Re: [PATCH bpf-next,v4 2/4] xfrm: interface: Add unstable helpers for setting/getting XFRM metadata from TC-BPF

On 12/2/22 1:59 AM, Eyal Birger wrote:

+__used noinline +int bpf_skb_set_xfrm_info(struct __sk_buff *skb_ctx,

• 	  const struct bpf_xfrm_info *from)
  

+{

• struct sk_buff *skb = (struct sk_buff *)skb_ctx;
• struct metadata_dst *md_dst;
• struct xfrm_md_info *info;
• if (unlikely(skb_metadata_dst(skb)))

• return -EINVAL;
  

• md_dst = this_cpu_ptr(xfrm_md_dst);
• info = &md_dst->u.xfrm_info;
• info->if_id = from->if_id;
• info->link = from->link;
• skb_dst_force(skb);
• info->dst_orig = skb_dst(skb);
• dst_hold((struct dst_entry *)md_dst);
• skb_dst_set(skb, (struct dst_entry *)md_dst);

I may be missed something obvious and this just came to my mind,

What stops cleanup_xfrm_interface_bpf() being run while skb is still holding the md_dst?

[ ... ]

+static const struct btf_kfunc_id_set xfrm_interface_kfunc_set = {

• .owner = THIS_MODULE,
• .set = &xfrm_ifc_kfunc_set,

+};

+int __init register_xfrm_interface_bpf(void) +{

• int err;
• xfrm_md_dst = metadata_dst_alloc_percpu(0, METADATA_XFRM,

• 				GFP_KERNEL);
  

• if (!xfrm_md_dst)

• return -ENOMEM;
  

• err = register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS,

• 			&xfrm_interface_kfunc_set);
  

• if (err < 0) {

• metadata_dst_free_percpu(xfrm_md_dst);
  

• return err;
  

• }
• return 0;

+}

+void cleanup_xfrm_interface_bpf(void) +{

• metadata_dst_free_percpu(xfrm_md_dst);

+}