How do I rewrite global variables before loading a BPF object?

[Ephemeraler]

Before that, I use libbpf and C to write bpf controller. In *.bpf.c, I declare a global variable (filter_pid). and I set value (skel->bss->filter_pid = 1234) in *.c. Now, what should I do in cilium/ebpf. when I executed go generate, I might not find this variable In bpf_**.go. 3Q.

[ti-mo]

@Ephemeraler ebpf-go implements this using CollectionSpec.RewriteConstants().

Given the following global variable in C:

volatile const uint32_t arg;

The following Go code takes care of rewriting that constant to hold the value 1 at runtime:

err := spec.RewriteConstants(map[string]interface{}{
	"arg":  uint32(1),
})

[cougarwww]

Thanks very much, I understand. I have solved.

How did you solved? i have the same error.

---

I have solved too. we can not use bpfObjects after have.RewriteConstants , but use have.LoadAndAssign to get objs

[g0dA]

Yes, i have solved too, tks!

[devendra-dhakad]

@Ephemeraler @g0dA @cougarwww Can you please share the code here?

[thediveo]

@g0dA can you please be so kind as to share the fixed code here? Thank you!

[thediveo]

For the benefit of others being stuck with the same problem here's how I solved it; it's a shame that the posters claiming to have a solution never shared their solutions in a clear manner, yet are expecting to get help in the future nevertheless. Anyway, here we Go, assuming that you want to use bpf2go:

The ebpf module examples usually use loadBpfObjects(obj, opts) in roughly this manner:

objs := bpfObjects{}
err := loadBpfObjects(&objs, nil)
// ...error handling...

Luckily, loadBpfObjects(...) is a very simple wrapper around getting a spec by calling the autogenerated loadBpf() and then doing spec.LoadAndAssign(...). Now, rewriting constants needs to be done after loading the spec and before LoadAndAssign:

spec, err := loadBpf()
// ...error handling...
err = spec.RewriteConstants(map[string]interface{}{
  "foobar": uint32(42),
})
// ...error handling...
objs := bpfObjects{}
err = spec.LoadAndAssign(&objs, nil)
// ...error handling...

If something is unclear in my example, please ask!