Re: [RFC PATCH] fanotify: notify on mount attach and detach

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Nov 28, 2024 at 03:39:59PM +0100, Miklos Szeredi wrote:

> Prior to this patch mount namespace changes could be monitored by polling
> /proc/self/mountinfo, which did not convey any information about what
> changed.
> 
> To monitor an entire mount namespace with this new interface, watches need
> to be added to all existing mounts.  This can be done by performing
> listmount()/statmount() recursively at startup and when a new mount is
> added.

First impression is that it's bloody awful, TBH.  You are calling fsnotify()
under mount_lock; in effect, *ANY* path_init() call done during that time
will be spinning in __read_seqcount_begin() until you are done with that
shite.

And it's _very_ easy to generate a lot of such events with a single syscall;
that doesn't even need sroot - a root in container will suffice.

So... why is it not a DoS?