Hi Jiri, 

On Thu, Jan 04, 2024 at 12:41:51PM +0100, Jiri Olsa wrote:

On Wed, Jan 03, 2024 at 04:31:56PM -0700, Daniel Xu wrote:

SNIP

quoted

diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
index 88f914579fa1..771e29762a2d 100644
--- a/include/linux/btf_ids.h
+++ b/include/linux/btf_ids.h

@@ -8,6 +8,9 @@ struct btf_id_set {
 	u32 ids[];
 };
 
+/* This flag implies BTF_SET8 holds kfunc(s) */
+#define BTF_SET8_KFUNC		(1 << 0)
+
 struct btf_id_set8 {
 	u32 cnt;
 	u32 flags;

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 51e8b4bee0c8..b8ba00a4179f 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -7769,6 +7769,9 @@ static int __register_btf_kfunc_id_set(enum btf_kfunc_hook hook,
 	struct btf *btf;
 	int ret, i;
 
+	/* All kfuncs need to be tagged as such in BTF */
+	WARN_ON(!(kset->set->flags & BTF_SET8_KFUNC));

__register_btf_kfunc_id_set gets called also from the 'hooks' path:

  bpf_mptcp_kfunc_init
    register_btf_fmodret_id_set
      __register_btf_kfunc_id_set

so it will hit the warn.. it should be probably in the register_btf_kfunc_id_set ?

Yeah, good catch.

also given that we can have modules calling register_btf_kfunc_id_set,
should we just return error instead of the warn?

It looks like quite a few registrations go through late_initcall(),
in which error codes are thrown away. I'm looking at
init/main.c:do_initcall_level:

        for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++)
                do_one_initcall(initcall_from_entry(fn));

Higher level question: if out of tree module does not follow convention,
it would still make sense to WARN(), right?

SNIP

quoted

diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
index 91907b321f91..32972334cd50 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -341,7 +341,7 @@ static struct bin_attribute bin_attr_bpf_testmod_file __ro_after_init = {
 	.write = bpf_testmod_test_write,
 };
 
-BTF_SET8_START(bpf_testmod_common_kfunc_ids)
+BTF_SET8_START(bpf_testmod_common_kfunc_ids, BTF_SET8_KFUNC)
 BTF_ID_FLAGS(func, bpf_iter_testmod_seq_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_testmod_seq_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_testmod_seq_destroy, KF_ITER_DESTROY)

we need to change also bpf_testmod_check_kfunc_ids set

Good catch, thanks.

Daniel