Re: [PATCH bpf-next v4 04/12] bpf: take the vmlinux BTF from the btf_vmlinux module

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu, Oct 01, 2026 at 10:52 PM Jay Wang <wanjay@amazon.com> wrote:
> +	err = __sys_bpf(cmd, USER_BPFPTR(uattr), size, USER_BPFPTR(uattr_common), size_common);
> +	if (IS_MODULE(CONFIG_DEBUG_INFO_BTF) && err < 0 && bpf_btf_vmlinux_retry(cmd, misses))
> +		err = __sys_bpf(cmd, USER_BPFPTR(uattr), size, USER_BPFPTR(uattr_common),
> +				size_common);

Running the command twice when it failed and a global counter moved
is not going to fly.
Missing BTF is not always an error.
With =m and BTF not loaded yet take
  int prog(struct xdp_md *ctx)
with func_info. That is everything libbpf loads.
do_check_common() calls btf_prepare_func_args(env, 0).
find_canonical_prog_ctx_type() returns NULL, btf_is_prog_ctx_type()
returns false, the arg becomes ARG_PTR_TO_MEM and
func_info_aux[0].unreliable is set.
The prog loads. err == 0. No retry.
From then on freplace of that prog fails with
"Cannot replace static functions" and fentry/fexit see its args
as scalars, even after BTF is loaded.
With =y both work.

That is the only reason why "a socket filter does not load it".
In v3 such prog loaded BTF.
systemd built with BPF_FRAMEWORK loads
  int sd_bind4(struct bpf_sock_addr *ctx)
from manager_setup_cgroup() at every boot.

The result of the verification cannot depend on who touched BTF first.

bpf-ci is right too. The first run writes into uattr.

It's starting to feel that this is dead end.
So much complexity to save few Mbyte.

pw-bot: cr