[PATCH bpf-next v1 8/8] resolve_btfids: Enforce consistent kfunc flags across BTF ID sets

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

A kfunc may be listed in several BTF ID sets, which is expected
because different kfuncs are available to BPF programs depending on
their type.

However kfunc flags across different BTF ID sets must be consistent [1].
The flags should be considered a part of the kfunc declaration,
because they influence its BTF representation and verifier handling.

Enforce the kfunc flag consistency in resolve_btifds by hard failing
on error and blocking kernel (or module) build.

[1] https://lore.kernel.org/bpf/9b2196dd-443b-4632-ae11-030cdbdc59b4@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/bpf/resolve_btfids/main.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index 338d0c0a8e58..85488935909d 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -982,17 +982,26 @@ static int push_kfunc(struct btf2btf_context *ctx, struct kfunc *kfunc)
 	struct rb_node *parent = NULL;
 	struct kfunc *k;
 
-	/* Dedup by BTF ID: collecting the same kfunc twice is a no-op. */
+	/*
+	 * Dedup by BTF ID: collecting the same kfunc twice is a no-op,
+	 * UNLESS the kfunc flags are inconsistent, in which case we
+	 * fail hard because it indicates a bug in a kfunc set declaration.
+	 */
 	while (*p) {
 		parent = *p;
 		k = rb_entry(parent, struct kfunc, rb_node);
 
-		if (kfunc->btf_id < k->btf_id)
+		if (kfunc->btf_id < k->btf_id) {
 			p = &(*p)->rb_left;
-		else if (kfunc->btf_id > k->btf_id)
+		} else if (kfunc->btf_id > k->btf_id) {
 			p = &(*p)->rb_right;
-		else
+		} else if (k->flags == kfunc->flags) {
 			return 0;
+		} else {
+			pr_err("ERROR: resolve_btfids: kfunc %s has inconsistent flags across BTF ID sets: 0x%x != 0x%x\n",
+			       kfunc->name, k->flags, kfunc->flags);
+			return -EINVAL;
+		}
 	}
 
 	k = zalloc(sizeof(*k));
-- 
2.55.0