netdev - Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

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Message-ID: <87lflom0xx.fsf@toke.dk>
Date:   Tue, 19 May 2020 16:52:42 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...il.com>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, brouer@...hat.com,
        john.fastabend@...il.com, ast@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, andriin@...com,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

Daniel Borkmann <daniel@...earbox.net> writes:

> On 5/19/20 2:02 AM, David Ahern wrote:
>> On 5/18/20 3:06 PM, Daniel Borkmann wrote:
>>> So given we neither call this hook on the skb path, nor XDP_TX nor
>>> AF_XDP's TX
>>> path, I was wondering also wrt the discussion with John if it makes
>>> sense to
>>> make this hook a property of the devmap _itself_, for example, to have a
>>> default
>>> BPF prog upon devmap creation or a dev-specific override that is passed
>>> on map
>>> update along with the dev. At least this would make it very clear where
>>> this is
>>> logically tied to and triggered from, and if needed (?) would provide
>>> potentially
>>> more flexibility on specifiying BPF progs to be called while also
>>> solving your
>>> use-case.
>> 
>> You lost me on the 'property of the devmap.' The programs need to be per
>> netdevice, and devmap is an array of devices. Can you elaborate?
>
> I meant that the dev{map,hash} would get extended in a way where the
> __dev_map_update_elem() receives an (ifindex, BPF prog fd) tuple from
> user space and holds the program's ref as long as it is in the map slot.
> Then, upon redirect to the given device in the devmap, we'd execute the
> prog as well in order to also allow for XDP_DROP policy in there. Upon
> map update when we drop the dev from the map slot, we also release the
> reference to the associated BPF prog. What I mean to say wrt 'property
> of the devmap' is that this program is _only_ used in combination with
> redirection to devmap, so given we are not solving all the other egress
> cases for reasons mentioned, it would make sense to tie it logically to
> the devmap which would also make it clear from a user perspective _when_
> the prog is expected to run.

I would be totally on board with this. Also makes sense for the
multicast map type, if you want to fix up the packet after the redirect,
just stick the fixer-upper program into the map along with the ifindex.

-Toke