netdev - Re: [PATCH net] act_bpf: allow non-default TC

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Date:	Tue, 17 Mar 2015 14:53:50 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Jiri Pirko <jiri@...nulli.us>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH net] act_bpf: allow non-default TC_ACT opcodes as BPF exec outcome

On Tue, Mar 17, 2015 at 12:25 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> Revisiting commit d23b8ad8ab23 ("tc: add BPF based action") with regards
> to eBPF support, I was thinking that it might be better to improve
> return semantics from a BPF program invoked through BPF_PROG_RUN().
>
> Currently, in case filter_res is 0, we overwrite the default action
> opcode with TC_ACT_SHOT. A default action opcode configured through tc's
> m_bpf can be: TC_ACT_RECLASSIFY, TC_ACT_PIPE, TC_ACT_SHOT, TC_ACT_UNSPEC,
> TC_ACT_OK.
>
> In cls_bpf, we have the possibility to overwrite the default class
> associated with the classifier in case filter_res is _not_ 0xffffffff
> (-1).
>
> That allows us to fold multiple [e]BPF programs into a single one, where
> they would otherwise need to be defined as a separate classifier with
> its own classid, needlessly redoing parsing work, etc.
>
> Similarly, we could do better in act_bpf: Since above TC_ACT* opcodes
> are exported to UAPI anyway, we reuse them for return-code-to-tc-opcode
> mapping, where we would allow above possibilities. Thus, like in cls_bpf,
> a filter_res of 0xffffffff (-1) means that the configured _default_ action
> is used. Any unkown return code from the BPF program would fail in
> tcf_bpf() with TC_ACT_UNSPEC.
>

So you allow bpf bytecode to override the action code specified
in cmdline, I doubt this is user-friendly since if I run different
bytecode I would see different behaviors even if I specify the same
action in cmdline. Allowing cmdline to override bytecode makes
more sense to me.

A even cleaner solution is not to override either of them, that is
either bytecode or cmdline fully controls the action code.
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