Re: [PATCH bpf-next v4 1/2] xsk: remove AF_XDP socket from map when the socket is released

["Björn Töpel" <bjorn.topel@intel.com>]

On 2019-08-15 01:17, Daniel Borkmann wrote:
> Seems reasonable to me, and inc as opposed to inc_not_zero is also fine
> here since at this point in time we're still holding one reference to
> the map.

Ok, good.

> But I think there's a catch with the current code that still
> needs fixing:
> 
> Imagine you do a xsk_map_update_elem() where we have a situation where
> xs == old_xs. There, we first do the xsk_map_sock_add() to add the new
> xsk map node at the tail of the socket's xs->map_list. We do the xchg()
> and then xsk_map_sock_delete() for old_xs which then walks xs->map_list
> again and purges all entries including the just newly created one. This
> means we'll end up with an xs socket at the given map slot, but the xs
> socket has empty xs->map_list. This means we could release the xs sock
> and the xsk_delete_from_maps() won't need to clean up anything anymore
> but yet the xs is still in the map slot, so if you redirect to that
> socket, it would be use-after-free, no?

Ah, correct. Checking against self-assignment, or doing the delete prior 
add. I'll spin a v5.

...and again, thanks for detailed review, Daniel!


Björn