Re: [PATCH net-next v8 3/5] net: stmmac: drop multi-buffer frames in XDP mode

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

On 18.10.2026 19:07, Lorenzo Bianconi wrote:
> stmmac_rx() runs the XDP program once per descriptor buffer and uses
> skb == NULL as the only marker for the start of a frame. A frame larger
> than one buffer is split by the hardware across several descriptors;
> GMAC4 enables jumbo frames unconditionally (GMAC_CONFIG_JE in
> GMAC_CORE_INIT), so an on-link peer can deliver such a frame even when
> the interface MTU is left at 1500.
> 
> When an XDP verdict ends a non-last descriptor, skb is cleared and the
> next descriptor of the same hardware frame is parsed as a new frame.
> For XDP_CONSUMED this happens via read_again with len preserved, so
> stmmac_rx_buf1_len() then returns min(dma_buf_sz, plen - len) for the
> last descriptor, which can be as small as 0-3 bytes. buf1_len -=
> ETH_FCS_LEN underflows and xdp_prepare_buff() puts data_end far past
> data; on XDP_PASS this produces an skb whose length is close to 4G that
> reaches the network stack (skb->tail and skb->len are 32-bit and wrap
> below skb->end, so skb_over_panic() does not fire). A 4+ byte tail is
> instead offered to the XDP program and delivered as a standalone
> mid-payload Ethernet frame on XDP_PASS. XDP_TX/XDP_REDIRECT instead
> falls through to the next iteration, resetting the running length, so
> the tail is re-parsed as a full-size new frame.
> 
> The same fault exists when a single descriptor uses its second buffer:
> GMAC4/XGMAC always provide a second data buffer per descriptor, so a
> frame up to 2 * dma_buf_sz can land on a single descriptor while the 
> XDP
> program still only sees the first buffer. XDP_TX then transmits a
> truncated frame, and XDP_PASS appends the second buffer only after the
> program has already run over the first one.
> 
> XDP does not support scatter-gather frames until the multi-buffer
> rework, so drop the whole frame instead: mark it as errored as soon as
> a non-last descriptor, or a descriptor using its second buffer, is seen
> while an XDP program is attached, drain and release every descriptor -
> including the last. Removing the re-parse lets both XDP verdict 
> branches
> simply complete the packet via next.
> 
> Note: the subsequent "net: stmmac: rework stmmac_rx to support XDP rx
> multi-buff" accumulates the whole frame into a single xdp_buff and
> supersedes this patch; the fix is kept so the net portion of the series
> stands on its own.
> 
> Fixes: 5fabb01207a2 ("net: stmmac: Add initial XDP support")
> Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
> ---
> [...]

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

Thanks,
Nicolai