netdev - Re: [PATCH bpf-next RFC v1 3/8] libbpf: Support kernel module function calls

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Date:   Tue, 31 Aug 2021 20:38:00 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next RFC v1 3/8] libbpf: Support kernel module
 function calls

On Tue, Aug 31, 2021 at 7:59 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Aug 31, 2021 at 7:27 PM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> > > > @@ -5327,6 +5340,7 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
> > > >                         ext = &obj->externs[relo->sym_off];
> > > >                         insn[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
> > > >                         insn[0].imm = ext->ksym.kernel_btf_id;
> > > > +                       insn[0].off = ext->ksym.offset;
> > >
> > > Just a few lines above we use insn[1].imm =
> > > ext->ksym.kernel_btf_obj_fd; for EXT_KSYM (for variables). Why are you
> > > inventing a new form if we already have a pretty consistent pattern?
> > >
> >
> > That makes sense. This is all new to me, so I went with what was described in
> > e6ac2450d6de (bpf: Support bpf program calling kernel function), but I'll rework
> > it to encode the btf fd like that in the next spin. It also makes the everything
> > far simpler.
>
> Hmm. kfunc call is a call insn. There is no imm[1].

Doh, right :( Never mind, we'll need to use fd_array for this.

Either way, I don't think hashmap use is warranted here to find a BTF
slot. Let's just do linear search, it's not like we are going to have
thousands of module BTFs used by any single BPF program, right?