linux-kernel - Re: [PATCH bpf-next 11/15] bpftool: add skeleton codegen command

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Date:   Thu, 12 Dec 2019 10:58:31 -0800
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Andrii Nakryiko <andriin@...com>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 11/15] bpftool: add skeleton codegen command

On 12/12, Jakub Kicinski wrote:
> On Thu, 12 Dec 2019 08:53:22 -0800, Andrii Nakryiko wrote:
> > > > > Btw, how hard it would be to do this generation with a new python
> > > > > script instead of bpftool? Something along the lines of
> > > > > scripts/bpf_helpers_doc.py that parses BTF and spits out this C header
> > > > > (shouldn't be that hard to write custom BTF parser in python, right)?
> > > > >  
> > > >
> > > > Not impossible, but harder than I'd care to deal with. I certainly
> > > > don't want to re-implement a good chunk of ELF and BTF parsing (maps,
> > > > progs, in addition to datasec stuff). But "it's hard to use bpftool in
> > > > our build system" doesn't seem like good enough reason to do all that.  
> > > You can replace "our build system" with some other project you care about,
> > > like systemd. They'd have the same problem with vendoring in recent enough
> > > bpftool or waiting for every distro to do it. And all this work is
> > > because you think that doing:
> > >
> > >         my_obj->rodata->my_var = 123;
> > >
> > > Is easier / more type safe than doing:
> > >         int *my_var = bpf_object__rodata_lookup(obj, "my_var");
> > >         *my_var = 123;  
> > 
> > Your arguments are confusing me. Did I say that we shouldn't add this
> > type of "dynamic" interface to variables? Or did I say that every
> > single BPF application has to adopt skeleton and bpftool? I made no
> > such claims and it seems like discussion is just based around where I
> > have to apply my time and efforts... You think it's not useful - don't
> > integrate bpftool into your build system, simple as that. Skeleton is
> > used for selftests, but it's up to maintainers to decide whether to
> > keep this, similar to all the BTF decisions.
> 
> Since we have two people suggesting this functionality to be a separate
> tool could you please reconsider my arguments from two days ago?
> 
>   There absolutely nothing this tool needs from [bpftool], no
>   JSON needed, no bpffs etc. It can be a separate tool like
>   libbpf-skel-gen or libbpf-c-skel or something, distributed with libbpf.
>   That way you can actually soften the backward compat. In case people
>   become dependent on it they can carry that little tool on their own.

[..]
> I'd honestly leave the distro packaging problem for people who actually
> work on that to complain about.
I'm representing a 'Google distro' :-D