netdev - Re: [PATCH bpf-next] libbpf: Add bpf_object_

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Message-ID: <875zepmykv.fsf@toke.dk>
Date:   Fri, 27 Mar 2020 19:39:44 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: Add bpf_object__rodata getter function

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

>> > But basically, why can't you use BPF skeleton?
>>
>> Couple of reasons:
>>
>> - I don't need any of the other features of the skeleton
>> - I don't want to depend on bpftool in the build process
>> - I don't want to embed the BPF bytecode into the C object
>
> Just curious, how are you intending to use global variables. Are you
> restricting to a single global var (a struct probably), so it's easier
> to work with it? Or are you resolving all the variables' offsets
> manually? It's really inconvenient to work with global variables
> without skeleton, which is why I'm curious.

Yeah, there's a single:

static volatile const struct xdp_dispatcher_config conf = {};

in the BPF file. Which is defined as:

struct xdp_dispatcher_config {
	__u8 num_progs_enabled;
	__u32 chain_call_actions[MAX_DISPATCHER_ACTIONS];
};

>> > Also, application can already find that map by looking at name.
>>
>> Yes, it can find the map, but it can't access the data. But I guess I
>> could just add a getter for that. Just figured this was easier to
>> consume; but I can see why it might impose restrictions on future
>> changes, so I'll send a v2 with such a map-level getter instead.
>
> Sounds good, I'll go review v2 now.

Great, thanks!

-Toke