[PATCH bpf-next v2 4/6] resolve_btfids: Emit bpf_kfunc and bpf_fastcall decl tags

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

Emit the bpf_kfunc decl tag for every discovered kfunc, and bpf_fastcall
for kfuncs flagged KF_FASTCALL. These were previously produced by pahole
under --btf_features=decl_tag_kfuncs.

resolve_btfids now discovers kfuncs from the BTF ID sets [1] and
becomes the source of truth for their annotations.

Drop decl_tag_kfuncs pahole feature flag from scripts/Makefile.btf

[1] https://lore.kernel.org/all/20260722233518.778854-1-ihor.solodrai@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 scripts/Makefile.btf            |  2 +-
 tools/bpf/resolve_btfids/main.c | 31 ++++++++++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
index 8f73c093d27a..a1812985a61a 100644
--- a/scripts/Makefile.btf
+++ b/scripts/Makefile.btf
@@ -14,7 +14,7 @@ pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsis
 else
 
 # Switch to using --btf_features for v1.26 and later.
-pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
+pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func
 
 pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=layout
 
diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index 30c7f95aa3b1..ea25ca34aa00 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -161,6 +161,10 @@ struct object {
 	u32 addr_syms_cap;
 };
 
+#define DECL_TAG_FASTCALL "bpf_fastcall"
+#define DECL_TAG_KFUNC "bpf_kfunc"
+
+#define KF_FASTCALL	(1 << 12)
 #define KF_ARENA_RET	(1 << 13)
 #define KF_ARENA_ARG1	(1 << 14)
 #define KF_ARENA_ARG2	(1 << 15)
@@ -1233,7 +1237,7 @@ static int process_kfunc_with_implicit_args(struct btf2btf_context *ctx, struct
 			continue;
 
 		tag_name = btf__name_by_offset(btf, t->name_off);
-		if (strcmp(tag_name, "bpf_kfunc") == 0)
+		if (strcmp(tag_name, DECL_TAG_KFUNC) == 0)
 			continue;
 
 		idx = btf_decl_tag(t)->component_idx;
@@ -1411,6 +1415,21 @@ static int process_kfunc_with_arena_flags(struct btf2btf_context *ctx,
 	return 0;
 }
 
+static int add_decl_tag(struct btf2btf_context *ctx, const char *tag_name,
+			u32 target_btf_id, int component_idx)
+{
+	s32 new_id;
+
+	new_id = btf__add_decl_tag(ctx->btf, tag_name, target_btf_id, component_idx);
+	if (new_id < 0) {
+		pr_err("ERROR: resolve_btfids: failed to add '%s' decl tag for BTF id %u: %d\n",
+		       tag_name, target_btf_id, new_id);
+		return new_id;
+	}
+
+	return push_decl_tag_id(ctx, new_id);
+}
+
 static int btf2btf(struct object *obj)
 {
 	struct btf2btf_context ctx = {};
@@ -1424,6 +1443,16 @@ static int btf2btf(struct object *obj)
 	for (next = rb_first(&ctx.kfuncs); next; next = rb_next(next)) {
 		struct kfunc *kfunc = rb_entry(next, struct kfunc, rb_node);
 
+		err = add_decl_tag(&ctx, DECL_TAG_KFUNC, kfunc->btf_id, -1);
+		if (err)
+			goto out;
+
+		if (kfunc->flags & KF_FASTCALL) {
+			err = add_decl_tag(&ctx, DECL_TAG_FASTCALL, kfunc->btf_id, -1);
+			if (err)
+				goto out;
+		}
+
 		if (kfunc->flags & KF_IMPLICIT_ARGS) {
 			err = process_kfunc_with_implicit_args(&ctx, kfunc);
 			if (err)
-- 
2.55.0