How to modify BTF to allow inject a new program?

[SirHao]

Does cilium/ebpf provides a method to update btf info when I try to add a program in a CollectionSpec?
What I do:

1. write raw instructions for a program copy from exist one.
2. insert to CollectionSpec.Programs
3. use ebpf.NewCollectionWithOptions ,try to load all programs

What I get return:
load fail: number of funcs in func_info doesn't mathch number of subprogs.

What I confused?
It seem that if I create a new program, I need to modify the .btf sec(maybe .btf.ext). Does there has any method, an what should I do?

[ti-mo]

Hi, thanks for the question. ebpf-go unmarshals all BTF info (including .btf.ext) into Go types and attaches them as metadata to individual asm.Instructions when loading an ELF. When loading the program into the kernel, the metadata is marshaled back into BTF wire format along with the program. This is why, when you inspect a loaded program's BTF with bpftool, it'll only contain the types used by your program's funcinfo, not the original BTF blob that came with the ELF. The same concept applies to maps created by ebpf-go.

Here's an example of how to attach your own funcinfo to an instruction:

ebpf/info_test.go

Line 38 in e3234a1

btf.WithFuncMetadata(asm.LoadImm(asm.R0, 0, asm.DWord), btfFn).

Unless you're using a program type that relies on correct funcinfo to validate function arguments, a dummy (like we're using in the test) should be fine to satisfy the verifier.

Good luck!