bpf, documentation: Add graph documentation for non-owning refs

It is difficult to intuit the semantics of owning and non-owning
references from verifier code. In order to keep the high-level details
from being lost in the mailing list, this patch adds documentation
explaining semantics and details.

The target audience of doc added in this patch is folks working on BPF
internals, as there's focus on "what should the verifier do here". Via
reorganization or copy-and-paste, much of the content can probably be
repurposed for BPF program writer audience as well.
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230214004017.2534011-9-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/graph_ds_impl.rst

+=========================
+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.
+
+.. contents::
+    :local:
+    :depth: 2
+
+Introduction
+------------
+
+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. Consequentially, 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.
+
+Locking
+-------
+
+The new-style data structures are intrusive and are defined similarly to their
+vanilla kernel counterparts:
+
+.. code-block:: c
+        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
+---------------------
+
+**Motivation**
+
+Consider the following BPF code:
+
+.. code-block:: c
+
+        struct node_data *n = bpf_obj_new(typeof(*n)); /* ACQUIRED */
+
+        bpf_spin_lock(&lock);
+
+        bpf_rbtree_add(&tree, n); /* PASSED */
+
+        bpf_spin_unlock(&lock);
+
+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.:
+
+.. code-block:: c
+
+        int x;
+        struct node_data *n = bpf_obj_new(typeof(*n)); /* ACQUIRED */
+
+        bpf_spin_lock(&lock);
+
+        bpf_rbtree_add(&tree, n); /* PASSED */
+        x = n->data;
+        n->data = 42;
+
+        bpf_spin_unlock(&lock);
+
+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.
+
+**Details**
+
+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:
+
+.. code-block:: c
+
+        struct node_data *n, *m, *o, *p;
+        n = bpf_obj_new(typeof(*n));     /* 1 */
+
+        bpf_spin_lock(&lock);
+
+        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_spin_unlock(&lock);
+
+        bpf_obj_drop(o);
+        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.

Documentation/bpf/other.rst

@@ -6,4 +6,5 @@ Other
    :maxdepth: 1
 
    ringbuf
-   llvm_reloc
\ No newline at end of file
+   llvm_reloc
+   graph_ds_impl