Re: [PATCH net-next v6 2/4] net: stmmac: drop split frames on AF_XDP zero-copy RX

[Nicolai Buchwitz <nb@tipi-net.de>]

On 12.10.2026 18:44, Lorenzo Bianconi wrote:
> stmmac_rx_zc() assumes a 1:1 mapping between a received frame and an
> XSK pool buffer and drops any frame that spans more than one RX
> descriptor:
> 
>         /* XSK pool expects RX frame 1:1 mapped to XSK buffer */
>         if (likely(status & rx_not_ls)) {
>                 xsk_buff_free(buf->xdp);
>                 buf->xdp = NULL;
>                 rx_dropped++;
>                 error = 1;
>                 dirty++;
>                 goto read_again;
>         }
> 
> The non-last descriptors are freed and dropped, but the frame is not
> marked as errored. When the poll reaches the last descriptor of the
> same frame, error is still 0, so the fragment is handled as a complete
> frame: it is passed to the XDP program, copied into an skb on XDP_PASS,
> or converted for XDP_TX. The tail starts in the middle of the original
> payload at a predictable offset and, parsed as a fresh frame (len == 
> 0),
> stmmac_rx_buf1_len() reports the full dma_buf_sz length for it even
> though the DMA engine only wrote the tail remainder into that buffer, 
> so
> the reads extend past the received bytes and can reach beyond the end 
> of
> the XSK pool buffer. GMAC4 always enables jumbo frames (GMAC_CONFIG_JE
> in GMAC_CORE_INIT), so an on-link peer can trigger such a split even
> when the interface MTU is left at 1500.
> 
> Set error when the 1:1 mapping check drops a descriptor, so the
> remaining descriptors of the frame - including the last one - are
> drained and released with xsk_buff_free() instead of being processed as
> a new frame. Account every freed buffer with dirty and count the
> dropped frame once in the poll budget / rx_pkt_n, and once in
> rx_dropped.
> 
> Fixes: bba2556efad6 ("net: stmmac: Enable RX via AF_XDP zero-copy")
> Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
> ---

> [...]

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>

Thanks,
Nicolai