btf: warn but return no error for NULL btf from __register_btf_kfunc_id_set()

__register_btf_kfunc_id_set() assumes .BTF to be part of the module's .ko file if CONFIG_DEBUG_INFO_BTF is enabled. If that's not the case, the function prints an error message and return an error. As a result, such modules cannot be loaded.

However, the section could be stripped out during a build process. It would be better to let the modules loaded, because their basic functionalities have no problem[1], though the BTF functionalities will not be supported. Make the function to lower the level of the message from error to warn, and return no error.

[1] https://lore.kernel.org/bpf/[email protected]/

---

Changes from v2
(https://lore.kernel.org/bpf/[email protected]/)
- Keep the error for vmlinux case.
Changes from v1
(https://lore.kernel.org/all/[email protected]/)
- Fix Fixes: tag (Jiri Olsa)
- Add 'Acked-by: ' from Jiri Olsa

Notes
- This is a fix. Hence, would better to merged into bpf tree, not bpf-next.
- This doesn't cleanly applied on 6.1.y. I will send the backport to stable@ as soon as this is merged into the mainline.

 kernel/bpf/btf.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

index 29fe21099298..817204d53372 100644
@@ -7891,10 +7891,8 @@ static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook,
 			pr_err("missing vmlinux BTF, cannot register kfuncs\n");
 			return -ENOENT;
 		}
-		if (kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES)) {
-			pr_err("missing module BTF, cannot register kfuncs\n");
-			return -ENOENT;
-		}
+		if (kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES))
+			pr_warn("missing module BTF, cannot register kfuncs\n");
 		return 0;
 	}
 	if (IS_ERR(btf))

2.25.1

Looks good, applied to bpf, thanks!