Reading constants from an ELF

[thediveo]

Describe the bug

Hopefully I've not missed the obvious here ... to the best of my knowledge there isn't currently "an official" way to keep constant/#define definitions synchronized between the eBPF C side and associated Go code. However, the opposite direction is supported via ebpf.CollectionSpec.RewriteConstants.

A potential ebpf.CollectionSpec.GetConstants(consts map[string]any) error would allow (albeit only at runtime) the Go side of code to pick up constant definitions and be always in sync; no more duplicate definition with #define in the eBPF program, and potentially out-of-sync Go consts.

For instance, in the eBPF C code:

#define MAX_FOOBAR 42
const volatile u64 max_foobar = MAX_FOOBAR;

SEC("very_raw_tracepoint/ohlala")
int BPF_PROG(ohlala) {
    /* ensure .rodata volatile constants get loaded */ __u32 _ = max_foobar;
}

Nota bene: I found out the hard way that elfCode.loadDataSections skips sections (including .rodata) when they aren't referenced in any way by program sections.

How to reproduce

n/a

Version information

github.com/cilium/ebpf v0.16.0

[ti-mo]

Reading constants is now possible with #1564 and will ship in 0.17. Manual is here: https://ebpf-go.dev/concepts/global-variables.

However, this strikes me as rather roundabout, esp. since we cull unreferenced symbols as you've mentioned. This cannot be worked around (aside from creating a reference..) and is still the case with the new CollectionSpec.Variables implementation.

For carrying e.g. some version info or other build-time info in an ELF, consider putting them in enums. Your MAX_ symbol in particular looks like it could be an enum in your actual code base. An ELF's enums are preserved in BTF, quick example in the repo's testdata:

λ  ebpf main ✓ bpftool btf dump file testdata/errors-el.elf
...
[4] STRUCT 'nonexist' size=4 vlen=1
	'non_exist' type_id=2 bits_offset=0

Try this in C:

enum metadata { bar = 1 };
// __hidden to omit from CollectionSpec.Variables. You could make this static,
// but then you'll need to mark it with __attribute__((used)), pick your poison..
// In the ELF, these would look exactly the same.
__hidden volatile enum metadata __meta;

bpftool:

[89] ENUM 'metadata' encoding=UNSIGNED size=4 vlen=1
        'bar' val=1

In Go:

var consts *btf.Enum
if err := spec.Types.TypeByName("metadata", &consts); err != nil {
    panic(err)
}

for _, v := range consts.Values {
    if v.Name == "non_exist" {
        return v.Value
    }
}

Hope this helps.