On Fri, 6 Mar 2026 at 01:14, Kumar Kartikeya Dwivedi <[email protected]> wrote:
>
> On Wed, 4 Mar 2026 at 15:50, Chengkaitao <[email protected]> wrote:
> >
> > From: Kaitao Cheng <[email protected]>
> >
> > If a user holds ownership of a node in the middle of a list, they
> > can directly remove it from the list without strictly adhering to
> > deletion rules from the head or tail.
> >
> > We have added an additional parameter bpf_list_head *head to
> > bpf_list_del, as the verifier requires the head parameter to
> > check whether the lock is being held.
> >
> > This is typically paired with bpf_refcount. After calling
> > bpf_list_del, it is generally necessary to drop the reference to
> > the list node twice to prevent reference count leaks.
> >
> > Signed-off-by: Kaitao Cheng <[email protected]>
> > ---
> > kernel/bpf/helpers.c | 27 +++++++++++++++++++++++++++
> > kernel/bpf/verifier.c | 6 +++++-
> > 2 files changed, 32 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 6eb6c82ed2ee..cc1a096a1f64 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -2459,6 +2459,32 @@ __bpf_kfunc struct bpf_list_node
> > *bpf_list_pop_back(struct bpf_list_head *head)
> > return __bpf_list_del(head, true);
> > }
> >
> > +__bpf_kfunc struct bpf_list_node *bpf_list_del(struct bpf_list_head *head,
> > + struct bpf_list_node *node)
> > +{
> > + struct bpf_list_node_kern *knode = (struct bpf_list_node_kern
> > *)node;
> > + struct list_head *h = (void *)head;
> > +
> > + /* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
> > + * called on its fields, so init here
> > + */
> > + if (unlikely(!h->next)) {
> > + INIT_LIST_HEAD(h);
> > + return NULL;
> > + }
>
> nit: I would avoid doing this here, just for symmetry.
> If we are part of the list after the owner checks, this should be
> initialized correctly anyway.
Hmm, looks like I should take my ack back (just in principle, there's
no bug here).
I see you copied the comment and check from __bpf_list_del.
I was looking at rbtree_remove to see whether it did RB_CLEAR_NODE(n).
I think it's fine to keep this init, but let's try not repeating the
same logic twice.
I would make all 3 (pop_front, pop_back, list_del) use __bpf_list_del.
Just pass h->next, h->prev, or node depending on which one is being called.
>
> > +
> > + if (unlikely(!knode))
> > + return NULL;
>
> nit: It seems like node can't (shouldn't) be NULL, so we can lose this check.
>
> > +
> > + if (WARN_ON_ONCE(READ_ONCE(knode->owner) != head))
> > + return NULL;
> > +
> > + list_del_init(&knode->list_head);
> > + WRITE_ONCE(knode->owner, NULL);
> > +
> > + return node;
> > +}
> > +
> > __bpf_kfunc struct bpf_list_node *bpf_list_front(struct bpf_list_head
> > *head)
> > {
> > struct list_head *h = (struct list_head *)head;
> > @@ -4545,6 +4571,7 @@ BTF_ID_FLAGS(func, bpf_list_push_front_impl)
> > BTF_ID_FLAGS(func, bpf_list_push_back_impl)
> > BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
> > BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
> > +BTF_ID_FLAGS(func, bpf_list_del, KF_ACQUIRE | KF_RET_NULL)
> > BTF_ID_FLAGS(func, bpf_list_front, KF_RET_NULL)
> > BTF_ID_FLAGS(func, bpf_list_back, KF_RET_NULL)
> > BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 67c09b43a497..c9557d3fb8dd 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -12461,6 +12461,7 @@ enum special_kfunc_type {
> > KF_bpf_list_push_back_impl,
> > KF_bpf_list_pop_front,
> > KF_bpf_list_pop_back,
> > + KF_bpf_list_del,
> > KF_bpf_list_front,
> > KF_bpf_list_back,
> > KF_bpf_cast_to_kern_ctx,
> > @@ -12521,6 +12522,7 @@ BTF_ID(func, bpf_list_push_front_impl)
> > BTF_ID(func, bpf_list_push_back_impl)
> > BTF_ID(func, bpf_list_pop_front)
> > BTF_ID(func, bpf_list_pop_back)
> > +BTF_ID(func, bpf_list_del)
> > BTF_ID(func, bpf_list_front)
> > BTF_ID(func, bpf_list_back)
> > BTF_ID(func, bpf_cast_to_kern_ctx)
> > @@ -12996,6 +12998,7 @@ static bool is_bpf_list_api_kfunc(u32 btf_id)
> > btf_id == special_kfunc_list[KF_bpf_list_push_back_impl] ||
> > btf_id == special_kfunc_list[KF_bpf_list_pop_front] ||
> > btf_id == special_kfunc_list[KF_bpf_list_pop_back] ||
> > + btf_id == special_kfunc_list[KF_bpf_list_del] ||
> > btf_id == special_kfunc_list[KF_bpf_list_front] ||
> > btf_id == special_kfunc_list[KF_bpf_list_back];
> > }
> > @@ -13118,7 +13121,8 @@ static bool check_kfunc_is_graph_node_api(struct
> > bpf_verifier_env *env,
> > switch (node_field_type) {
> > case BPF_LIST_NODE:
> > ret = (kfunc_btf_id ==
> > special_kfunc_list[KF_bpf_list_push_front_impl] ||
> > - kfunc_btf_id ==
> > special_kfunc_list[KF_bpf_list_push_back_impl]);
> > + kfunc_btf_id ==
> > special_kfunc_list[KF_bpf_list_push_back_impl] ||
> > + kfunc_btf_id == special_kfunc_list[KF_bpf_list_del]);
> > break;
> > case BPF_RB_NODE:
> > ret = (kfunc_btf_id ==
> > special_kfunc_list[KF_bpf_rbtree_remove] ||
> > --
> > 2.50.1 (Apple Git-155)
> >
> >
