[PATCH bpf v1 2/8] selftests/bpf: Check rbtree callback restrictions in subprogs

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Add a verifier failure case where an rbtree comparator enters two nested
static subprograms and the innermost subprogram unlocks and relocks the
tree. Restoring the lock keeps the surrounding callback state balanced,
so the test specifically exercises whether the callback restriction follows
the nested calls.

Also add a load-only positive control whose comparator calls a harmless
static subprogram. This preserves the intended support for verified static
subprogram calls while holding the tree lock.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../testing/selftests/bpf/progs/rbtree_fail.c | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/rbtree_fail.c b/tools/testing/selftests/bpf/progs/rbtree_fail.c
index 803419a47c62..4504608196ab 100644
--- a/tools/testing/selftests/bpf/progs/rbtree_fail.c
+++ b/tools/testing/selftests/bpf/progs/rbtree_fail.c
@@ -272,6 +272,47 @@ static bool less__bad_res_spin_unlock(struct bpf_rb_node *a, const struct bpf_rb
 	return false;
 }
 
+static __noinline void rbtree_cb_unlock_relock(void)
+{
+	bpf_spin_unlock(&glock);
+	bpf_spin_lock(&glock);
+}
+
+static __noinline void rbtree_cb_nested_unlock(void)
+{
+	rbtree_cb_unlock_relock();
+	asm volatile ("");
+}
+
+static bool less__bad_subprog_unlock(struct bpf_rb_node *a, const struct bpf_rb_node *b)
+{
+	struct node_data *node_a;
+	struct node_data *node_b;
+
+	node_a = container_of(a, struct node_data, node);
+	node_b = container_of(b, struct node_data, node);
+	rbtree_cb_nested_unlock();
+
+	return node_a->key < node_b->key;
+}
+
+static __noinline void rbtree_cb_noop(void)
+{
+	asm volatile ("");
+}
+
+static bool less__subprog_allowed(struct bpf_rb_node *a, const struct bpf_rb_node *b)
+{
+	struct node_data *node_a;
+	struct node_data *node_b;
+
+	node_a = container_of(a, struct node_data, node);
+	node_b = container_of(b, struct node_data, node);
+	rbtree_cb_noop();
+
+	return node_a->key < node_b->key;
+}
+
 static __always_inline
 long add_with_cb(bool (cb)(struct bpf_rb_node *a, const struct bpf_rb_node *b))
 {
@@ -330,4 +371,18 @@ long rbtree_api_add_bad_cb_res_spin_unlock(void *ctx)
 	return 0;
 }
 
+SEC("?tc")
+__failure __msg("can't spin_{lock,unlock} in rbtree cb")
+long rbtree_api_add_bad_cb_subprog_unlock(void *ctx)
+{
+	return add_with_cb(less__bad_subprog_unlock);
+}
+
+SEC("?tc")
+__success
+long rbtree_api_add_cb_subprog_allowed(void *ctx)
+{
+	return add_with_cb(less__subprog_allowed);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.53.0