Re: [PATCH net-next v10 4/5] net: devmem: document NETDEV_A_DMABUF_AUTORELEASE netlink attribute

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Mon, Jan 26, 2026 at 06:44:40PM -0800, Jakub Kicinski wrote:
> On Mon, 26 Jan 2026 18:30:45 -0800 Bobby Eshleman wrote:
> > > > I'm not a fan of the existing cmsg scheme, but we already have userspace
> > > > using it, so getting more performance out of it seems like an easy win?  
> > > 
> > > I don't like:
> > >  - the fact that we have to add the binding to a socket (extra field)
> > >    - single socket can only serve single binding, there's no technical
> > >      reason for this really, AFAICT, just the fact that we have a single
> > >      pointer in the sock struct  
> > 
> > The core reason is that sockets lose the ability to map a given token to
> > a given binding by no longer storing the niov ptr.
> > 
> > One proposal I had was to encode some number of bits in the token that
> > can be used to lookup the binding in an array, I could reboot that
> > approach.
> > 
> > With 32 bits, we can represent:
> > 
> > dmabuf max size = 512 GB, max dmabuf count = 8
> > dmabuf max size = 256 GB, max dmabuf count = 16
> > dmabuf max size = 128 GB, max dmabuf count = 32
> > 
> > etc...
> > 
> > Then, if the dmabuf count encoding space is exhausted, the socket would
> > have to wait until the user returns all of the tokens from one of the
> > dmabufs and frees the ID (or err out is another option).
> > 
> > This wouldn't change adding a field to the socket, we'd have to add one
> > or two more for allocating the dmabuf ID and fetching the dmabuf with
> > it. But it does fix the single binding thing.
> 
> I think the bigger problem (than space exhaustion) is that we'd also
> have some understanding of permissions. If an application guesses 
> the binding ID of another app it can mess up its buffers. ENOBUENO..

I was thinking it would be per-socket, effectively:

sk->sk_devmem_info.bindings[binding_id_from_token(token)]

So sockets could only access those that they have already recv'd on.