feat: added BPFReplaceExistedObject

[ianchen0119]

All of the variables located in the rodata/bss section will be mapped into an eBPF map during the eBPF program's runtime.
And there are some known limitations:

1. The rodata section can only be overwritten before the eBPF program is attached.
2. The BSS section can be modified via MAP Update. But the data synchronization can't be guaranteed.

It was a significant challenge when I attempted to implement the user-space scheduler in Go.
My solution is to use the cgo to call eBPF skeleton APIs, so I need an additional wrapper to replace the existing eBPF object before the program attaches.

=== example ===

// ...

func LoadSkel() unsafe.Pointer {
	return C.open_skel()
}

func LoadSched(objPath string) *Sched {
	obj := LoadSkel()
	bpfModule, err := bpf.NewModuleFromFileArgs(bpf.NewModuleArgs{
		BPFObjPath:     "",
		KernelLogLevel: 0,
	})
	if err != nil {
		panic(err)
	}
	if err := bpfModule.BPFReplaceExistedObject(obj); err != nil {
		panic(err)
	}

	s := &Sched{
		mod: bpfModule,
	}

	return s
}

func (s *Sched) SetBuiltinIdle(enabled bool) {
	C.set_builtin_idle(C.bool(enabled))
}

func (s *Sched) Attach() error {
	_, err := s.structOps.AttachStructOps()
	return err
}

void set_builtin_idle(bool enabled) {
    global_obj->rodata->builtin_idle = enabled;
}

Therefore, I can call SetBuiltinIdle() before Attach() to modify the target variable builtin_idle in the rodata section.

[Copilot]

Pull Request Overview

This PR adds a new method BPFReplaceExistedObject to the Module struct that allows replacing an existing BPF object with one passed from external code (e.g., CGO). This enables modifying rodata section variables before program attachment, which is particularly useful for user-space schedulers that need to configure BPF programs through C skeleton APIs.

• Adds BPFReplaceExistedObject method to replace the internal BPF object pointer
• Enables modification of rodata section variables before program attachment
• Supports integration with CGO-based BPF skeleton workflows

---

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[Copilot]

The method name contains a spelling error. 'ExistedObject' should be 'ExistingObject'. The correct name should be BPFReplaceExistingObject.

Suggested change

	func (m *Module) BPFReplaceExistedObject(obj unsafe.Pointer) error {
	func (m *Module) BPFReplaceExistingObject(obj unsafe.Pointer) error {

[Copilot]

The method lacks input validation. It should check if the obj parameter is nil before casting and assignment to prevent potential runtime panics.

Suggested change

	func (m *Module) BPFReplaceExistedObject(obj unsafe.Pointer) error {
	func (m *Module) BPFReplaceExistedObject(obj unsafe.Pointer) error {
	if obj == nil {
	return errors.New("obj parameter is nil")
	}

[ianchen0119]

Hi @geyslan

The Gthulhu is now able to be used in 5G Data Plane optimization! For more details, please visit https://free5gc.org/blog/20250726/index.en.
Additionally, if you can help promote the Gthulhu, it would be great! We're currently aiming for CNCF Landscape recognition — the maintainers are happy to accept the project, but it needs at least 300 ⭐

Thanks a lot!

[geyslan]

Hi @ianchen0119 thanks again for contributing to libbpfgo.

1. The rodata section can only be overwritten before the eBPF program is attached.

Wouldn't InitGlobalVariable() be a solution for this case?

void set_builtin_idle(bool enabled) {
    global_obj->rodata->builtin_idle = enabled;
}

Do you have plans of bringing such functionality into the libbpfgo?

[ianchen0119]

Hi @ianchen0119 thanks again for contributing to libbpfgo.

1. The rodata section can only be overwritten before the eBPF program is attached.

Wouldn't InitGlobalVariable() be a solution for this case?

void set_builtin_idle(bool enabled) {
    global_obj->rodata->builtin_idle = enabled;
}

Do you have plans of bringing such functionality into the libbpfgo?

Sorry for the late reply.
I didn't try the InitGlobalVariable in this case, cuz I think the skeleton is more flexible to update the global data for me.
To use the example above, we need to generate the skeleton file based on the compiled eBPF program, then use an additional file to wrap the static library. I have no idea how to make these things generic. Do you have any suggestion on this?

Thank you!

[ianchen0119]

Hi @geyslan
I forgot to tag you in previous comment.
Just want to make sure you've received the status update :)

Thank you!

[geyslan]

@ianchen0119 no worries. I'll revisit this soon but I think, for now, that perhaps we could provide a NewModuleFromSkel() which could straightforwardly instantiate the module taking care of setting *C.struct_bpf_object. Please be my guest for testing it.