Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing program when attaching XDP

Alexei Starovoitov
2020-03-26T19:59:05Z

On Thu, Mar 26, 2020 at 01:35:13PM +0100, Toke Høiland-Jørgensen wrote:
>
> Additionally, in the case where there is *not* a central management
> daemon (i.e., what I'm implementing with libxdp), this would be the flow
> implemented by the library without bpf_link:
>
> 1. Query kernel for current BPF prog loaded on $IFACE
> 2. Sanity-check that this program is a dispatcher program installed by
> libxdp
> 3. Create a new dispatcher program with whatever changes we want to do
> (such as adding another component program).
> 4. Atomically replace the old program with the new one using the netlink
> API in this patch series.

in this model what stops another application that is not using libdispatcher to
nuke dispatcher program ?

> Whereas with bpf_link, it would be:
>
> 1. Find the pinned bpf_link for $IFACE (e.g., load from
> /sys/fs/bpf/iface-links/$IFNAME).
> 2. Query kernel for current BPF prog linked to $LINK
> 3. Sanity-check that this program is a dispatcher program installed by
> libxdp
> 4. Create a new dispatcher program with whatever changes we want to do
> (such as adding another component program).
> 5. Atomically replace the old program with the new one using the
> LINK_UPDATE bpf() API.

whereas here dispatcher program is only accessible to libdispatcher.
Instance of bpffs needs to be known to libdispatcher only.
That's the ownership I've been talking about.

As discussed early we need a way for _human_ to nuke dispatcher program,
but such api shouldn't be usable out of application/task.