support StructOps skeleton generation in bpf2go

[shun159]

bpftool gen skeleton provides "shadow" for StructOps, allowing C developers to assign BPF programs to struct fields directly. In Go, cilium/ebpf users currently have to manually handle ebpf.Map.Update with binary buffers or raw slices, which is error-prone.

1. Manual Offset Management: In projects like sched_ext, the memory layout of struct_ops changes across kernel versions. Manually calculating offsets in Go is difficult.
2. Lack of Abstraction: There is no bridge between *ebpf.Program and the StructOps map fields in the generated code.

Proposal:

• Enhance bpf2go to generate StructOps Shadow Types.
• Source Generation: bpf2go scans for StructOps maps and generates Go structs mirroring the C structure.
• Dynamic Mapping: Use User BTF (or vmlinux BTF) at runtime to resolve member offsets and sizes, ensuring compatibility across different kernels.
• Declarative Assignment: Allow users to assign *ebpf.Program and scalar values directly to struct fields before/during load.

A mockup of the generated Go struct

// generated shadow struct by bpf2go
type bpfStructOps struct {
	MinimalSched *bpfMinimalSchedStructOps `ebpf:"minimal_sched"`
}

type bpfMinimalSchedStructOps struct {
	// Function pointers are defined as *ebpf.Program.
	SelectCpu *ebpf.Program `ebpf:"select_cpu"`
	Enqueue   *ebpf.Program `ebpf:"enqueue"`
	Dispatch  *ebpf.Program `ebpf:"dispatch"`
	Running   *ebpf.Program `ebpf:"running"`

	// Scalar values are mapped to appropriate Go types.
	Flags     uint64 `ebpf:"flags"`
	TimeoutMs uint32 `ebpf:"timeout_ms"`
}

main.go

func main() {
	objs := bpfObjects{}
	// LoadAndAssign internally binds the shadow struct with the map's initial values.
	if err := loadBpfObjects(&objs, nil); err != nil {
		log.Fatal(err)
	}
	defer objs.Close()

	// --- Highly intuitive API for users ---
	ops := objs.StructOps.MinimalSched
	
	// Dynamic program assignment at runtime.
	ops.SelectCpu = objs.Programs.MyCustomSelectCpu
	ops.Enqueue = objs.Programs.MyGlobalEnqueue
	
	// Fine-tuning parameters via scalar fields.
	ops.Flags = 0x1 // e.g., SCX_OPS_KEEP_BUILTIN_IDLE
	ops.TimeoutMs = 100

	// Map update: BTF-based offset resolution occurs internally.
	if err := objs.MinimalSched.PutStructOps(ops); err != nil {
		log.Fatalf("failed to set struct_ops: %v", err)
	}

	// Attachment.
	l, err := link.AttachStructOps(link.StructOpsOptions{Map: objs.MinimalSched})
	if err != nil {
		log.Fatal(err)
	}
	defer l.Close()
	
	// ... (Wait for signal)
}