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

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Date: Thu, 4 Jan 2024 21:23:08 +0100
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
CC: <netdev@...r.kernel.org>, <magnus.karlsson@...el.com>, <bjorn@...nel.org>,
	<echaudro@...hat.com>, <lorenzo@...nel.org>, <tirthendu.sarkar@...el.com>,
	<bpf@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
	<andrii@...nel.org>
Subject: Re: [PATCH v3 bpf 2/4] xsk: fix usage of multi-buffer BPF helpers
 for ZC XDP

On Wed, Jan 03, 2024 at 02:53:20PM -0800, Martin KaFai Lau wrote:
> On 1/3/24 4:04 AM, Maciej Fijalkowski wrote:
> > On Tue, Jan 02, 2024 at 02:58:00PM -0800, Martin KaFai Lau wrote:
> > > 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?
> > 
> > Hey Martin thanks for taking a look, I had to do this in order to satisfy
> > !CONFIG_XDP_SOCKETS builds :/
> 
> There is compilation/checker warning if it does not check for NULL?
> 
> hmm... but it still should not reach here in the runtime and call
> xsk_buff_get_tail() in the !CONFIG_XDP_SOCKETS build. Can the NULL test on
> the get_tail() return value be removed? The above "mem_info->type ==
> MEM_TYPE_XSK_BUFF_POOL" should have avoided the get_tail() call for the
> !CONFIG_XDP_SOCKETS build. Otherwise, it could be passing NULL to the
> __xdp_return() and hit the same bug again. The NULL check here is pretty
> hard to reason logically.

Thanks for bringing this up, you are of course right. I'll address that.

> 
> > 
> > > 
> > > > +				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.
> > 
> > Generally I don't see the need for xdp_return_buff() (which calls in the
> > end __xdp_return() being discussed) to handle MEM_TYPE_XSK_BUFF_POOL, this
> > could be refactored later and then probably this fix would look different,
> > but this is out of the scope now.
> > 
> > > 
> > > > +		return true;
> > > > +	}
> > > > +	__shrink_data(xdp, mem_info, frag, shrink);
> > > > +	return false;
> > > > +}
> > > > +
> > > 
> > > 
> > 
> 
>