[PATCH net-next 1/2] net: stmmac: fix XDP TX DMA address for compound RX pages

[Daniel Machon]

Hi Lorenzo,

> When RX buffers are compound pages (order > 0), only the head page
> carries the DMA address programmed by the page pool
> (page_pool_get_dma_addr()). stmmac_xdp_xmit_xdpf() computed the head
> buffer's bus address from the page that happens to contain the data:
> 
> 	page = frag ? skb_frag_page(frag)
> 		    : virt_to_page(xdpf->data);
> 	dma_addr = page_pool_get_dma_addr(page);
> 	...
> 
> RX pages become compound as soon as dma_buf_sz needs more than one page
> (pp_params.order = order_base_2(num_pages) > 0), which happens from an
> MTU around 3000 bytes once dma_buf_sz reaches 4096. If an XDP program
> moves the data forward with bpf_xdp_adjust_head() by more than about a
> page, xdpf->data points into a tail page whose struct page has no DMA
> address: page_pool_get_dma_addr() then returns a bogus bus address, and
> dma_sync_single_for_device() together with the TX descriptor transmit
> from the wrong location.

Looks to me like mtu > ETH_DATA_LAN is rejected when setting a prog, and also
on mtu change when a prog is attached. So can this even happen today?

> 
> Use virt_to_head_page() to read the DMA base of the buffer and keep the
> offset relative to the head page (sizeof(*xdpf) + xdpf->headroom), which
> is correct no matter which page the data ends up in.

I think the fix looks correct.

> 
> Fixes: be8b38a722e6 ("net: stmmac: Add support for XDP_TX action")
> Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi at oss.qualcomm.com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index b8b4de4c5b1a..ca61ab449e32 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -5301,7 +5301,7 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
>  			struct page *page;
>  
>  			page = frag ? skb_frag_page(frag)
> -				    : virt_to_page(xdpf->data);
> +				    : virt_to_head_page(xdpf->data);
>  			dma_addr = page_pool_get_dma_addr(page);
>  			if (frag)
>  				dma_addr += skb_frag_off(frag);
> 
> -- 
> 2.56.0
> 

/Daniel