Re: [PATCH net] igb: only strip Rx timestamp header on the first buffer of a frame

[Kurt Kanzenbach <kurt@linutronix.de>]

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Hi,

On Fri Jun 12 2026, Tjerk Kusters wrote:
> Hi,
>
> The patch is attached (0001-igb-only-strip-Rx-timestamp-header-on-the-first-buff.patch)
> as my mail setup cannot send it inline via git send-email; apologies for the
> attachment.

b4 has a web submission endpoint. Maybe you can use that one:

https://b4.docs.kernel.org/en/latest/contributor/send.html

[snip]

> From fee3e3452dfcd7e109332369672a3e0090cadeb3 Mon Sep 17 00:00:00 2001
> From: T Kusters <tkusters@aweta.nl>
> Date: Tue, 9 Jun 2026 14:06:24 +0200
> Subject: [PATCH net] igb: only strip Rx timestamp header on the first buffer
>  of a frame
>
> When Rx hardware timestamping is enabled (e.g. ptp4l, which configures
> HWTSTAMP_FILTER_ALL), the NIC prepends a 16-byte timestamp header to the
> first Rx buffer of every received frame. igb_clean_rx_irq() strips this
> header inside its per-buffer loop:
>
> 	if (igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
> 		ts_hdr_len = igb_ptp_rx_pktstamp(rx_ring->q_vector,
> 						 pktbuf, &timestamp);
> 		pkt_offset += ts_hdr_len;
> 		size -= ts_hdr_len;
> 	}
>
> For a frame that spans more than one Rx buffer (e.g. a jumbo frame), this
> block runs once per buffer. The timestamp header only exists at the start
> of the first buffer, but igb_ptp_rx_pktstamp() is called for every buffer.
>
> On a continuation buffer the data is packet payload, not a timestamp
> header. igb_ptp_rx_pktstamp() already has two guards against acting on a
> non-header buffer: it returns 0 if PTP is disabled, and returns 0 if the
> reserved dwords (the first 8 bytes) are non-zero. Neither is sufficient
> here: PTP is enabled, and a continuation buffer whose payload happens to
> begin with 8 zero bytes passes the reserved-dword check. In that case the
> payload is mistaken for a valid timestamp header and igb_ptp_rx_pktstamp()
> returns IGB_TS_HDR_LEN, so the caller strips 16 bytes of real data from
> that buffer. A frame spanning N buffers whose continuation buffers start
> with zero bytes therefore loses 16 * (N - 1) bytes from its tail.
>
> This is easily triggered by a GigE Vision camera streaming dark frames
> (mostly 0x00 pixel data) over jumbo UDP with PTP active on the receiver:
> the all-zero frames arrive truncated while frames with non-zero content
> are fine. There is no error indication.
>
> No content-based check can reliably tell a continuation buffer that begins
> with zero bytes from a real timestamp header, because both are all zero.
> Fix it structurally instead: only attempt the strip on the first buffer of
> a frame, which is the only buffer that can contain a timestamp header. In
> igb_clean_rx_irq() skb is NULL until the first buffer has been processed,
> so guarding the strip with !skb restricts it to the first buffer
> regardless of payload content.
>
> Fixes: 5379260852b0 ("igb: Fix XDP with PTP enabled")
> Cc: stable@vger.kernel.org
> Signed-off-by: T Kusters <tkusters@aweta.nl>

Great explanation! igb_clean_rx_irq_zc() does not need the same
treatment, correct?

Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>

> ---
>  drivers/net/ethernet/intel/igb/igb_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index ce91dda00ec0..abb55cd589a9 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -9061,7 +9061,8 @@ static int igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
>  		pktbuf = page_address(rx_buffer->page) + rx_buffer->page_offset;
>  
>  		/* pull rx packet timestamp if available and valid */
> -		if (igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
> +		if (!skb &&
> +		    igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
>  			int ts_hdr_len;
>  
>  			ts_hdr_len = igb_ptp_rx_pktstamp(rx_ring->q_vector,
> -- 
> 2.27.0
>

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