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

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 <redacted>
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[...]

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

Thanks,
Nicolai