On Mon, Feb 12, 2024 at 4:34 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Thu, 2024-02-08 at 20:06 -0800, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > 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.
Right. That's expected, because __uint(max_entries, 1);
The test creates an area on 1 page and it's consumed
by int __arena * __arena bar; variable.
Of course, one variable doesn't take the whole page.
There could have been many arena global vars.
But that page is not available anymore to bpf_arena_alloc_pages,
so it returns NULL.
