How to load BTF style maps?

How to load BTF style maps?

[Elerion]

I have maps written in the old style like this:

struct bpf_map_def SEC("maps") xdp_stats_map = {
.type        = BPF_MAP_TYPE_ARRAY,
.key_size    = sizeof(__u32),
.value_size  = sizeof(struct datarec),
.max_entries = XDP_ACTION_MAX,
};

I changed it to the new BTF style like this but now the example BPF
loader from the kernel doesn't work anymore.

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __uint(max_entries,  XDP_ACTION_MAX);
    __type(key,  __u32);
    __type(value,  struct datarec);
}  xdp_stats_map SEC(".maps");

I used this to load my program
https://github.com/torvalds/linux/blob/master/samples/bpf/bpf_load.c

But now it fails to load. First thing I noticed was the I had to
change "maps" to ".maps"
https://github.com/torvalds/linux/blob/master/samples/bpf/bpf_load.c#L563

But then bpf_create_map_node fails because all the arguments are 0. I
dumped the buffer here
https://github.com/torvalds/linux/blob/master/samples/bpf/bpf_load.c#L489
and it just copies 28 bytes of zeroes for each map I have.

How do you load BTF style maps? bpf_load.c doesn't seem to work on them.

Re: How to load BTF style maps?

[Toke Høiland-Jørgensen]

Elerion <elerion1000@gmail.com> writes:

> I have maps written in the old style like this:
>
> struct bpf_map_def SEC("maps") xdp_stats_map = {
> .type        = BPF_MAP_TYPE_ARRAY,
> .key_size    = sizeof(__u32),
> .value_size  = sizeof(struct datarec),
> .max_entries = XDP_ACTION_MAX,
> };
>
> I changed it to the new BTF style like this but now the example BPF
> loader from the kernel doesn't work anymore.
>
> struct {
>     __uint(type, BPF_MAP_TYPE_ARRAY);
>     __uint(max_entries,  XDP_ACTION_MAX);
>     __type(key,  __u32);
>     __type(value,  struct datarec);
> }  xdp_stats_map SEC(".maps");
>
> I used this to load my program
> https://github.com/torvalds/linux/blob/master/samples/bpf/bpf_load.c
>
> But now it fails to load. First thing I noticed was the I had to
> change "maps" to ".maps"
> https://github.com/torvalds/linux/blob/master/samples/bpf/bpf_load.c#L563
>
> But then bpf_create_map_node fails because all the arguments are 0. I
> dumped the buffer here
> https://github.com/torvalds/linux/blob/master/samples/bpf/bpf_load.c#L489
> and it just copies 28 bytes of zeroes for each map I have.
>
> How do you load BTF style maps? bpf_load.c doesn't seem to work on them.

Hmm, bpf_load.c seems to be using old-style loading (not libbpf) and so
wouldn't understand BTF-defined maps. I guess we should fix that.

In the meantime, you can try one of the following options:

Use xdp-loader from xdp-tools (since this seems to be an XDP use case?):
https://github.com/xdp-project/xdp-tools

Use bpftool (in tools/bpf/bpftool in the kernel source tree).

-Toke