BPF Graph Data Structures

This document describes implementation details of new-style "graph" data structures (linked_list, rbtree), with particular focus on the verifier's implementation of semantics specific to those data structures.

Although no specific verifier code is referred to in this document, the document assumes that the reader has general knowledge of BPF verifier internals, BPF maps, and BPF program writing.

Note that the intent of this document is to describe the current state of these graph data structures. No guarantees of stability for either semantics or APIs are made or implied here.


The BPF map API has historically been the main way to expose data structures of various types for use within BPF programs. Some data structures fit naturally with the map API (HASH, ARRAY), others less so. Consequently, programs interacting with the latter group of data structures can be hard to parse for kernel programmers without previous BPF experience.

Luckily, some restrictions which necessitated the use of BPF map semantics are no longer relevant. With the introduction of kfuncs, kptrs, and the any-context BPF allocator, it is now possible to implement BPF data structures whose API and semantics more closely match those exposed to the rest of the kernel.

Two such data structures - linked_list and rbtree - have many verification details in common. Because both have "root"s ("head" for linked_list) and "node"s, the verifier code and this document refer to common functionality as "graph_api", "graph_root", "graph_node", etc.

Unless otherwise stated, examples and semantics below apply to both graph data structures.

Unstable API

Data structures implemented using the BPF map API have historically used BPF helper functions - either standard map API helpers like bpf_map_update_elem or map-specific helpers. The new-style graph data structures instead use kfuncs to define their manipulation helpers. Because there are no stability guarantees for kfuncs, the API and semantics for these data structures can be evolved in a way that breaks backwards compatibility if necessary.

Root and node types for the new data structures are opaquely defined in the uapi/linux/bpf.h header.


The new-style data structures are intrusive and are defined similarly to their vanilla kernel counterparts:

struct node_data {
  long key;
  long data;
  struct bpf_rb_node node;

struct bpf_spin_lock glock;
struct bpf_rb_root groot __contains(node_data, node);

The "root" type for both linked_list and rbtree expects to be in a map_value which also contains a bpf_spin_lock - in the above example both global variables are placed in a single-value arraymap. The verifier considers this spin_lock to be associated with the bpf_rb_root by virtue of both being in the same map_value and will enforce that the correct lock is held when verifying BPF programs that manipulate the tree. Since this lock checking happens at verification time, there is no runtime penalty.

Non-owning references


Consider the following BPF code:

struct node_data *n = bpf_obj_new(typeof(*n)); /* ACQUIRED */


bpf_rbtree_add(&tree, n); /* PASSED */


From the verifier's perspective, the pointer n returned from bpf_obj_new has type PTR_TO_BTF_ID | MEM_ALLOC, with a btf_id of struct node_data and a nonzero ref_obj_id. Because it holds n, the program has ownership of the pointee's (object pointed to by n) lifetime. The BPF program must pass off ownership before exiting - either via bpf_obj_drop, which free's the object, or by adding it to tree with bpf_rbtree_add.

(ACQUIRED and PASSED comments in the example denote statements where "ownership is acquired" and "ownership is passed", respectively)

What should the verifier do with n after ownership is passed off? If the object was free'd with bpf_obj_drop the answer is obvious: the verifier should reject programs which attempt to access n after bpf_obj_drop as the object is no longer valid. The underlying memory may have been reused for some other allocation, unmapped, etc.

When ownership is passed to tree via bpf_rbtree_add the answer is less obvious. The verifier could enforce the same semantics as for bpf_obj_drop, but that would result in programs with useful, common coding patterns being rejected, e.g.:

int x;
struct node_data *n = bpf_obj_new(typeof(*n)); /* ACQUIRED */


bpf_rbtree_add(&tree, n); /* PASSED */
x = n->data;
n->data = 42;


Both the read from and write to n->data would be rejected. The verifier can do better, though, by taking advantage of two details:

  • Graph data structure APIs can only be used when the bpf_spin_lock associated with the graph root is held

  • Both graph data structures have pointer stability

    • Because graph nodes are allocated with bpf_obj_new and adding / removing from the root involves fiddling with the bpf_{list,rb}_node field of the node struct, a graph node will remain at the same address after either operation.

Because the associated bpf_spin_lock must be held by any program adding or removing, if we're in the critical section bounded by that lock, we know that no other program can add or remove until the end of the critical section. This combined with pointer stability means that, until the critical section ends, we can safely access the graph node through n even after it was used to pass ownership.

The verifier considers such a reference a non-owning reference. The ref returned by bpf_obj_new is accordingly considered an owning reference. Both terms currently only have meaning in the context of graph nodes and API.


Let's enumerate the properties of both types of references.

owning reference

  • This reference controls the lifetime of the pointee

  • Ownership of pointee must be 'released' by passing it to some graph API kfunc, or via bpf_obj_drop, which free's the pointee

    • If not released before program ends, verifier considers program invalid

  • Access to the pointee's memory will not page fault

non-owning reference

  • This reference does not own the pointee

    • It cannot be used to add the graph node to a graph root, nor free'd via bpf_obj_drop

  • No explicit control of lifetime, but can infer valid lifetime based on non-owning ref existence (see explanation below)

  • Access to the pointee's memory will not page fault

From verifier's perspective non-owning references can only exist between spin_lock and spin_unlock. Why? After spin_unlock another program can do arbitrary operations on the data structure like removing and free-ing via bpf_obj_drop. A non-owning ref to some chunk of memory that was remove'd, free'd, and reused via bpf_obj_new would point to an entirely different thing. Or the memory could go away.

To prevent this logic violation all non-owning references are invalidated by the verifier after a critical section ends. This is necessary to ensure the "will not page fault" property of non-owning references. So if the verifier hasn't invalidated a non-owning ref, accessing it will not page fault.

Currently bpf_obj_drop is not allowed in the critical section, so if there's a valid non-owning ref, we must be in a critical section, and can conclude that the ref's memory hasn't been dropped-and- free'd or dropped-and-reused.

Any reference to a node that is in an rbtree _must_ be non-owning, since the tree has control of the pointee's lifetime. Similarly, any ref to a node that isn't in rbtree _must_ be owning. This results in a nice property: graph API add / remove implementations don't need to check if a node has already been added (or already removed), as the ownership model allows the verifier to prevent such a state from being valid by simply checking types.

However, pointer aliasing poses an issue for the above "nice property". Consider the following example:

struct node_data *n, *m, *o, *p;
n = bpf_obj_new(typeof(*n));     /* 1 */


bpf_rbtree_add(&tree, n);        /* 2 */
m = bpf_rbtree_first(&tree);     /* 3 */

o = bpf_rbtree_remove(&tree, n); /* 4 */
p = bpf_rbtree_remove(&tree, m); /* 5 */


bpf_obj_drop(p); /* 6 */

Assume the tree is empty before this program runs. If we track verifier state changes here using numbers in above comments:

  1. n is an owning reference

  2. n is a non-owning reference, it's been added to the tree

  3. n and m are non-owning references, they both point to the same node

  4. o is an owning reference, n and m non-owning, all point to same node

  5. o and p are owning, n and m non-owning, all point to the same node

  6. a double-free has occurred, since o and p point to same node and o was free'd in previous statement

States 4 and 5 violate our "nice property", as there are non-owning refs to a node which is not in an rbtree. Statement 5 will try to remove a node which has already been removed as a result of this violation. State 6 is a dangerous double-free.

At a minimum we should prevent state 6 from being possible. If we can't also prevent state 5 then we must abandon our "nice property" and check whether a node has already been removed at runtime.

We prevent both by generalizing the "invalidate non-owning references" behavior of bpf_spin_unlock and doing similar invalidation after bpf_rbtree_remove. The logic here being that any graph API kfunc which:

  • takes an arbitrary node argument

  • removes it from the data structure

  • returns an owning reference to the removed node

May result in a state where some other non-owning reference points to the same node. So remove-type kfuncs must be considered a non-owning reference invalidation point as well.