[PATCH bpf v3 2/4] selftests/bpf: Test refcount_acquire return nullability

[Ning Ding <dingning04@gmail.com>]

The verifier could accept an unchecked bpf_refcount_acquire() result for a
borrowed RCU-loaded map kptr. If the call returns NULL, passing the result
to bpf_obj_drop() can crash the kernel.

Add tests showing that an owned input remains non-NULL, a checked borrowed
result is accepted, and an unchecked borrowed result is rejected.

Assisted-by: Codex:gpt-5.5
Assisted-by: ChatGPT:GPT-5.6-Thinking
Signed-off-by: Ning Ding <dingning04@gmail.com>
---
 .../selftests/bpf/progs/refcounted_kptr.c     | 61 +++++++++++++++++++
 .../bpf/progs/refcounted_kptr_fail.c          | 47 ++++++++++++++
 2 files changed, 108 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr.c b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
index 61906f48025cc..fd35093285c0d 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
@@ -23,6 +23,15 @@ struct map_value {
 	struct node_data __kptr *node;
 };
 
+struct node_refcount_only {
+	long key;
+	struct bpf_refcount refcount;
+};
+
+struct map_value_refcount_only {
+	struct node_refcount_only __kptr *node;
+};
+
 struct {
 	__uint(type, BPF_MAP_TYPE_ARRAY);
 	__type(key, int);
@@ -30,6 +39,13 @@ struct {
 	__uint(max_entries, 2);
 } stashed_nodes SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value_refcount_only);
+	__uint(max_entries, 1);
+} stashed_refcount_only SEC(".maps");
+
 struct node_acquire {
 	long key;
 	long data;
@@ -832,6 +848,51 @@ long rbtree_refcounted_node_ref_escapes_owning_input(void *ctx)
 	return 0;
 }
 
+SEC("tc")
+__success
+long refcount_acquire_owning_input_no_null_check(void *ctx)
+{
+	struct node_refcount_only *n, *m;
+
+	n = bpf_obj_new(typeof(*n));
+	if (!n)
+		return 1;
+
+	m = bpf_refcount_acquire(n);
+	bpf_obj_drop(m);
+	bpf_obj_drop(n);
+
+	return 0;
+}
+
+SEC("tc")
+__success
+long refcount_acquire_rcu_map_kptr_null_checked(void *ctx)
+{
+	struct map_value_refcount_only *mapval;
+	struct node_refcount_only *n, *m;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_refcount_only, &idx);
+	if (!mapval)
+		return 1;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 2;
+	}
+	m = bpf_refcount_acquire(n);
+	bpf_rcu_read_unlock();
+
+	if (!m)
+		return 3;
+	bpf_obj_drop(m);
+
+	return 0;
+}
+
 static long __stash_map_empty_xchg(struct node_data *n, int idx)
 {
 	struct map_value *mapval = bpf_map_lookup_elem(&stashed_nodes, &idx);
diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
index 024ef2aae2008..acd3e81a39168 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
@@ -19,6 +19,15 @@ struct node_refcounted {
 	struct bpf_refcount refcount;
 };
 
+struct node_refcount_only {
+	long key;
+	struct bpf_refcount refcount;
+};
+
+struct map_value_refcount_only {
+	struct node_refcount_only __kptr *node;
+};
+
 extern void bpf_rcu_read_lock(void) __ksym;
 extern void bpf_rcu_read_unlock(void) __ksym;
 
@@ -28,6 +37,13 @@ private(A) struct bpf_rb_root groot __contains(node_acquire, node);
 private(B) struct bpf_spin_lock lock;
 private(B) struct bpf_list_head head __contains(node_refcounted, list);
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value_refcount_only);
+	__uint(max_entries, 1);
+} stashed_refcount_only SEC(".maps");
+
 static bool less(struct bpf_rb_node *a, const struct bpf_rb_node *b)
 {
 	struct node_acquire *node_a;
@@ -80,6 +96,37 @@ long refcount_acquire_maybe_null(void *ctx)
 	return 0;
 }
 
+SEC("?tc")
+__failure __msg("Possibly NULL pointer passed to trusted R1")
+long refcount_acquire_rcu_map_kptr_unchecked_drop(void *ctx)
+{
+	struct map_value_refcount_only *mapval;
+	struct node_refcount_only *tmp, *n, *m;
+	int idx = 0;
+
+	tmp = bpf_obj_new(typeof(*tmp));
+	if (!tmp)
+		return 3;
+	bpf_obj_drop(tmp);
+
+	mapval = bpf_map_lookup_elem(&stashed_refcount_only, &idx);
+	if (!mapval)
+		return 1;
+
+	bpf_rcu_read_lock();
+	n = mapval->node;
+	if (!n) {
+		bpf_rcu_read_unlock();
+		return 2;
+	}
+	m = bpf_refcount_acquire(n);
+	bpf_rcu_read_unlock();
+
+	bpf_obj_drop(m);
+
+	return 0;
+}
+
 SEC("?tc")
 __failure __msg("Unreleased reference id=3 alloc_insn={{[0-9]+}}")
 long rbtree_refcounted_node_ref_escapes_owning_input(void *ctx)
-- 
2.43.0