Re: [PATCH 1/4] rust: netlink: add raw netlink abstraction

[Andrew Lunn <andrew@lunn.ch>]

> We can make a kernel::net module (rust/kernel/net/) and put it there
> instead? I guess netlink.rs can also be a submodule of that.
> 
> Hmm ... but I'm currently using genlmsg_new() / nlmsg_free(), and I
> assume the other use-cases do not go through those methods, since they
> sound netlink specific.
> 
> To be honest, I'm new to the kernel's networking stack, so I probably
> can't design Rust wrapper for sk_buff that supports all those different
> usecases without someone walking me through how it works. It may or may
> not be best to write a netlink-specific struct now and expand it later.

At its core, an skbuff represents a block of memory. Normally that
memory would be packet data flowing through the network stack. You can
append to the front and back of the data in the buffer. You can add
the buffer to linked lists, there are reference counting operations,
so the buffer can have multiple users, there are places you can store
per protocol data, and places you can store data while within one
protocol layer, etc.

When using sk_buff for packet data, you are generally on a fast path,
so much of the code is in header files as inline functions. This is
going to make rust binding harder, my current understanding is that it
is hard to make use of such functions from rust.

However, for netlink, its is slow path code, it seems like all the
functions you need to call are in object files, not inline headers.

I also suspect, but don't know, that the netlink upper API is disjoint
to the packet handling upper API. How you allocate a skbuff for
netlink is different to packet data. It is very likely an error if you
try to transmit out an interface an skbuff allocated for netlink
usage, etc.

So, maybe you can have a base definition of skbuff in rust/kernel/net,
and build on top of that to make a netlink specific skbuff, with a
limited list of methods which can access it, those needed for netlink?
Make use of the Rust type system? And leave the messy fast path packet
data things for somebody else.

     Andrew