Re: [PATCH net] igc: only strip RX timestamp header from first buffer

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Tue, Aug 25, 2026 at 11:55:40PM +0000, Paul Moses wrote:
> igc_clean_rx_irq() strips IGC_TS_HDR_LEN whenever a descriptor reports
> IGC_RXDADV_STAT_TSIP. For multi-buffer packets, continuation descriptors
> retain TSIP even though the inline timestamp is present only in the first
> RX buffer.
> 
> Subtracting the header length from each continuation buffer truncates
> jumbo packets by 16 bytes per continuation and leaves the packet length
> larger than the received data.
> 
> Only consume the timestamp header when skb is NULL, which identifies the
> first buffer of a new packet. An skb carried in rx_ring->skb remains
> non-NULL when packet assembly resumes in a later NAPI poll.
> 
> Link: https://lore.kernel.org/all/20260625-igb-rx-ts-fix-v3-1-99b3efa08dca@aweta.nl/
> Fixes: e1ed4f92a625 ("igc: Refactor Rx timestamp handling")
> Cc: stable@vger.kernel.org
> Signed-off-by: Paul Moses <p@1g4.org>

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

> ---
> #
> # REPRO
> # 
> # Receiver: igc / enp1s0 / 10.0.5.175
> # Sender:   RTL8125B / enP4p65s0 / 10.0.5.165
> 
> # Both hosts
> sudo ip link set dev enp1s0 mtu 9000
> sudo ip link set dev enP4p65s0 mtu 9000
> 
> # Baseline: RX timestamping off
> sudo hwstamp_ctl -i enp1s0 -t 0 -r 0
> ping -n -M do -c 5 -s 8972 10.0.5.175
> # 5/5 received
> 
> # Enable RX timestamping only
> sudo hwstamp_ctl -i enp1s0 -t 0 -r 1
> 
> # Receiver
> sudo timeout 20 tcpdump -ni enp1s0 -s 0 -w /tmp/igc-jumbo.pcap \
>   'icmp and host 10.0.5.165'
> 
> # Sender
> ping -n -M do -c 5 -W 2 -s 8972 10.0.5.175
> # 0/5 received
> 
> # Receiver
> sudo tcpdump -nn -e -vvv -r /tmp/igc-jumbo.pcap
> # length 8950
> # [total length 9000 > length 8936] (invalid)
> # wrong icmp cksum
> 
> # Expected Ethernet length: 9014
> # Captured length:          8950
> # Loss:                       64 = 4 continuation buffers * 16 bytes
> 
> # Disable RX timestamping again
> sudo hwstamp_ctl -i enp1s0 -t 0 -r 0
> ping -n -M do -c 5 -s 8972 10.0.5.175
> # 5/5 received
> 
> ---
> 
>  drivers/net/ethernet/intel/igc/igc_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> index 1fb5f3cbe93c..41b293dc10c3 100644
> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> @@ -2639,7 +2639,8 @@ static int igc_clean_rx_irq(struct igc_q_vector *q_vector, const int budget)
>  
>  		pktbuf = page_address(rx_buffer->page) + rx_buffer->page_offset;
>  
> -		if (igc_test_staterr(rx_desc, IGC_RXDADV_STAT_TSIP)) {
> +		if (!skb &&
> +		    igc_test_staterr(rx_desc, IGC_RXDADV_STAT_TSIP)) {
>  			ctx.rx_ts = pktbuf;
>  			pkt_offset = IGC_TS_HDR_LEN;
>  			size -= IGC_TS_HDR_LEN;
> -- 
> 2.55.GIT
> 
>