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

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Date:   Sun, 11 Oct 2020 16:30:28 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, 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,
        eyal.birger@...il.com
Subject: Re: [PATCH bpf-next V3 0/6] bpf: New approach for BPF MTU handling

On Sat, 10 Oct 2020 16:52:48 -0700 John Fastabend wrote:
> Jakub Kicinski wrote:
> > FWIW I took a quick swing at testing it with the HW I have and it did
> > exactly what hardware should do. The TX unit entered an error state 
> > and then the driver detected that and reset it a few seconds later.  
> 
> Ths seems like the right thing to do in my opinion. If the
> stack gives the NIC garbage entering error state and reset
> sounds expected. Thanks for actually trying it by the way.
> 
> We might have come to different conclusions though from my side
> the conclusion is, good nothing horrible happened no MTU check needed.
> If the user spews garbage at the nic from the BPF program great it
> gets dropped and causes the driver/nic to punish you a bit by staying
> hung. Fix your BPF program.

Right probably difference of perspective. I understand that from
Cilium's POV you can probably feel pretty confident about the BPF
programs that are running. I bet Maciej is even more confident with
Android!

But in principle BPF was supposed to make the kernel end user
programmable. We have to ensure it's safe.

> > > > And all this for what? Saving 2 cycles on a branch that will almost
> > > > never be taken?    
> 
> 2 cycles here and 2 cycles there .... plus complexity to think about
> it. Eventually it all adds up. At the risk of entering bike shedding
> territory maybe.

Not sure it's a bike shedding territory but I doubt you want to be
making either the complexity or the performance argument to a fellow 
TLS maintainer.. cough cough.. ;)