[PATCH v2 dwarves 2/4] btf_encoder: emit type tags for bpf_arena pointers

[Ihor Solodrai <ihor.solodrai@linux.dev>]

When adding a kfunc prototype to BTF, check for the flags indicating
bpf_arena pointers and emit a type tag encoding
__attribute__((address_space(1))) for them. This also requires
updating BTF type ids in the btf_encoder_func_state, which is done as
a side effect in the tagging functions.

This feature depends on recent update in libbpf, supporting arbitrarty
attribute encoding [1].

[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
---
 btf_encoder.c | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 85 insertions(+), 1 deletion(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 965e8f0..3cec106 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -40,7 +40,13 @@
 #define BTF_SET8_KFUNCS		(1 << 0)
 #define BTF_KFUNC_TYPE_TAG	"bpf_kfunc"
 #define BTF_FASTCALL_TAG       "bpf_fastcall"
-#define KF_FASTCALL            (1 << 12)
+#define BPF_ARENA_ATTR         "address_space(1)"
+
+/* kfunc flags, see include/linux/btf.h in the kernel source */
+#define KF_FASTCALL   (1 << 12)
+#define KF_ARENA_RET  (1 << 13)
+#define KF_ARENA_ARG1 (1 << 14)
+#define KF_ARENA_ARG2 (1 << 15)
 
 struct btf_id_and_flag {
 	uint32_t id;
@@ -731,6 +737,78 @@ static int32_t btf_encoder__tag_type(struct btf_encoder *encoder, uint32_t tag_t
 	return encoder->type_id_off + tag_type;
 }
 
+#if LIBBPF_MAJOR_VERSION >= 1 && LIBBPF_MINOR_VERSION >= 6
+static int btf__tag_bpf_arena_ptr(struct btf *btf, int ptr_id)
+{
+	const struct btf_type *ptr;
+	int tagged_type_id;
+
+	ptr = btf__type_by_id(btf, ptr_id);
+	if (!btf_is_ptr(ptr))
+		return -EINVAL;
+
+	tagged_type_id = btf__add_type_attr(btf, BPF_ARENA_ATTR, ptr->type);
+	if (tagged_type_id < 0)
+		return tagged_type_id;
+
+	return btf__add_ptr(btf, tagged_type_id);
+}
+
+static int btf__tag_bpf_arena_arg(struct btf *btf, struct btf_encoder_func_state *state, int idx)
+{
+	int id;
+
+	if (state->nr_parms <= idx)
+		return -EINVAL;
+
+	id = btf__tag_bpf_arena_ptr(btf, state->parms[idx].type_id);
+	if (id < 0) {
+		btf__log_err(btf, BTF_KIND_TYPE_TAG, BPF_ARENA_ATTR, true, id,
+			"Error adding BPF_ARENA_ATTR for an argument of kfunc '%s'", state->elf->name);
+		return id;
+	}
+	state->parms[idx].type_id = id;
+
+	return id;
+}
+
+static int btf__add_bpf_arena_type_tags(struct btf *btf, struct btf_encoder_func_state *state)
+{
+	uint32_t flags = state->elf->kfunc_flags;
+	int ret_type_id;
+	int err;
+
+	if (KF_ARENA_RET & flags) {
+		ret_type_id = btf__tag_bpf_arena_ptr(btf, state->ret_type_id);
+		if (ret_type_id < 0) {
+			btf__log_err(btf, BTF_KIND_TYPE_TAG, BPF_ARENA_ATTR, true, ret_type_id,
+				"Error adding BPF_ARENA_ATTR for return type of kfunc '%s'", state->elf->name);
+			return ret_type_id;
+		}
+		state->ret_type_id = ret_type_id;
+	}
+
+	if (KF_ARENA_ARG1 & flags) {
+		err = btf__tag_bpf_arena_arg(btf, state, 0);
+		if (err < 0)
+			return err;
+	}
+
+	if (KF_ARENA_ARG2 & flags) {
+		err = btf__tag_bpf_arena_arg(btf, state, 1);
+		if (err < 0)
+			return err;
+	}
+
+	return 0;
+}
+#endif // LIBBPF_MAJOR_VERSION >= 1 && LIBBPF_MINOR_VERSION >= 6
+
+static inline bool is_kfunc_state(struct btf_encoder_func_state *state)
+{
+	return state && state->elf && state->elf->kfunc;
+}
+
 static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct ftype *ftype,
 					   struct btf_encoder_func_state *state)
 {
@@ -744,6 +822,12 @@ static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct f
 
 	assert(ftype != NULL || state != NULL);
 
+#if LIBBPF_MAJOR_VERSION >= 1 && LIBBPF_MINOR_VERSION >= 6
+	if (is_kfunc_state(state) && encoder->tag_kfuncs)
+		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
+			return -1;
+#endif
+
 	/* add btf_type for func_proto */
 	if (ftype) {
 		btf = encoder->btf;
-- 
2.48.1