On Wed, 12 May 2021 at 19:50, Andrii Nakryiko [off-list ref] wrote:

...

So at least for BPF skeleton, the flow I was imagining would be
like this.

Thank you for the worked out example, it's really helpful.

1. BPF library abc consists of abc1.bpf.c and abc2.bpf.c. It also has
user-space component in abc.c.
2. BPF app uses abs library and has its own app1.bpf.c and app2.bpf.c
and app.c for user-space.
3. BPF library author sets up its Makefile to do
  a. clang -target bpf -g -O2 -c abc1.bpf.c -o abc1.bpf.o
  b. clang -target bpf -g -O2 -c abc2.bpf.c -o abc2.bpf.o
  c. bpftool gen lib libabc.bpf.o abc1.bpf.o abc2.bpf.o

I think we can plug this into bpf2go [1] on our side in the best case,
which would avoid duplicating the static linker.

  d. bpftool gen subskeleton libabc.bpf.o > libabc.subskel.h
  e. abc.c (user-space library) is of the form

#include "libabc.subskel.h"

static struct libabc_bpf *subskel;

int libabc__init(struct bpf_object *obj)
{
    subskel = libabc_bpf__open_subskel(obj);

    subskel->data->abc_my_var = 123;
}

int libabc__attach()
{
    libabc_bpf__attach(subskel);
}

  f. cc abc.c into libabc.a and then libabc.a and libabc.bpf.o are
distributed to end user

3. Now, from BPF application author side:
  a. clang -target bpf -g -O2 -c app1.bpf.c -o app1.bpf.o
  b. clang -target bpf -g -O2 -c app2.bpf.c -o app2.bpf.o
  c. bpftool gen object app.bpf.o app1.bpf.o app2.bpf.o libabc.bpf.o

I haven't worked out exactly how things would work, but on the Go side
it might be possible to distribute libabc.bpf.o plus the Go "library"
code as a package. So the Go toolchain would never create this merged
object, but instead do

    bpftool gen object app.bpf.o app1.bpf.o app2.bpf.o

and later link app.bpf.o and libabc.bpf.o at runtime. It would be
simpler from our side if bpftool gen object could link both libraries
and "programs", maybe we can discuss the details of this during office
hours.

1: https://pkg.go.dev/github.com/cilium/ebpf/cmd/bpf2go

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