netdev - Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions

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Date:   Wed, 20 Jan 2021 13:44:24 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        ciara.loftus@...el.com, weqaar.a.janjua@...el.com
Subject: Re: [PATCH bpf-next v2 1/8] xdp: restructure redirect actions

Björn Töpel <bjorn.topel@...il.com> writes:

> From: Björn Töpel <bjorn.topel@...el.com>
>
> The XDP_REDIRECT implementations for maps and non-maps are fairly
> similar, but obviously need to take different code paths depending on
> if the target is using a map or not. Today, the redirect targets for
> XDP either uses a map, or is based on ifindex.
>
> Future commits will introduce yet another redirect target via the a
> new helper, bpf_redirect_xsk(). To pave the way for that, we introduce
> an explicit redirect type to bpf_redirect_info. This makes the code
> easier to follow, and makes it easier to add new redirect targets.
>
> Further, using an explicit type in bpf_redirect_info has a slight
> positive performance impact by avoiding a pointer indirection for the
> map type lookup, and instead use the hot cacheline for
> bpf_redirect_info.
>
> The bpf_redirect_info flags member is not used by XDP, and not
> read/written any more. The map member is only written to when
> required/used, and not unconditionally.

I like the simplification. However, the handling of map clearing becomes
a bit murky with this change:

You're not changing anything in bpf_clear_redirect_map(), and you're
removing most of the reads and writes of ri->map. Instead,
bpf_xdp_redirect_map() will store the bpf_dtab_netdev pointer in
ri->tgt_value, which xdp_do_redirect() will just read and use without
checking. But if the map element (or the entire map) has been freed in
the meantime that will be a dangling pointer. I *think* the RCU callback
in dev_map_delete_elem() and the rcu_barrier() in dev_map_free()
protects against this, but that is by no means obvious. So confirming
this, and explaining it in a comment would be good.

Also, as far as I can tell after this, ri->map is only used for the
tracepoint. So how about just storing the map ID and getting rid of the
READ/WRITE_ONCE() entirely?

(Oh, and related to this I think this patch set will conflict with
Hangbin's multi-redirect series, so maybe you two ought to coordinate? :))

-Toke