Re: [PATCH 2/2 bpf] xsk: avoid data corruption on cq descriptor number

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 10/28/25 5:02 PM, Fernando Fernandez Mancera wrote:
> Since commit 30f241fcf52a ("xsk: Fix immature cq descriptor
> production"), the descriptor number is stored in skb control block and
> xsk_cq_submit_addr_locked() relies on it to put the umem addrs onto
> pool's completion queue.
>[...]
>   
>   	len = desc->len;
> @@ -804,6 +823,11 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>   			if (unlikely(err))
>   				goto free_err;
>   
> +			if (!skb_ext_add(skb, SKB_EXT_XDP)) {
> +				err = -ENOMEM;
> +				goto free_err;
> +			}
> +

This is a leftover. Without it, the logic simplified and indeed the 
performance for non-fragmented traffic is not affected at all.

While reviewing this, consider this line is dropped. I will send a V2 in 
24 hours anyway as this is indeed introducing a buggy behavior.

>   			xsk_skb_init_misc(skb, xs, desc->addr);
>   			if (desc->options & XDP_TX_METADATA) {
>   				err = xsk_skb_metadata(skb, buffer, desc,
> @@ -814,6 +838,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>   		} else {
>   			int nr_frags = skb_shinfo(skb)->nr_frags;
>   			struct xsk_addr_node *xsk_addr;
> +			struct xdp_skb_ext *ext;
>   			struct page *page;
>   			u8 *vaddr;
>   
> @@ -828,6 +853,22 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>   				goto free_err;
>   			}
>   
> +			ext = skb_ext_find(skb, SKB_EXT_XDP);
> +			if (!ext) {
> +				ext = skb_ext_add(skb, SKB_EXT_XDP);
> +				if (!ext) {
> +					__free_page(page);
> +					err = -ENOMEM;
> +					goto free_err;
> +				}
> +				memset(ext, 0, sizeof(*ext));
> +				INIT_LIST_HEAD(&ext->addrs_list);
> +				ext->num_descs = 1;
> +			} else if (ext->num_descs == 0) {
> +				INIT_LIST_HEAD(&ext->addrs_list);
> +				ext->num_descs = 1;
> +			}
> +
>   			xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
>   			if (!xsk_addr) {
>   				__free_page(page);
> @@ -843,12 +884,11 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>   			refcount_add(PAGE_SIZE, &xs->sk.sk_wmem_alloc);
>   
>   			xsk_addr->addr = desc->addr;
> -			list_add_tail(&xsk_addr->addr_node, &XSKCB(skb)->addrs_list);
> +			list_add_tail(&xsk_addr->addr_node, &ext->addrs_list);
> +			xsk_inc_num_desc(skb);
>   		}
>   	}
>   
> -	xsk_inc_num_desc(skb);
> -
>   	return skb;
>   
>   free_err:
> @@ -857,7 +897,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>   
>   	if (err == -EOVERFLOW) {
>   		/* Drop the packet */
> -		xsk_inc_num_desc(xs->skb);
>   		xsk_drop_skb(xs->skb);
>   		xskq_cons_release(xs->tx);
>   	} else {