On Mon, 9 Mar 2026 at 07:34, Leon Hwang <[email protected]> wrote:
>
> On 8/3/26 21:46, Chengkaitao 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 | 30 +++++++++++++++++++++++-------
> > kernel/bpf/verifier.c | 6 +++++-
> > 2 files changed, 28 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 6eb6c82ed2ee..01b74c4ac00d 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -2426,20 +2426,23 @@ __bpf_kfunc int bpf_list_push_back_impl(struct
> > bpf_list_head *head,
> > return __bpf_list_add(n, head, true, meta ? meta->record : NULL, off);
> > }
> >
> > -static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head,
> > bool tail)
> > +static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head,
> > + struct list_head *n)
> > {
> > - struct list_head *n, *h = (void *)head;
> > + struct list_head *h = (void *)head;
> > struct bpf_list_node_kern *node;
> >
> > /* 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))
> > + if (unlikely(!h->next)) {
> > INIT_LIST_HEAD(h);
> > - if (list_empty(h))
> > + return NULL;
> > + }
> > +
> > + if (n == h)
> > return NULL;
Couldn't you keep the list_empty(h) check after INIT_LIST_HEAD(h) as before?
And we don't need n == h either. You could add a comment that n will
never match h.
The verifier should ensure it, since it distinguishes the head and node types.
Let's say the head is uninit. Then list_empty(h) will catch that case.
Otherwise n might be NULL.
After list_empty(h) says false, we definitely have non-null n.
We just need to check list membership using the owner check, and then
we're good.
It will be a less noisy diff.
> >
> > - n = tail ? h->prev : h->next;
> > node = container_of(n, struct bpf_list_node_kern, list_head);
> > if (WARN_ON_ONCE(READ_ONCE(node->owner) != head))
> > return NULL;
>
> This refactoring is worth, because the "struct list_head *n" seems
> better than "bool tail".
>
> But, such refactoring should be a preparatory patch. Importantly,
> refactoring should not introduce functional changes.
>
I think it's fine. Let's address this and avoid too many respins now.
It isn't a lot of code anyway. You could mention in the commit log
that you chose to refactor __bpf_list_del though.
> Thanks,
> Leon
>
> > @@ -2451,12 +2454,24 @@ static struct bpf_list_node *__bpf_list_del(struct
> > bpf_list_head *head, bool tai
> >
> > __bpf_kfunc struct bpf_list_node *bpf_list_pop_front(struct bpf_list_head
> > *head)
> > {
> > - return __bpf_list_del(head, false);
> > + struct list_head *h = (void *)head;
> > +
> > + return __bpf_list_del(head, h->next);
> > }
> >
> > __bpf_kfunc struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head
> > *head)
> > {
> > - return __bpf_list_del(head, true);
> > + struct list_head *h = (void *)head;
> > +
> > + return __bpf_list_del(head, h->prev);
> > +}
> > +
> > +__bpf_kfunc struct bpf_list_node *bpf_list_del(struct bpf_list_head *head,
> > + struct bpf_list_node *node)
> > +{
> > + struct bpf_list_node_kern *kn = (void *)node;
> > +
> > + return __bpf_list_del(head, &kn->list_head);
> > }
> >
> > __bpf_kfunc struct bpf_list_node *bpf_list_front(struct bpf_list_head
> > *head)
> > @@ -4545,6 +4560,7 @@ 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] ||
>
>
