Re: [PATCH 4/7] vfs: Add superblock notifications

Jann Horn
2019-05-28T20:28:18Z

On Tue, May 28, 2019 at 6:05 PM David Howells <dhowells@redhat.com> wrote:
> Add a superblock event notification facility whereby notifications about
> superblock events, such as I/O errors (EIO), quota limits being hit
> (EDQUOT) and running out of space (ENOSPC) can be reported to a monitoring
> process asynchronously. Note that this does not cover vfsmount topology
> changes. mount_notify() is used for that.
[...]
> +#ifdef CONFIG_SB_NOTIFICATIONS
> +/*
> + * Post superblock notifications.
> + */
> +void post_sb_notification(struct super_block *s, struct superblock_notification *n)
> +{
> + post_watch_notification(s->s_watchers, &n->watch, current_cred(),
> + s->s_unique_id);
> +}

You're using current_cred() here? So the idea is that if some random
process runs into a disk I/O error, the I/O error will come from that
task's credentials? In general, you're not supposed to look at task
credentials in ->read/->write handlers.

> +static void release_sb_watch(struct watch_list *wlist, struct watch *watch)
> +{
> + struct super_block *s = watch->private;
> +
> + put_super(s);
> +}
> +
> +/**
> + * sys_sb_notify - Watch for superblock events.
> + * @dfd: Base directory to pathwalk from or fd referring to superblock.
> + * @filename: Path to superblock to place the watch upon
> + * @at_flags: Pathwalk control flags
> + * @watch_fd: The watch queue to send notifications to.
> + * @watch_id: The watch ID to be placed in the notification (-1 to remove watch)
> + */
> +SYSCALL_DEFINE5(sb_notify,
> + int, dfd,
> + const char __user *, filename,
> + unsigned int, at_flags,
> + int, watch_fd,
> + int, watch_id)
> +{
> + struct watch_queue *wqueue;
> + struct super_block *s;
> + struct watch_list *wlist = NULL;
> + struct watch *watch;
> + struct path path;
> + int ret;
> +
> + if (watch_id < -1 || watch_id > 0xff)
> + return -EINVAL;
> +
> + ret = user_path_at(dfd, filename, at_flags, &path);

As in the other patch, I don't think userspace is supposed to be able
to supply user_path_at()'s third argument.

It might make sense to require that the path points to the root inode
of the superblock? That way you wouldn't be able to do this on a bind
mount that exposes part of a shared filesystem to a container.

> + if (ret)
> + return ret;
> +
> + wqueue = get_watch_queue(watch_fd);
> + if (IS_ERR(wqueue))
> + goto err_path;
> +
> + s = path.dentry->d_sb;
> + if (watch_id >= 0) {
> + if (!s->s_watchers) {
> + wlist = kzalloc(sizeof(*wlist), GFP_KERNEL);
> + if (!wlist)
> + goto err_wqueue;
> + INIT_HLIST_HEAD(&wlist->watchers);
> + spin_lock_init(&wlist->lock);
> + wlist->release_watch = release_sb_watch;
> + }
> +
> + watch = kzalloc(sizeof(*watch), GFP_KERNEL);
> + if (!watch)
> + goto err_wlist;
> +
> + init_watch(watch, wqueue);
> + watch->id = s->s_unique_id;
> + watch->private = s;
> + watch->info_id = (u32)watch_id << 24;
> +
> + down_write(&s->s_umount);
> + ret = -EIO;
> + if (atomic_read(&s->s_active)) {
> + if (!s->s_watchers) {
> + s->s_watchers = wlist;
> + wlist = NULL;
> + }
> +
> + ret = add_watch_to_object(watch, s->s_watchers);
> + if (ret == 0) {
> + spin_lock(&sb_lock);
> + s->s_count++;
> + spin_unlock(&sb_lock);

Why do watches hold references on the superblock they're watching?

> + }
> + }
> + up_write(&s->s_umount);
> + if (ret < 0)
> + kfree(watch);
> + } else if (s->s_watchers) {

This should probably have something like a READ_ONCE() for clarity?

> + down_write(&s->s_umount);
> + ret = remove_watch_from_object(s->s_watchers, wqueue,
> + s->s_unique_id, false);
> + up_write(&s->s_umount);
> + } else {
> + ret = -EBADSLT;
> + }
> +
> +err_wlist:
> + kfree(wlist);
> +err_wqueue:
> + put_watch_queue(wqueue);
> +err_path:
> + path_put(&path);
> + return ret;
> +}
> +#endif