Re: [PATCH bpf-next 2/3] bpf: add kfunc for skb refcounting

[Amery Hung <ameryhung@gmail.com>]

On 2/20/2025 5:45 AM, Maciej Fijalkowski wrote:
> These have been mostly taken from Amery Hung's work related to bpf qdisc
> implementation. bpf_skb_{acquire,release}() are for increment/decrement
> sk_buff::users whereas bpf_skb_destroy() is called for map entries that
> have not been released and map is being wiped out from system.
> 
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> ---
>   net/core/filter.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 62 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 2ec162dd83c4..9bd2701be088 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -12064,6 +12064,56 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
>   
>   __bpf_kfunc_end_defs();
>   
> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +		  "Global functions as their definitions will be in vmlinux BTF");
> +
> +/* bpf_skb_acquire - Acquire a reference to an skb. An skb acquired by this
> + * kfunc which is not stored in a map as a kptr, must be released by calling
> + * bpf_skb_release().
> + * @skb: The skb on which a reference is being acquired.
> + */
> +__bpf_kfunc struct sk_buff *bpf_skb_acquire(struct sk_buff *skb)
> +{
> +	if (refcount_inc_not_zero(&skb->users))

Any reason to use refcount_inc_not_zero instead of refcount_inc here?

> +		return skb;
> +	return NULL;
> +}
> +
> +/* bpf_skb_release - Release the reference acquired on an skb.
> + * @skb: The skb on which a reference is being released.
> + */
> +__bpf_kfunc void bpf_skb_release(struct sk_buff *skb)
> +{
> +	skb_unref(skb);
> +}
> +
> +/* bpf_skb_destroy - Release an skb reference acquired and exchanged into
> + * an allocated object or a map.
> + * @skb: The skb on which a reference is being released.
> + */
> +__bpf_kfunc void bpf_skb_destroy(struct sk_buff *skb)
> +{
> +	(void)skb_unref(skb);
Actually, there might be a dtor that work for both cls and qdisc. This 
skb_unref() seems redundant, consume_skb() already unref once.

> +	consume_skb(skb);

consume_skb() indicates that the skb is consumed, but if the skb here is 
being dropped by dtor. kfree_skb() or maybe  kfree_skb_reason() with a 
proper reason should be better.

Here is the comments for consume_skb() in skbuff.c
  *      Functions identically to kfree_skb, but kfree_skb assumes that 
the frame
  *      is being dropped after a failure and notes that

So the dtor would be basically the same as the qdisc one.

> +}
> +
> +__diag_pop();
> +
> +BTF_KFUNCS_START(skb_kfunc_btf_ids)
> +BTF_ID_FLAGS(func, bpf_skb_acquire, KF_ACQUIRE | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_skb_release, KF_RELEASE)
> +BTF_KFUNCS_END(skb_kfunc_btf_ids)
> +
> +static const struct btf_kfunc_id_set skb_kfunc_set = {
> +	.owner = THIS_MODULE,
> +	.set   = &skb_kfunc_btf_ids,
> +};
> +
> +BTF_ID_LIST(skb_kfunc_dtor_ids)
> +BTF_ID(struct, sk_buff)
> +BTF_ID_FLAGS(func, bpf_skb_destroy, KF_RELEASE)
> +
>   int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
>   			       struct bpf_dynptr *ptr__uninit)
>   {
> @@ -12117,6 +12167,13 @@ static const struct btf_kfunc_id_set bpf_kfunc_set_tcp_reqsk = {
>   
>   static int __init bpf_kfunc_init(void)
>   {
> +	const struct btf_id_dtor_kfunc skb_kfunc_dtors[] = {
> +		{
> +			.btf_id       = skb_kfunc_dtor_ids[0],
> +			.kfunc_btf_id = skb_kfunc_dtor_ids[1]
> +		},
> +	};
> +
>   	int ret;
>   
>   	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_skb);
> @@ -12133,6 +12190,11 @@ static int __init bpf_kfunc_init(void)
>   	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &bpf_kfunc_set_xdp);
>   	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
>   					       &bpf_kfunc_set_sock_addr);
> +	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &skb_kfunc_set);
> +
> +	ret = ret ?: register_btf_id_dtor_kfuncs(skb_kfunc_dtors,
> +						 ARRAY_SIZE(skb_kfunc_dtors),
> +						 THIS_MODULE);
>   	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_tcp_reqsk);
>   }
>   late_initcall(bpf_kfunc_init);