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

[SeongJae Park <sj@kernel.org>]

__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/20220219082037.ow2kbq5brktf4f2u@apollo.legion/

Reported-by: Alexander Egorenkov <Alexander.Egorenkov@ibm.com>
Link: https://lore.kernel.org/bpf/87y228q66f.fsf@oc8242746057.ibm.com/
Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20220219082037.ow2kbq5brktf4f2u@apollo.legion/
Fixes: dee872e124e8 ("bpf: Populate kfunc BTF ID sets in struct btf")
Cc: <stable@vger.kernel.org> # 5.17.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 kernel/bpf/btf.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 6b682b8e4b50..d683f034996f 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -7848,14 +7848,10 @@ static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook,
 
 	btf = btf_get_module_btf(kset->owner);
 	if (!btf) {
-		if (!kset->owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) {
-			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))
+			pr_warn("missing vmlinux BTF, cannot register kfuncs\n");
+		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