Subject: Re: [PATCHv4 bpf-next 1/2] xdp: add a new helper for dev map multicast support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 10 Jun 2020 12:18:59 +0200

On Tue, 26 May 2020 22:05:38 +0800
Hangbin Liu <[email-masked]> wrote:

> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 90f44f382115..acdc63833b1f 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -475,3 +475,29 @@ void xdp_warn(const char *msg, const char *func, const int line)
>  	WARN(1, "XDP_WARN: %s(line:%d): %s\n", func, line, msg);
>  };
>  EXPORT_SYMBOL_GPL(xdp_warn);
> +
> +struct xdp_frame *xdpf_clone(struct xdp_frame *xdpf)
> +{
> +	unsigned int headroom, totalsize;
> +	struct xdp_frame *nxdpf;
> +	struct page *page;
> +	void *addr;
> +
> +	headroom = xdpf->headroom + sizeof(*xdpf);
> +	totalsize = headroom + xdpf->len;
> +
> +	if (unlikely(totalsize > PAGE_SIZE))
> +		return NULL;
> +	page = dev_alloc_page();
> +	if (!page)
> +		return NULL;
> +	addr = page_to_virt(page);
> +
> +	memcpy(addr, xdpf, totalsize);

I don't think this will work.  You are assuming that the memory model
(xdp_mem_info) is the same.

You happened to use i40, that have MEM_TYPE_PAGE_SHARED, and you should
have changed this to MEM_TYPE_PAGE_ORDER0, but it doesn't crash as they
are compatible.  If you were using mlx5, I suspect that this would
result in memory leaking.

You also need to update xdpf->frame_sz, as you also cannot assume it is
the same.

> +
> +	nxdpf = addr;
> +	nxdpf->data = addr + headroom;
> +
> +	return nxdpf;
> +}
> +EXPORT_SYMBOL_GPL(xdpf_clone);

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

struct xdp_frame {
	void *data;
	u16 len;
	u16 headroom;
	u32 metasize:8;
	u32 frame_sz:24;
	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
	 * while mem info is valid on remote CPU.
	 */
	struct xdp_mem_info mem;
	struct net_device *dev_rx; /* used by cpumap */
};