> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 1e8754877dd1..51b6ea4bb8cb 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2593,6 +2593,15 @@ __bpf_kfunc struct bpf_list_node
> *bpf_list_pop_back(struct bpf_list_head *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;
> +
> + /* verifier guarantees node is a list node rather than list head */
> + return __bpf_list_del(head, &kn->list_head);
> +}
> +
Looking at bpf_list_pop_front() and bpf_list_pop_back() in the same
file, both functions read from the list_head (h->next or h->prev)
before passing it to __bpf_list_del(). If the list is uninitialized,
they end up passing potentially invalid pointer values to __bpf_list_del(),
which then has to handle this by initializing the list and returning NULL.
While __bpf_list_del() handles this correctly, would it be clearer to
move the initialization check into the top-level kfuncs (like
bpf_list_pop_front/back) so that __bpf_list_del() always receives a
valid list_head pointer rather than potentially garbage values that
get ignored later?
Reference:
https://lore.kernel.org/bpf/af50d188f6bc2de81fe3c321b26a645a9425ba9f2644cb6f8525d5bb63738...@mail.kernel.org/
> @@ -4725,6 +4734,7 @@ BTF_ID_FLAGS(func, bpf_list_push_back, KF_IMPLICIT_ARGS)
> 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 11054ad89c14..a6bcbe68cd72 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -10744,6 +10744,7 @@ enum special_kfunc_type {
> KF_bpf_list_push_back,
> 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,
[ ... ]
> @@ -11456,7 +11459,8 @@ static bool check_kfunc_is_graph_node_api(struct
> bpf_verifier_env *env,
>
> switch (node_field_type) {
> case BPF_LIST_NODE:
> - ret = is_bpf_list_push_kfunc(kfunc_btf_id);
> + ret = is_bpf_list_push_kfunc(kfunc_btf_id) ||
> + kfunc_btf_id == special_kfunc_list[KF_bpf_list_del];
> break;
> case BPF_RB_NODE:
> ret = (is_bpf_rbtree_add_kfunc(kfunc_btf_id) ||
Can bpf_list_del actually be called from BPF programs? Looking at the
verifier's handling of KF_ARG_PTR_TO_LIST_NODE arguments in
check_kfunc_args():
case KF_ARG_PTR_TO_LIST_NODE:
...
if (!reg->ref_obj_id) {
verbose(env, "allocated object must be referenced\n");
return -EINVAL;
}
This code path unconditionally requires an owning reference (ref_obj_id
must be non-zero). However, once a node is added to the list via
bpf_list_push_front/back, the owning reference is converted to a
non-owning reference (ref_obj_id becomes 0). This means bpf_list_del
cannot be called on nodes that are actually in the list.
The sister kfunc bpf_rbtree_remove handles this by having a special
branch in check_kfunc_args() that accepts non-owning references for
that specific kfunc. Does bpf_list_del need similar handling to allow
removing nodes that are actually in the list?
Reference:
https://lore.kernel.org/bpf/294242698d576e4fedf243ab055c6f9a984e55033933a9bd09808f62a7018...@mail.kernel.org/
---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/25716874656
