On 2021-02-15 18:07, Toke Høiland-Jørgensen wrote:

Maciej Fijalkowski [off-list ref] writes:

quoted

Currently, if there are multiple xdpsock instances running on a single
interface and in case one of the instances is terminated, the rest of
them are left in an inoperable state due to the fact of unloaded XDP
prog from interface.

To address that, step away from setting bpf prog in favour of bpf_link.
This means that refcounting of BPF resources will be done automatically
by bpf_link itself.

When setting up BPF resources during xsk socket creation, check whether
bpf_link for a given ifindex already exists via set of calls to
bpf_link_get_next_id -> bpf_link_get_fd_by_id -> bpf_obj_get_info_by_fd
and comparing the ifindexes from bpf_link and xsk socket.

One consideration here is that bpf_link_get_fd_by_id() is a privileged
operation (privileged as in CAP_SYS_ADMIN), so this has the side effect
of making AF_XDP privileged as well. Is that the intention?

We're already using, e.g., bpf_map_get_fd_by_id() which has that
as well. So we're assuming that for XDP setup already!

Another is that the AF_XDP code is in the process of moving to libxdp
(see in-progress PR [0]), and this approach won't carry over as-is to
that model, because libxdp has to pin the bpf_link fds.

I was assuming there were two modes of operations for AF_XDP in libxdp.
One which is with the multi-program support (which AFAIK is why the
pinning is required), and one "like the current libbpf" one. For the
latter Maciej's series would be a good fit, no?

However, in libxdp we can solve the original problem in a different way,
and in fact I already suggested to Magnus that we should do this (see
[1]); so one way forward could be to address it during the merge in
libxdp? It should be possible to address the original issue (two
instances of xdpsock breaking each other when they exit), but
applications will still need to do an explicit unload operation before
exiting (i.e., the automatic detach on bpf_link fd closure will take
more work, and likely require extending the bpf_link kernel support)...

I'd say it's depending on the libbpf 1.0/libxdp merge timeframe. If
we're months ahead, then I'd really like to see this in libbpf until the
merge. However, I'll leave that for Magnus/you to decide!

Bottom line; I'd *really* like bpf_link behavior (process scoped) for
AF_XDP sooner than later! ;-)


Thanks for the input!
Björn

-Toke

[0] https://github.com/xdp-project/xdp-tools/pull/92
[1] https://github.com/xdp-project/xdp-tools/pull/92#discussion_r576204719