Issue eBPF kernel programs with ldc?

[data pulverizer]

On Sunday, 5 January 2025 at 14:27:57 UTC, Richard (Rikki) Andrew 
Cattermole wrote:
> On 06/01/2025 3:17 AM, data pulverizer wrote:
>
>> long bpf_trace_printk(const(char)* fmt, uint fmt_size, ...);
>
>> libbpf: failed to find BTF for extern 'bpf_trace_printk': -2
>
>
> Compare your D definition against:
>
> ```c
> static long (* const bpf_trace_printk)(const char *fmt, __u32 
> fmt_size, ...) = (void *) 6;
> ```
>
> https://github.com/libbpf/libbpf/blob/c5f22aca0f3aa855daa159b2777472b35e721804/src/bpf_helper_defs.h#L185
>
> The reason it cannot find it, is because its looking for a 
> function, when it is a global variable.

I have updated the kernel code to:

```d
@system:
@nogc:
extern(C):

@(ldc.attributes.section("license")) __gshared immutable(char)[3] 
LICENSE = "GPL";

import core.stdc.stdlib;
import core.stdc.stdarg;
import ldc.attributes;


// Define a type alias for the function signature
alias BpfTracePrintk = long function(const(char)* fmt, uint 
fmt_size, ...);

// Declare and initialize the function pointer
__gshared static BpfTracePrintk bpf_trace_printk = 
cast(BpfTracePrintk) 6;


@(ldc.attributes.section("xdp_md"))
struct xdp_md {
	uint data;
	uint data_end;
	uint data_meta;
	uint ingress_ifindex;
	uint rx_queue_index;
	uint egress_ifindex;
}

@(ldc.attributes.section("xdp")) int xdp_prog(xdp_md* ctx)
{
     bpf_trace_printk("Hello, eBPF!\n", 14);
     return 0;
}
```

And getting a different error:

```sh
$ ldc2 --O2 --betterC -c -g --nogc --fno-moduleinfo --march=bpf 
hello_world.d -of hello_world_d.o
$ ldc2 --betterC -L-lbpf -L-lelf -L-lz -of=loader_d loader_d.d
$ sudo ./loader_d

libbpf: elf: skipping unrecognized data section(21) .eh_frame
libbpf: elf: skipping relo section(22) .rel.eh_frame for 
section(21) .eh_frame
libbpf: BTF loading error: -22
libbpf: -- BEGIN BTF LOAD LOG ---
magic: 0xeb9f
version: 1
flags: 0x0
hdr_len: 24
type_off: 0
type_len: 312
str_off: 312
str_len: 374
btf_total_size: 710
[1] PTR hello_world.xdp_md* type_id=2 Invalid name

-- END BTF LOAD LOG --
libbpf: Error loading .BTF into kernel: -22. BTF is optional, 
ignoring.
libbpf: prog 'xdp_prog': BPF program load failed: Invalid argument
libbpf: prog 'xdp_prog': -- BEGIN PROG LOAD LOG --
unknown opcode 8d
processed 0 insns (limit 1000000) max_states_per_insn 0 
total_states 0 peak_states 0 mark_read 0
-- END PROG LOAD LOG --
libbpf: prog 'xdp_prog': failed to load: -22
libbpf: failed to load object 'hello_world_d.o'
ERROR: loading BPF object file failed
```

I updated `xdp_md` when it complained about it, it looks like it 
might be having trouble with the type name being prefixed with 
the module name `hello_world`?