How do I re-use a pinned map when loading an ELF?

[d0u9]

I have a map, say Global, which is pinned to /sys/fs/bpf/global.

I want two different BPF programs to reference this pinned map. Is that possible?

[chenhengqi]

Yes, you can do that by specifying MapOptions.PinPath.

[d0u9]

Hi @chenhengqi, thank you for your quick response.

I have an eBpf elf file that has multiple map sections. I only want to replace one of them with a pinned map. I checked the example and saw a snippet:

ebpf/examples/kprobepin/main.go

Lines 44 to 55 in 34be694

	var objs bpfObjects
	if err := loadBpfObjects(&objs, &ebpf.CollectionOptions{
	Maps: ebpf.MapOptions{
	// Pin the map to the BPF filesystem and configure the
	// library to automatically re-write it in the BPF
	// program so it can be re-used if it already exists or
	// create it if not
	PinPath: pinPath,
	},
	}); err != nil {
	log.Fatalf("loading objects: %v", err)
	}

Seems that the PinPath is applied to all maps in that ELF object. Is there a way to replace a single map?

[chenhengqi]

Try this:

ebpf/map.go

Line 215 in 3418f34

func NewMapWithOptions(spec *MapSpec, opts MapOptions) (*Map, error) {

[ti-mo]

Hi @d0u9, thanks for the question.

In order for the ELF loader to automatically pin or re-use a pinned map, the map definition needs to have its pinned flag set.

See the example below:

ebpf/examples/kprobepin/kprobe_pin.c

Line 13 in 34be694

__uint(pinning, LIBBPF_PIN_BY_NAME);

If you attempt to load this ELF (containing a map definition with the 'pinned' flag) without CollectionOptions.Maps.PinPath set, map creation will fail with an error.

Specifying /sys/fs/bpf will cause a map called global to be created and pinned to /sys/fs/bpf/global. If an map at that path already exists, it will automatically open it and use that existing map in any programs loaded from the ELF. The existing pinned map can then also be interacted with using the resulting Collection.Maps["global"].

[mrpre]

The normal map can be declared like

struct bpf_map_def SEC("maps") xxmap {
...
}

I wonder is it a BTF format declare in Cilium?

struct {
        __uint(type, BPF_MAP_TYPE_HASH);
       ...
} xxmap SEC(".maps");

and LIBBPF_PIN_BY_NAME only can be used along with BTF ?

[ti-mo]

@mrpre Correct, the legacy bpf_map_def does not have a pinning flag, so the flag cannot be set from C. If you wish to use bpf_map_def, you'll have to set the MapSpec.Pinning field in Go code after reading the ELF.