[PATCH bpf-next v8 4/8] bpf: refactor __bpf_list_add to take insertion point via **prev_ptr

[Chengkaitao]

From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>

Refactor __bpf_list_add to accept (new, head, struct list_head **prev_ptr,
..) instead of (node, head, bool tail, ..). Load prev from *prev_ptr after
INIT_LIST_HEAD(h), so we never dereference an uninitialized h->prev when
head was 0-initialized (e.g. push_back passes &h->prev).

When prev is not the list head, validate that prev is in the list via
its owner.

Prepares for bpf_list_add_impl(head, new, prev, ..) to insert after a
given list node.

Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
---
 kernel/bpf/helpers.c | 44 ++++++++++++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index dac346eb1e2f..a9665f97b3bc 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2379,11 +2379,13 @@ __bpf_kfunc void *bpf_refcount_acquire_impl(void *p__refcounted_kptr, void *meta
 	return (void *)p__refcounted_kptr;
 }
 
-static int __bpf_list_add(struct bpf_list_node_kern *node,
+static int __bpf_list_add(struct bpf_list_node_kern *new,
 			  struct bpf_list_head *head,
-			  bool tail, struct btf_record *rec, u64 off)
+			  struct list_head **prev_ptr,
+			  struct btf_record *rec, u64 off)
 {
-	struct list_head *n = &node->list_head, *h = (void *)head;
+	struct list_head *n = &new->list_head, *h = (void *)head;
+	struct list_head *prev;
 
 	/* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
 	 * called on its fields, so init here
@@ -2391,39 +2393,49 @@ static int __bpf_list_add(struct bpf_list_node_kern *node,
 	if (unlikely(!h->next))
 		INIT_LIST_HEAD(h);
 
-	/* node->owner != NULL implies !list_empty(n), no need to separately
+	prev = *prev_ptr;
+
+	/* When prev is not the list head, it must be a node in this list. */
+	if (prev != h && WARN_ON_ONCE(READ_ONCE(container_of(
+	    prev, struct bpf_list_node_kern, list_head)->owner) != head))
+		goto fail;
+
+	/* new->owner != NULL implies !list_empty(n), no need to separately
 	 * check the latter
 	 */
-	if (cmpxchg(&node->owner, NULL, BPF_PTR_POISON)) {
-		/* Only called from BPF prog, no need to migrate_disable */
-		__bpf_obj_drop_impl((void *)n - off, rec, false);
-		return -EINVAL;
-	}
-
-	tail ? list_add_tail(n, h) : list_add(n, h);
-	WRITE_ONCE(node->owner, head);
+	if (cmpxchg(&new->owner, NULL, BPF_PTR_POISON))
+		goto fail;
 
+	list_add(n, prev);
+	WRITE_ONCE(new->owner, head);
 	return 0;
+
+fail:
+	/* Only called from BPF prog, no need to migrate_disable */
+	__bpf_obj_drop_impl((void *)n - off, rec, false);
+	return -EINVAL;
 }
 
 __bpf_kfunc int bpf_list_push_front_impl(struct bpf_list_head *head,
 					 struct bpf_list_node *node,
 					 void *meta__ign, u64 off)
 {
-	struct bpf_list_node_kern *n = (void *)node;
+	struct bpf_list_node_kern *new = (void *)node;
 	struct btf_struct_meta *meta = meta__ign;
+	struct list_head *h = (void *)head;
 
-	return __bpf_list_add(n, head, false, meta ? meta->record : NULL, off);
+	return __bpf_list_add(new, head, &h, meta ? meta->record : NULL, off);
 }
 
 __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
 					struct bpf_list_node *node,
 					void *meta__ign, u64 off)
 {
-	struct bpf_list_node_kern *n = (void *)node;
+	struct bpf_list_node_kern *new = (void *)node;
 	struct btf_struct_meta *meta = meta__ign;
+	struct list_head *h = (void *)head;
 
-	return __bpf_list_add(n, head, true, meta ? meta->record : NULL, off);
+	return __bpf_list_add(new, head, &h->prev, meta ? meta->record : NULL, off);
 }
 
 static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head,
-- 
2.50.1 (Apple Git-155)