* Re: Bridge zeros out skb->cb in 5.2 (not in 5.1)
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@ 2019-08-23 9:12 ` Florian Westphal
0 siblings, 0 replies; only message in thread
From: Florian Westphal @ 2019-08-23 9:12 UTC (permalink / raw)
To: Dan Siemon; +Cc: netdev, bpf
Dan Siemon <dan@coverfire.com> wrote:
[ CCing bpf maling list ]
> Commit f12064d1b402c60c5db9c4b63d5ed6d7facb33f6 zeros out skb->cb in
> br_input.c:
>
> memset(skb->cb, 0, sizeof(struct br_input_skb_cb));
>
> https://github.com/torvalds/linux/commit/f12064d1b402c60c5db9c4b63d5ed6d7facb33f6
>
> Prior to 5.2, it was possible to store information in skb->cb and have it
> pass through the bridge to the output link.
I did not know this was even possible.
Any owner of the skb (bridge, ip stack, etc.) use skb->cb[] as they see fit.
> We leveraged this to have a BPF
> prog that runs on ingress and does custom packet parsing and stores the
> output qdisc:class in skb->cb. This enabled the egress BPF filter to be
> super simple and avoid having to parse the entire packet again.
>
> Note I haven't built with this patch removed so it's possible this isn't
> the problem but the memset is unconditional...
You're not exactly saying what the problem is, so I have no idea.
> Is this a regression? Is it expected that the bridge would wipe this field
> when just passing frames?
Even if you remove the memset, that commit br_input_skb_cb
has existed, and is used. Fields were just cleared on-demand rather
than unconditionally at the start.
I think the latter is better practice and also what other owners do.
So please explain what exactly the problem is and/or check that the
cb clearing "is the problem".
If it is, I have no idea how to fix it.
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2019-08-23 9:12 ` Bridge zeros out skb->cb in 5.2 (not in 5.1) Florian Westphal
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