Re: [PATCH 3/4] watch_queue: Implement mount topology and attribute change notifications

[Linus Torvalds <torvalds@linux-foundation.org>]

This just can't be right.

On Fri, Jul 24, 2020 at 6:12 AM David Howells <dhowells@redhat.com> wrote:
>
> +
> +/**
> + * sys_watch_mount - Watch for mount topology/attribute changes
> + * @dfd: Base directory to pathwalk from or fd referring to mount.
> + * @filename: Path to mount 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(watch_mount, [...]
> +               int, watch_id)
...
> +       if (watch_id < -1 || watch_id > 0xff)
> +               return -EINVAL;
...
> +       ret = inode_permission(path.dentry->d_inode, MAY_EXEC);
> +       if (ret)
> +               goto err_path;
...
> +       if (watch_id >= 0) {
...
> +               watch = kzalloc(sizeof(*watch), GFP_KERNEL);
> +               if (!watch)
> +                       goto err_wlist;

So now you can basically allocate as much kernel memory as you want as
a regular user, as long as you have a mounted directory you can walk
(ie everybody).

Is there any limiting of watches anywhere? I don't see it.

I notice we already have this pattern elsewhere. I think we need to
fix this before we add more watch types.

Watch allocation shouldn't just be a kzalloc(). I think you should
have a "watch_allocate()" that does the initialization of id etc, but
also does some basic per-user watch resource tracking or something.

              Linus