fanotify.c

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#include <linux/fanotify.h>
#include <linux/fdtable.h>
#include <linux/fsnotify_backend.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kernel.h> /* UINT_MAX */
#include <linux/mount.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <linux/wait.h>

static bool should_merge(struct fsnotify_event *old, struct fsnotify_event *new)
{
    pr_debug("%s: old=%p new=%p\n", __func__, old, new);

    if (old->to_tell == new->to_tell &&
        old->data_type == new->data_type &&
        old->tgid == new->tgid) {
        switch (old->data_type) {
        case (FSNOTIFY_EVENT_PATH):
            if ((old->path.mnt == new->path.mnt) &&
                (old->path.dentry == new->path.dentry))
                return true;
            break;
        case (FSNOTIFY_EVENT_NONE):
            return true;
        default:
            BUG();
        };
    }
    return false;
}

/* and the list better be locked by something too! */
static struct fsnotify_event *fanotify_merge(struct list_head *list,
                         struct fsnotify_event *event)
{
    struct fsnotify_event_holder *test_holder;
    struct fsnotify_event *test_event = NULL;
    struct fsnotify_event *new_event;

    pr_debug("%s: list=%p event=%p\n", __func__, list, event);


    list_for_each_entry_reverse(test_holder, list, event_list) {
        if (should_merge(test_holder->event, event)) {
            test_event = test_holder->event;
            break;
        }
    }

    if (!test_event)
        return NULL;

    fsnotify_get_event(test_event);

    /* if they are exactly the same we are done */
    if (test_event->mask == event->mask)
        return test_event;

    /*
     * if the refcnt == 2 this is the only queue
     * for this event and so we can update the mask
     * in place.
     */
    if (atomic_read(&test_event->refcnt) == 2) {
        test_event->mask |= event->mask;
        return test_event;
    }

    new_event = fsnotify_clone_event(test_event);

    /* done with test_event */
    fsnotify_put_event(test_event);

    /* couldn't allocate memory, merge was not possible */
    if (unlikely(!new_event))
        return ERR_PTR(-ENOMEM);

    /* build new event and replace it on the list */
    new_event->mask = (test_event->mask | event->mask);
    fsnotify_replace_event(test_holder, new_event);

    /* we hold a reference on new_event from clone_event */
    return new_event;
}

#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
static int fanotify_get_response_from_access(struct fsnotify_group *group,
                         struct fsnotify_event *event)
{
    int ret;

    pr_debug("%s: group=%p event=%p\n", __func__, group, event);

    wait_event(group->fanotify_data.access_waitq, event->response ||
                atomic_read(&group->fanotify_data.bypass_perm));

    if (!event->response) /* bypass_perm set */
        return 0;

    /* userspace responded, convert to something usable */
    spin_lock(&event->lock);
    switch (event->response) {
    case FAN_ALLOW:
        ret = 0;
        break;
    case FAN_DENY:
    default:
        ret = -EPERM;
    }
    event->response = 0;
    spin_unlock(&event->lock);

    pr_debug("%s: group=%p event=%p about to return ret=%d\n", __func__,
         group, event, ret);
    
    return ret;
}
#endif

static int fanotify_handle_event(struct fsnotify_group *group,
                 struct fsnotify_mark *inode_mark,
                 struct fsnotify_mark *fanotify_mark,
                 struct fsnotify_event *event)
{
    int ret = 0;
    struct fsnotify_event *notify_event = NULL;

    BUILD_BUG_ON(FAN_ACCESS != FS_ACCESS);
    BUILD_BUG_ON(FAN_MODIFY != FS_MODIFY);
    BUILD_BUG_ON(FAN_CLOSE_NOWRITE != FS_CLOSE_NOWRITE);
    BUILD_BUG_ON(FAN_CLOSE_WRITE != FS_CLOSE_WRITE);
    BUILD_BUG_ON(FAN_OPEN != FS_OPEN);
    BUILD_BUG_ON(FAN_EVENT_ON_CHILD != FS_EVENT_ON_CHILD);
    BUILD_BUG_ON(FAN_Q_OVERFLOW != FS_Q_OVERFLOW);
    BUILD_BUG_ON(FAN_OPEN_PERM != FS_OPEN_PERM);
    BUILD_BUG_ON(FAN_ACCESS_PERM != FS_ACCESS_PERM);
    BUILD_BUG_ON(FAN_ONDIR != FS_ISDIR);

    pr_debug("%s: group=%p event=%p\n", __func__, group, event);

    notify_event = fsnotify_add_notify_event(group, event, NULL, fanotify_merge);
    if (IS_ERR(notify_event))
        return PTR_ERR(notify_event);

#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
    if (event->mask & FAN_ALL_PERM_EVENTS) {
        /* if we merged we need to wait on the new event */
        if (notify_event)
            event = notify_event;
        ret = fanotify_get_response_from_access(group, event);
    }
#endif

    if (notify_event)
        fsnotify_put_event(notify_event);

    return ret;
}

static bool fanotify_should_send_event(struct fsnotify_group *group,
                       struct inode *to_tell,
                       struct fsnotify_mark *inode_mark,
                       struct fsnotify_mark *vfsmnt_mark,
                       __u32 event_mask, void *data, int data_type)
{
    __u32 marks_mask, marks_ignored_mask;
    struct path *path = data;

    pr_debug("%s: group=%p to_tell=%p inode_mark=%p vfsmnt_mark=%p "
         "mask=%x data=%p data_type=%d\n", __func__, group, to_tell,
         inode_mark, vfsmnt_mark, event_mask, data, data_type);

    /* if we don't have enough info to send an event to userspace say no */
    if (data_type != FSNOTIFY_EVENT_PATH)
        return false;

    /* sorry, fanotify only gives a damn about files and dirs */
    if (!S_ISREG(path->dentry->d_inode->i_mode) &&
        !S_ISDIR(path->dentry->d_inode->i_mode))
        return false;

    if (inode_mark && vfsmnt_mark) {
        marks_mask = (vfsmnt_mark->mask | inode_mark->mask);
        marks_ignored_mask = (vfsmnt_mark->ignored_mask | inode_mark->ignored_mask);
    } else if (inode_mark) {
        /*
         * if the event is for a child and this inode doesn't care about
         * events on the child, don't send it!
         */
        if ((event_mask & FS_EVENT_ON_CHILD) &&
            !(inode_mark->mask & FS_EVENT_ON_CHILD))
            return false;
        marks_mask = inode_mark->mask;
        marks_ignored_mask = inode_mark->ignored_mask;
    } else if (vfsmnt_mark) {
        marks_mask = vfsmnt_mark->mask;
        marks_ignored_mask = vfsmnt_mark->ignored_mask;
    } else {
        BUG();
    }

    if (S_ISDIR(path->dentry->d_inode->i_mode) &&
        (marks_ignored_mask & FS_ISDIR))
        return false;

    if (event_mask & marks_mask & ~marks_ignored_mask)
        return true;

    return false;
}

static void fanotify_free_group_priv(struct fsnotify_group *group)
{
    struct user_struct *user;

    user = group->fanotify_data.user;
    atomic_dec(&user->fanotify_listeners);
    free_uid(user);
}

const struct fsnotify_ops fanotify_fsnotify_ops = {
    .handle_event = fanotify_handle_event,
    .should_send_event = fanotify_should_send_event,
    .free_group_priv = fanotify_free_group_priv,
    .free_event_priv = NULL,
    .freeing_mark = NULL,
};