Re: [PATCH v2 bpf-next 14/20] libbpf: Recognize __arena global varaibles.

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-02-08 at 20:06 -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> LLVM automatically places __arena variables into ".arena.1" ELF section.
> When libbpf sees such section it creates internal 'struct bpf_map' LIBBPF_MAP_ARENA
> that is connected to actual BPF_MAP_TYPE_ARENA 'struct bpf_map'.
> They share the same kernel's side bpf map and single map_fd.
> Both are emitted into skeleton. Real arena with the name given by bpf program
> in SEC(".maps") and another with "__arena_internal" name.
> All global variables from ".arena.1" section are accessible from user space
> via skel->arena->name_of_var.

[...]

I hit a strange bug when playing with patch. Consider a simple example [0].
When the following BPF global variable:

    int __arena * __arena bar;

- is commented -- the test passes;
- is uncommented -- in the test fails because global variable 'shared' is NULL.

Note: the second __arena is necessary to put 'bar' to .arena.1 section.

[0] https://github.com/kernel-patches/bpf/commit/6d95c8557c25d01ef3f13e6aef2bda9ac2516484