t size;             /* size of the mapping, in bytes */
    unsigned long prot_mask;     /* allowed prot bits, as vm_flags */
};

/*
 * ashmem_range - represents an interval of unpinned (evictable) pages
 * Lifecycle: From unpin to pin
 * Locking: Protected by `ashmem_mutex'
 */
struct ashmem_range {
    struct list_head lru;       /* entry in LRU list */
    struct list_head unpinned;  /* entry in its area's unpinned list */
    struct ashmem_area *asma;   /* associated area */
    size_t pgstart;         /* starting page, inclusive */
    size_t pgend;           /* ending page, inclusive */
    unsigned int purged;        /* ASHMEM_NOT or ASHMEM_WAS_PURGED */
};

/* LRU list of unpinned pages, protected by ashmem_mutex */
static LIST_HEAD(ashmem_lru_list);

/* Count of pages on our LRU list, protected by ashmem_mutex */
static unsigned long lru_count;

/*
 * ashmem_mutex - protects the list of and each individual ashmem_area
 *
 * Lock Ordering: ashmex_mutex -> i_mutex -> i_alloc_sem
 */
static DEFINE_MUTEX(ashmem_mutex);

static struct kmem_cache *ashmem_area_cachep __read_mostly;
static struct kmem_cache *ashmem_range_cachep __read_mostly;

#define range_size(range) \
    ((range)->pgend - (range)->pgstart + 1)

#define range_on_lru(range) \
    ((range)->purged == ASHMEM_NOT_PURGED)

#define page_range_subsumes_range(range, start, end) \
    (((range)->pgstart >= (start)) && ((range)->pgend <= (end)))

#define page_range_subsumed_by_range(range, start, end) \
    (((range)->pgstart <= (start)) && ((range)->pgend >= (end)))

#define page_in_range(range, page) \
    (((range)->pgstart <= (page)) && ((range)->pgend >= (page)))

#define page_range_in_range(range, start, end) \
    (page_in_range(range, start) || page_in_range(range, end) || \
        page_range_subsumes_range(range, start, end))

#define range_before_page(range, page) \
    ((range)->pgend < (page))

#define PROT_MASK       (PROT_EXEC | PROT_READ | PROT_WRITE)

static inline void lru_add(struct ashmem_range *range)
{
    list_add_tail(&range->lru, &ashmem_lru_list);
    lru_count += range_size(range);
}

static inline void lru_del(struct ashmem_range *range)
{
    list_del(&range->lru);
    lru_count -= range_size(range);
}

/*
 * range_alloc - allocate and initialize a new ashmem_range structure
 *
 * 'asma' - associated ashmem_area
 * 'prev_range' - the previous ashmem_range in the sorted asma->unpinned list
 * 'purged' - initial purge value (ASMEM_NOT_PURGED or ASHMEM_WAS_PURGED)
 * 'start' - starting page, inclusive
 * 'end' - ending page, inclusive
 *
 * Caller must hold ashmem_mutex.
 */
static int range_alloc(struct ashmem_area *asma,
               struct ashmem_range *prev_range, unsigned int purged,
               size_t start, size_t end)
{
    struct ashmem_range *range;

    range = kmem_cache_zalloc(ashmem_range_cachep, GFP_KERNEL);
    if (unlikely(!range))
        return -ENOMEM;

    range->asma = asma;
    range->pgstart = start;
    range->pgend = end;
    range->purged = purged;

    list_add_tail(&range->unpinned, &prev_range->unpinned);

    if (range_on_lru(range))
        lru_add(range);

    return 0;
}

static void range_del(struct ashmem_range *range)
{
    list_del(&range->unpinned);
    if (range_on_lru(range))
        lru_del(range);
    kmem_cache_free(ashmem_range_cachep, range);
}

/*
 * range_shrink - shrinks a range
 *
 * Caller must hold ashmem_mutex.
 */
static inline void range_shrink(struct ashmem_range *range,
                size_t start, size_t end)
{
    size_t pre = range_size(range);

    range->pgstart = start;
    range->pgend = end;

    if (range_on_lru(range))
        lru_count -= pre - range_size(range);
}

static int ashmem_open(struct inode *inode, struct file *file)
{
    struct ashmem_area *asma;
    int ret;

    ret = generic_file_open(inode, file);
    if (unlikely(ret))
        return ret;

    asma = kmem_cache_zalloc(ashmem_area_cachep, GFP_KERNEL);
    if (unlikely(!asma))
        return -ENOMEM;

    INIT_LIST_HEAD(&asma->unpinned_list);
    memcpy(asma->name, ASHMEM_NAME_PREFIX, ASHMEM_NAME_PREFIX_LEN);
    asma->prot_mask = PROT_MASK;
    file->private_data = asma;

    return 0;
}

static int ashmem_release(struct inode *ignored, struct file *file)
{
    struct ashmem_area *asma = file->private_data;
    struct ashmem_range *range, *next;

    mutex_lock(&ashmem_mutex);
    list_for_each_entry_safe(range, next, &asma->unpinned_list, unpinned)
        range_del(range);
    mutex_unlock(&ashmem_mutex);

    if (asma->file)
        fput(asma->file);
    kmem_cache_free(ashmem_area_cachep, asma);

    return 0;
}

static ssize_t ashmem_read(struct file *file, char __user *buf,
               size_t len, loff_t *pos)
{
    struct ashmem_area *asma = file->private_data;
    int ret = 0;

    mutex_lock(&ashmem_mutex);

    /* If size is not set, or set to 0, always return EOF. */
    if (asma->size == 0)
        goto out;

    if (!asma->file) {
        ret = -EBADF;
        goto out;
    }

    ret = asma->file->f_op->read(asma->file, buf, len, pos);
    if (ret < 0)
        goto out;

    asma->file->f_pos = *pos;

out:
    mutex_unlock(&ashmem_mutex);
    return ret;
}

static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin)
{
    struct ashmem_area *asma = file->private_data;
    int ret;

    mutex_lock(&ashmem_mutex);

    if (asma->size == 0) {
        ret = -EINVAL;
        goto out;
    }

    if (!asma->file) {
        ret = -EBADF;
        goto out;
    }

    ret = asma->file->f_op->llseek(asma->file, offset, origin);
    if (ret < 0)
        goto out;

    file->f_pos = asma->file->f_pos;

out:
    mutex_unlock(&ashmem_mutex);
    return ret;
}

static inline vm_flags_t calc_vm_may_flags(unsigned long prot)
{
    return _calc_vm_trans(prot, PROT_READ,  VM_MAYREAD) |
           _calc_vm_trans(prot, PROT_WRITE, VM_MAYWRITE) |
           _calc_vm_trans(prot, PROT_EXEC,  VM_MAYEXEC);
}

static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
{
    struct ashmem_area *asma = file->private_data;
    int ret = 0;

    mutex_lock(&ashmem_mutex);

    /* user needs to SET_SIZE before mapping */
    if (unlikely(!asma->size)) {
        ret = -EINVAL;
        goto out;
    }

    /* requested protection bits must match our allowed protection mask */
    if (unlikely((vma->vm_flags & ~calc_vm_prot_bits(asma->prot_mask)) &
             calc_vm_prot_bits(PROT_MASK))) {
        ret = -EPERM;
        goto out;
    }
    vma->vm_flags &= ~calc_vm_may_flags(~asma->prot_mask);

    if (!asma->file) {
        char *name = ASHMEM_NAME_DEF;
        struct file *vmfile;

        if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0')
            name = asma->name;

        /* ... and allocate the backing shmem file */
        vmfile = shmem_file_setup(name, asma->size, vma->vm_flags);
        if (unlikely(IS_ERR(vmfile))) {
            ret = PTR_ERR(vmfile);
            goto out;
        }
        asma->file = vmfile;
    }
    get_file(asma->file);

    /*
     * XXX - Reworked to use shmem_zero_setup() instead of
     * shmem_set_file while we're in staging. -jstultz
     */
    if (vma->vm_flags & VM_SHARED) {
        ret = shmem_zero_setup(vma);
        if (ret) {
            fput(asma->file);
            goto out;
        }
    }

    if (vma->vm_file)
        fput(vma->vm_file);
    vma->vm_file = asma->file;

out:
    mutex_unlock(&ashmem_mutex);
    return ret;
}

/*
 * ashmem_shrink - our cache shrinker, called from mm/vmscan.c :: shrink_slab
 *
 * 'nr_to_scan' is the number of objects (pages) to prune, or 0 to query how
 * many objects (pages) we have in total.
 *
 * 'gfp_mask' is the mask of the allocation that got us into this mess.
 *
 * Return value is the number of objects (pages) remaining, or -1 if we cannot
 * proceed without risk of deadlock (due to gfp_mask).
 *
 * We approximate LRU via least-recently-unpinned, jettisoning unpinned partial
 * chunks of ashmem regions LRU-wise one-at-a-time until we hit 'nr_to_scan'
 * pages freed.
 */
static int ashmem_shrink(struct shrinker *s, struct shrink_control *sc)
{
    struct ashmem_range *range, *next;

    /* We might recurse into filesystem code, so bail out if necessary */
    if (sc->nr_to_scan && !(sc->gfp_mask & __GFP_FS))
        return -1;
    if (!sc->nr_to_scan)
        return lru_count;

    mutex_lock(&ashmem_mutex);
    list_for_each_entry_safe(range, next, &ashmem_lru_list, lru) {
        loff_t start = range->pgstart * PAGE_SIZE;
        loff_t end = (range->pgend + 1) * PAGE_SIZE;

        do_fallocate(range->asma->file,
                FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                start, end - start);
        range->purged = ASHMEM_WAS_PURGED;
        lru_del(range);

        sc->nr_to_scan -= range_size(range);
        if (sc->nr_to_scan <= 0)
            break;
    }
    mutex_unlock(&ashmem_mutex);

    return lru_count;
}

static struct shrinker ashmem_shrinker = {
    .shrink = ashmem_shrink,
    .seeks = DEFAULT_SEEKS * 4,
};

static int set_prot_mask(struct ashmem_area *asma, unsigned long prot)
{
    int ret = 0;

    mutex_lock(&ashmem_mutex);

    /* the user can only remove, not add, protection bits */
    if (unlikely((asma->prot_mask & prot) != prot)) {
        ret = -EINVAL;
        goto out;
    }

    /* does the application expect PROT_READ to imply PROT_EXEC? */
    if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
        prot |= PROT_EXEC;

    asma->prot_mask = prot;

out:
    mutex_unlock(&ashmem_mutex);
    return ret;
}

static int set_name(struct ashmem_area *asma, void __user *name)
{
    int ret = 0;

    mutex_lock(&ashmem_mutex);

    /* cannot change an existing mapping's name */
    if (unlikely(asma->file)) {
        ret = -EINVAL;
        goto out;
    }

    if (unlikely(copy_from_user(asma->name + ASHMEM_NAME_PREFIX_LEN,
                    name, ASHMEM_NAME_LEN)))
        ret = -EFAULT;
    asma->name[ASHMEM_FULL_NAME_LEN-1] = '\0';

out:
    mutex_unlock(&ashmem_mutex);

    return ret;
}

static int get_name(struct ashmem_area *asma, void __user *name)
{
    int ret = 0;

    mutex_lock(&ashmem_mutex);
    if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0') {
        size_t len;

        /*
         * Copying only `len', instead of ASHMEM_NAME_LEN, bytes
         * prevents us from revealing one user's stack to another.
         */
        len = strlen(asma->name + ASHMEM_NAME_PREFIX_LEN) + 1;
        if (unlikely(copy_to_user(name,
                asma->name + ASHMEM_NAME_PREFIX_LEN, len)))
            ret = -EFAULT;
    } else {
        if (unlikely(copy_to_user(name, ASHMEM_NAME_DEF,
                      sizeof(ASHMEM_NAME_DEF))))
            ret = -EFAULT;
    }
    mutex_unlock(&ashmem_mutex);

    return ret;
}

/*
 * ashmem_pin - pin the given ashmem region, returning whether it was
 * previously purged (ASHMEM_WAS_PURGED) or not (ASHMEM_NOT_PURGED).
 *
 * Caller must hold ashmem_mutex.
 */
static int ashmem_pin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
{
    struct ashmem_range *range, *next;
    int ret = ASHMEM_NOT_PURGED;

    list_for_each_entry_safe(range, next, &asma->unpinned_list, unpinned) {
        /* moved past last applicable page; we can short circuit */
        if (range_before_page(range, pgstart))
            break;

        /*
         * The user can ask us to pin pages that span multiple ranges,
         * or to pin pages that aren't even unpinned, so this is messy.
         *
         * Four cases:
         * 1. The requested range subsumes an existing range, so we
         *    just remove the entire matching range.
         * 2. The requested range overlaps the start of an existing
         *    range, so we just update that range.
         * 3. The requested range overlaps the end of an existing
         *    range, so we just update that range.
         * 4. The requested range punches a hole in an existing range,
         *    so we have to update one side of the range and then
         *    create a new range for the other side.
         */
        if (page_range_in_range(range, pgstart, pgend)) {
            ret |= range->purged;

            /* Case #1: Easy. Just nuke the whole thing. */
            if (page_range_subsumes_range(range, pgstart, pgend)) {
                range_del(range);
                continue;
            }

            /* Case #2: We overlap from the start, so adjust it */
            if (range->pgstart >= pgstart) {
                range_shrink(range, pgend + 1, range->pgend);
                continue;
            }

            /* Case #3: We overlap from the rear, so adjust it */
            if (range->pgend <= pgend) {
                range_shrink(range, range->pgstart, pgstart-1);
                continue;
            }

            /*
             * Case #4: We eat a chunk out of the middle. A bit
             * more complicated, we allocate a new range for the
             * second half and adjust the first chunk's endpoint.
             */
            range_alloc(asma, range, range->purged,
                    pgend + 1, range->pgend);
            range_shrink(range, range->pgstart, pgstart - 1);
            break;
        }
    }

    return ret;
}

/*
 * ashmem_unpin - unpin the given range of pages. Returns zero on success.
 *
 * Caller must hold ashmem_mutex.
 */
static int ashmem_unpin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
{
    struct ashmem_range *range, *next;
    unsigned int purged = ASHMEM_NOT_PURGED;

restart:
    list_for_each_entry_safe(range, next, &asma->unpinned_list, unpinned) {
        /* short circuit: this is our insertion point */
        if (range_before_page(range, pgstart))
            break;

        /*
         * The user can ask us to unpin pages that are already entirely
         * or partially pinned. We handle those two cases here.
         */
        if (page_range_subsumed_by_range(range, pgstart, pgend))
            return 0;
        if (page_range_in_range(range, pgstart, pgend)) {
            pgstart = min_t(size_t, range->pgstart, pgstart),
            pgend = max_t(size_t, range->pgend, pgend);
            purged |= range->purged;
            range_del(range);
            goto restart;
        }
    }

    return range_alloc(asma, range, purged, pgstart, pgend);
}

/*
 * ashmem_get_pin_status - Returns ASHMEM_IS_UNPINNED if _any_ pages in the
 * given interval are unpinned and ASHMEM_IS_PINNED otherwise.
 *
 * Caller must hold ashmem_mutex.
 */
static int ashmem_get_pin_status(struct ashmem_area *asma, size_t pgstart,
                 size_t pgend)
{
    struct ashmem_range *range;
    int ret = ASHMEM_IS_PINNED;

    list_for_each_entry(range, &asma->unpinned_list, unpinned) {
        if (range_before_page(range, pgstart))
            break;
        if (page_range_in_range(range, pgstart, pgend)) {
            ret = ASHMEM_IS_UNPINNED;
            break;
        }
    }

    return ret;
}

static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
                void __user *p)
{
    struct ashmem_pin pin;
    size_t pgstart, pgend;
    int ret = -EINVAL;

    if (unlikely(!asma->file))
        return -EINVAL;

    if (unlikely(copy_from_user(&pin, p, sizeof(pin))))
        return -EFAULT;

    /* per custom, you can pass zero for len to mean "everything onward" */
    if (!pin.len)
        pin.len = PAGE_ALIGN(asma->size) - pin.offset;

    if (unlikely((pin.offset | pin.len) & ~PAGE_MASK))
        return -EINVAL;

    if (unlikely(((__u32) -1) - pin.offset < pin.len))
        return -EINVAL;

    if (unlikely(PAGE_ALIGN(asma->size) < pin.offset + pin.len))
        return -EINVAL;

    pgstart = pin.offset / PAGE_SIZE;
    pgend = pgstart + (pin.len / PAGE_SIZE) - 1;

    mutex_lock(&ashmem_mutex);

    switch (cmd) {
    case ASHMEM_PIN:
        ret = ashmem_pin(asma, pgstart, pgend);
        break;
    case ASHMEM_UNPIN:
        ret = ashmem_unpin(asma, pgstart, pgend);
        break;
    case ASHMEM_GET_PIN_STATUS:
        ret = ashmem_get_pin_status(asma, pgstart, pgend);
        break;
    }

    mutex_unlock(&ashmem_mutex);

    return ret;
}

static long ashmem_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    struct ashmem_area *asma = file->private_data;
    long ret = -ENOTTY;

    switch (cmd) {
    case ASHMEM_SET_NAME:
        ret = set_name(asma, (void __user *) arg);
        break;
    case ASHMEM_GET_NAME:
        ret = get_name(asma, (void __user *) arg);
        break;
    case ASHMEM_SET_SIZE:
        ret = -EINVAL;
        if (!asma->file) {
            ret = 0;
            asma->size = (size_t) arg;
        }
        break;
    case ASHMEM_GET_SIZE:
        ret = asma->size;
        break;
    case ASHMEM_SET_PROT_MASK:
        ret = set_prot_mask(asma, arg);
        break;
    case ASHMEM_GET_PROT_MASK:
        ret = asma->prot_mask;
        break;
    case ASHMEM_PIN:
    case ASHMEM_UNPIN:
    case ASHMEM_GET_PIN_STATUS:
        ret = ashmem_pin_unpin(asma, cmd, (void __user *) arg);
        break;
    case ASHMEM_PURGE_ALL_CACHES:
        ret = -EPERM;
        if (capable(CAP_SYS_ADMIN)) {
            struct shrink_control sc = {
                .gfp_mask = GFP_KERNEL,
                .nr_to_scan = 0,
            };
            ret = ashmem_shrink(&ashmem_shrinker, &sc);
            sc.nr_to_scan = ret;
            ashmem_shrink(&ashmem_shrinker, &sc);
        }
        break;
    }

    return ret;
}

static const struct file_operations ashmem_fops = {
    .owner = THIS_MODULE,
    .open = ashmem_open,
    .release = ashmem_release,
    .read = ashmem_read,
    .llseek = ashmem_llseek,
    .mmap = ashmem_mmap,
    .unlocked_ioctl = ashmem_ioctl,
    .compat_ioctl = ashmem_ioctl,
};

static struct miscdevice ashmem_misc = {
    .minor = MISC_DYNAMIC_MINOR,
    .name = "ashmem",
    .fops = &ashmem_fops,
};

static int __init ashmem_init(void)
{
    int ret;

    ashmem_area_cachep = kmem_cache_create("ashmem_area_cache",
                      sizeof(struct ashmem_area),
                      0, 0, NULL);
    if (unlikely(!ashmem_area_cachep)) {
        pr_err("failed to create slab cache\n");
        return -ENOMEM;
    }

    ashmem_range_cachep = kmem_cache_create("ashmem_range_cache",
                      sizeof(struct ashmem_range),
                      0, 0, NULL);
    if (unlikely(!ashmem_range_cachep)) {
        pr_err("failed to create slab cache\n");
        return -ENOMEM;
    }

    ret = misc_register(&ashmem_misc);
    if (unlikely(ret)) {
        pr_err("failed to register misc device!\n");
        return ret;
    }

    register_shrinker(&ashmem_shrinker);

    pr_info("initialized\n");

    return 0;
}

static void __exit ashmem_exit(void)
{
    int ret;

    unregister_shrinker(&ashmem_shrinker);

    ret = misc_deregister(&ashmem_misc);
    if (unlikely(ret))
        pr_err("failed to unregister misc device!\n");

    kmem_cache_destroy(ashmem_range_cachep);
    kmem_cache_destroy(ashmem_area_cachep);

    pr_info("unloaded\n");
}

module_init(ashmem_init);
module_exit(ashmem_exit);

MODULE_LICENSE("GPL");