Re: [PATCH v3 bpf 2/4] xsk: fix usage of multi-buffer BPF helpers for ZC XDP

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

On 12/21/23 5:26 AM, Maciej Fijalkowski wrote:
> This comes from __xdp_return() call with xdp_buff argument passed as
> NULL which is supposed to be consumed by xsk_buff_free() call.
> 
> To address this properly, in ZC case, a node that represents the frag
> being removed has to be pulled out of xskb_list. Introduce
> appriopriate xsk helpers to do such node operation and use them
> accordingly within bpf_xdp_adjust_tail().

[ ... ]

> +static inline struct xdp_buff *xsk_buff_get_tail(struct xdp_buff *first)
> +{
> +	struct xdp_buff_xsk *xskb = container_of(first, struct xdp_buff_xsk, xdp);
> +	struct xdp_buff_xsk *frag;
> +
> +	frag = list_last_entry(&xskb->pool->xskb_list, struct xdp_buff_xsk,
> +			       xskb_list_node);
> +	return &frag->xdp;
> +}
> +

[ ... ]

> +static void __shrink_data(struct xdp_buff *xdp, struct xdp_mem_info *mem_info,
> +			  skb_frag_t *frag, int shrink)
> +{
> +	if (mem_info->type == MEM_TYPE_XSK_BUFF_POOL) {
> +		struct xdp_buff *tail = xsk_buff_get_tail(xdp);
> +
> +		if (tail)
> +			tail->data_end -= shrink;
> +	}
> +	skb_frag_size_sub(frag, shrink);
> +}
> +
> +static bool shrink_data(struct xdp_buff *xdp, skb_frag_t *frag, int shrink)
> +{
> +	struct xdp_mem_info *mem_info = &xdp->rxq->mem;
> +
> +	if (skb_frag_size(frag) == shrink) {
> +		struct page *page = skb_frag_page(frag);
> +		struct xdp_buff *zc_frag = NULL;
> +
> +		if (mem_info->type == MEM_TYPE_XSK_BUFF_POOL) {
> +			zc_frag = xsk_buff_get_tail(xdp);
> +
> +			if (zc_frag) {

Based on the xsk_buff_get_tail(), would zc_frag ever be NULL?

> +				xdp_buff_clear_frags_flag(zc_frag);
> +				xsk_buff_del_tail(zc_frag);
> +			}
> +		}
> +
> +		__xdp_return(page_address(page), mem_info, false, zc_frag);

and iiuc, this patch is fixing a bug when zc_frag is NULL and 
MEM_TYPE_XSK_BUFF_POOL.

> +		return true;
> +	}
> +	__shrink_data(xdp, mem_info, frag, shrink);
> +	return false;
> +}
> +