Re: [PATCH bpf-next v3 2/2] selftests/bpf: Cover refcount acquire node offsets

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-06-22 at 02:25 +0000, Yiyang Chen wrote:
> Add regression coverage for bpf_refcount_acquire() on graph-node-derived
> pointers.
> 
> The rejected cases pass popped list and rbtree node pointers directly to
> bpf_refcount_acquire(), which must fail because those pointers carry
> non-zero fixed offsets.
> 
> Do not add a positive container_of() case here. Existing refcounted_kptr
> coverage already exercises valid base-pointer acquisitions, and this patch
> only checks the rejected offset forms.

This paragraph is completely unnecessary.

> Fixes: 7c50b1cb76aca ("bpf: Add bpf_refcount_acquire kfunc")

I don't think the fixes tag applies to the test.

> Signed-off-by: Yiyang Chen <chenyy23@mails.tsinghua.edu.cn>
> ---
>  .../bpf/progs/refcounted_kptr_fail.c          | 77 +++++++++++++++++++
>  1 file changed, 77 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
> index 7247a20c0..77cda5ca2 100644
> --- a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
> +++ b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
> @@ -13,12 +13,22 @@ struct node_acquire {
>  	struct bpf_refcount refcount;
>  };
>  
> +struct node_refcounted {
> +	long key;
> +	struct bpf_rb_node rb;
> +	struct bpf_list_node list;
> +	struct bpf_refcount refcount;
> +};
> +
>  extern void bpf_rcu_read_lock(void) __ksym;
>  extern void bpf_rcu_read_unlock(void) __ksym;
>  
>  #define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8)))
>  private(A) struct bpf_spin_lock glock;
>  private(A) struct bpf_rb_root groot __contains(node_acquire, node);
> +private(B) struct bpf_spin_lock lock;
> +private(B) struct bpf_rb_root root __contains(node_refcounted, rb);
> +private(B) struct bpf_list_head head __contains(node_refcounted, list);
>  
>  static bool less(struct bpf_rb_node *a, const struct bpf_rb_node *b)
>  {
> @@ -31,6 +41,17 @@ static bool less(struct bpf_rb_node *a, const struct bpf_rb_node *b)
>  	return node_a->key < node_b->key;
>  }
>  
> +static bool less_refcounted(struct bpf_rb_node *a, const struct bpf_rb_node *b)
> +{
> +	struct node_refcounted *node_a;
> +	struct node_refcounted *node_b;
> +
> +	node_a = container_of(a, struct node_refcounted, rb);
> +	node_b = container_of(b, struct node_refcounted, rb);
> +
> +	return node_a->key < node_b->key;
> +}
> +
>  SEC("?tc")
>  __failure __msg("Unreleased reference id=4 alloc_insn={{[0-9]+}}")
>  long rbtree_refcounted_node_ref_escapes(void *ctx)
> @@ -93,6 +114,62 @@ long rbtree_refcounted_node_ref_escapes_owning_input(void *ctx)
>  	return 0;
>  }
>  
> +SEC("?tc")
> +__failure __msg("dereference of modified ptr_ ptr R1")
> +long refcount_acquire_list_node_offset(void *ctx)
> +{
> +	struct node_refcounted *node, *base, *ref;
> +	struct bpf_list_node *list_node;
> +
> +	node = bpf_obj_new(typeof(*node));
> +	if (!node)
> +		return 1;
> +
> +	bpf_spin_lock(&lock);
> +	bpf_list_push_front(&head, &node->list);
> +	list_node = bpf_list_pop_front(&head);
> +	bpf_spin_unlock(&lock);
> +	if (!list_node)
> +		return 2;
> +
> +	base = container_of(list_node, struct node_refcounted, list);
> +	ref = bpf_refcount_acquire(list_node);
> +	if (ref)
> +		bpf_obj_drop(ref);
> +	bpf_obj_drop(base);
> +	return 0;
> +}
> +
> +SEC("?tc")
> +__failure __msg("dereference of modified ptr_ ptr R1")
> +long refcount_acquire_rbtree_node_offset(void *ctx)

Why is this test necessary? Does it exercise any paths that the
previous one does not check?

> +{
> +	struct node_refcounted *node, *base, *ref;
> +	struct bpf_rb_node *rb_node;
> +
> +	node = bpf_obj_new(typeof(*node));
> +	if (!node)
> +		return 1;
> +
> +	node->key = 1;
> +
> +	bpf_spin_lock(&lock);
> +	bpf_rbtree_add(&root, &node->rb, less_refcounted);
> +	rb_node = bpf_rbtree_first(&root);
> +	if (rb_node)
> +		rb_node = bpf_rbtree_remove(&root, rb_node);
> +	bpf_spin_unlock(&lock);
> +	if (!rb_node)
> +		return 2;
> +
> +	base = container_of(rb_node, struct node_refcounted, rb);
> +	ref = bpf_refcount_acquire(rb_node);
> +	if (ref)
> +		bpf_obj_drop(ref);
> +	bpf_obj_drop(base);
> +	return 0;
> +}
> +
>  SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
>  __failure __msg("function calls are not allowed while holding a lock")
>  int BPF_PROG(rbtree_fail_sleepable_lock_across_rcu,