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

[Martin KaFai Lau <martin.lau@linux.dev>]

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);
> +}