netdev - Re: [PATCH bpf-next V3 0/6] bpf: New approach for BPF MTU handling

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Date:   Fri, 09 Oct 2020 13:49:14 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        eyal.birger@...il.com
Subject: Re: [PATCH bpf-next V3 0/6] bpf: New approach for BPF MTU handling

Jakub Kicinski wrote:
> On Thu, 08 Oct 2020 16:08:57 +0200 Jesper Dangaard Brouer wrote:
> > V3: Drop enforcement of MTU in net-core, leave it to drivers
> 
> Sorry for being late to the discussion.
> 
> I absolutely disagree. We had cases in the past where HW would lock up
> if it was sent a frame with bad geometry.
> 
> We will not be sprinkling validation checks across the drivers because
> some reconfiguration path may occasionally yield a bad packet, or it's
> hard to do something right with BPF.

This is a driver bug then. As it stands today drivers may get hit with
skb with MTU greater than set MTU as best I can tell. Generally I
expect drivers use MTU to configure RX buffers not sure how it is going
to be used on TX side? Any examples? I just poked around through the
driver source to see and seems to confirm its primarily for RX side
configuration with some drivers throwing the event down to the firmware
for something that I can't see in the code?

I'm not suggestiong sprinkling validation checks across the drivers.
I'm suggesting if the drivers hang we fix them.