netdev - Re: [PATCH net] bpf: expose netns inode to bpf programs

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Date:   Sat, 4 Feb 2017 19:49:44 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        "David S . Miller" <davem@...emloft.net>,
        David Ahern <dsa@...ulusnetworks.com>,
        Tejun Heo <tj@...nel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Thomas Graf <tgraf@...g.ch>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] bpf: expose netns inode to bpf programs

On Sat, Feb 4, 2017 at 7:35 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Sat, Feb 04, 2017 at 07:22:03PM -0800, Andy Lutomirski wrote:
>> On Sat, Feb 4, 2017 at 7:18 PM, Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>> > On Sat, Feb 04, 2017 at 09:08:38AM -0800, Andy Lutomirski wrote:
>> >> > So use-case would be that someone wants to attach the very same
>> >> > prog via tc to various netdevs sitting in different netns, and
>> >> > that prog looks up a map, controlled by initns, with skb->netns_inum
>> >> > as key and the resulting value could contain allowed feature bits
>> >> > for that specific netns prog the skbs goes through? That would be
>> >> > a feature, not "concern", no? At the same time, it's up to the
>> >> > user or mgmt app what gets loaded so f.e. it might just as well
>> >> > tailor/optimize the progs individually for the devs sitting in
>> >> > netns-es to avoid such map lookup.
>> >>
>> >> Agreed.  I don't see why you would install the exact same program on
>> >> two sockets in different netnses if the program contains, say, an
>> >> ifindex.  Why not just install a variant with the right ifindex into
>> >> each socket?
>> >
>> > In other cases people prefer to have one program compiled once
>> > and thoroughly tested offline, since some folks are worried
>> > that on-the-fly compilation may cause generated code to
>> > be rejected by the verifier.
>>
>> I would be rather surprised if someone wrote a program that did had an
>> expression like (ifindex == 17), tested it offline, and refused to
>> update the 17 based on the actual ifindex in use.
>
> All programs have bugs. What bpf subsytem is trying to do is
> to be flexible and satisfy as many use cases as possible.
> Boxing users into "one way to do things" isn't a successful
> strategy in my opinion. perl vs python, if you like :)
> bpf is perl like. We don't enforce spaces vs tabs :)
>

Daniel's asking what the netns query is good for in programs that are
attached to sockets.  I think your example isn't relevant here.  In
fact, I think your example of pre-tested programs is even less
relevant, since a there's no way to know what the netns id will be
prior to the creation of a netns, so you can't usefully hard-code a
netns id into a precompiled BPF program.