netdev - Re: [PATCH bpf-next 2/6] bpf, net: rework cookie generator as per-cpu one

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Message-ID: <c9fee768-4f35-e596-001b-2e2a0e4f48a1@gmail.com>
Date:   Fri, 25 Sep 2020 17:15:17 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, ast@...nel.org,
        john.fastabend@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/6] bpf, net: rework cookie generator as per-cpu
 one



On 9/25/20 5:00 PM, Jakub Kicinski wrote:
                    unlikely((val & (COOKIE_LOCAL_BATCH - 1)) == 0)) {
> 
> Can we reasonably assume we won't have more than 4k CPUs and just
> statically divide this space by encoding CPU id in top bits?

This might give some food to side channel attacks, since this would
give an indication of cpu that allocated the id.

Also, I hear that some distros enabled 8K cpus.