bpf: refactor __bpf_list_add to take insertion point via **prev_ptr

Refactor __bpf_list_add to accept (node, 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(head, new, prev, ..) to insert after a given
list node.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260521032306.97118-6-kaitao.cheng@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 804c201c28f318928c76bab4357108f0e8401762..1c69476c8a09480ef71b0fecb7b16fe909521e1e 100644 (file)
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2478,9 +2478,11 @@ __bpf_kfunc void *bpf_refcount_acquire_impl(void *p__refcounted_kptr, void *meta
 
 static int __bpf_list_add(struct bpf_list_node_kern *node,
                          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 *prev;
 
        /* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
         * called on its fields, so init here
@@ -2488,19 +2490,31 @@ static int __bpf_list_add(struct bpf_list_node_kern *node,
        if (unlikely(!h->next))
                INIT_LIST_HEAD(h);
 
+       prev = *prev_ptr;
+
+       /* When prev is not the list head, it must be a node in this list. */
+       if (prev != h) {
+               struct bpf_list_node_kern *prev_kn =
+                       container_of(prev, struct bpf_list_node_kern, list_head);
+
+               if (unlikely(READ_ONCE(prev_kn->owner) != head))
+                       goto fail;
+       }
+
        /* node->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;
-       }
+       if (cmpxchg(&node->owner, NULL, BPF_PTR_POISON))
+               goto fail;
 
-       tail ? list_add_tail(n, h) : list_add(n, h);
+       list_add(n, prev);
        WRITE_ONCE(node->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;
 }
 
 /**
@@ -2521,8 +2535,9 @@ __bpf_kfunc int bpf_list_push_front(struct bpf_list_head *head,
                                    u64 off)
 {
        struct bpf_list_node_kern *n = (void *)node;
+       struct list_head *h = (void *)head;
 
-       return __bpf_list_add(n, head, false, meta ? meta->record : NULL, off);
+       return __bpf_list_add(n, head, &h, meta ? meta->record : NULL, off);
 }
 
 __bpf_kfunc int bpf_list_push_front_impl(struct bpf_list_head *head,
@@ -2550,8 +2565,9 @@ __bpf_kfunc int bpf_list_push_back(struct bpf_list_head *head,
                                   u64 off)
 {
        struct bpf_list_node_kern *n = (void *)node;
+       struct list_head *h = (void *)head;
 
-       return __bpf_list_add(n, head, true, meta ? meta->record : NULL, off);
+       return __bpf_list_add(n, head, &h->prev, meta ? meta->record : NULL, off);
 }
 
 __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,