[PATCH net v3] igb: only strip Rx timestamp header on the first buffer of a frame

[Tjerk Kusters via B4 Relay]

From: Tjerk Kusters <tkusters@xxxxxxxx>

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@xxxxxxxxxxxxxxx
Reviewed-by: Piotr Kwapulinski <piotr.kwapulinski@xxxxxxxxx>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@xxxxxxxxx>
Reviewed-by: Kurt Kanzenbach <kurt@xxxxxxxxxxxxx>
Signed-off-by: Tjerk Kusters <tkusters@xxxxxxxx>
---
Changes in v3:
- update the rx-timestamp comment to note it only applies to the first
  buffer of a frame (Piotr Kwapulinski)
- add Reviewed-by from Aleksandr Loktionov and Piotr Kwapulinski
- no functional change
- Link to v2: https://patch.msgid.link/20260619-igb-rx-ts-fix-v2-1-d3b8d605ca62@xxxxxxxx

igb: only strip Rx timestamp header on the first buffer of a frame

Changes in v2:
 - resend via b4 (v1 was sent with a mail client)
 - use full author name "Tjerk Kusters" (Jacob Keller)
 - add Reviewed-by from Kurt Kanzenbach
 - no functional change

Link to v1: https://lore.kernel.org/all/PAWPR05MB1069106D52F4E17F1EDB99C67B9182@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
---
 drivers/net/ethernet/intel/igb/igb_main.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index ce91dda00ec0..539bf5389a24 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -9060,8 +9060,11 @@ static int igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
 		rx_buffer = igb_get_rx_buffer(rx_ring, size, &rx_buf_pgcnt);
 		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)) {
+		/* pull rx packet timestamp if available and valid; it is only
+		 * present on the first buffer of a frame
+		 */
+		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,

---
base-commit: 2d3090a8aeb596a26935db0955d46c9a5db5c6ce
change-id: 20260619-igb-rx-ts-fix-cd70585ee316

Best regards,
--  
Tjerk Kusters <tkusters@xxxxxxxx>