Re: [PATCH v20 09/25] net/pcap: fix error accounting and backpressure on transmit

Bruce Richardson
2026-03-16T12:24:34Z

On Tue, Mar 10, 2026 at 09:09:47AM -0700, Stephen Hemminger wrote:
> The error handling when pcap_sendpacket() was incorrect.
> When underlying kernel socket buffer got full the send was counted as
> an error. Malformed multi-segment mbufs where pkt_len exceeds actual
> data were silently accepted.
>
> On Linux, pcap_sendpacket() calls send() on a blocking PF_PACKET
> socket with default kernel buffer sizes and no TX ring (PACKET_TX_RING).
> The send() call only blocks when the kernel socket send buffer is full,
> providing limited backpressure. Backpressure is not an error.
>

I think we need a clearer explanation of what this patch does, this didn't
make sense to me on reading it. See also comments inline below - I think we
can get a better patch with a clearer explanation by just focusing on the
changes to eth_pcap_tx. If you want to update the tx_dumper function put it
in a different patch with a different explanation.

> Fixes: fbbbf553f268 ("net/pcap: fix concurrent multiseg Tx")
> Cc: stable@dpdk.org
>
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

With commit log clarified, I'm ok with this, but would prefer a smaller
patch.

Acked-by: Bruce Richardson <bruce.richardson@intel.com>

> ---
> drivers/net/pcap/pcap_ethdev.c | 58 +++++++++++++++++++++-------------
> 1 file changed, 36 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/net/pcap/pcap_ethdev.c b/drivers/net/pcap/pcap_ethdev.c
> index 72a297d423..47a050df11 100644
> --- a/drivers/net/pcap/pcap_ethdev.c
> +++ b/drivers/net/pcap/pcap_ethdev.c
> @@ -407,7 +407,8 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
> * dumper */
> for (i = 0; i < nb_pkts; i++) {
> struct rte_mbuf *mbuf = bufs[i];
> - size_t len, caplen;
> + uint32_t len, caplen;
> + const uint8_t *data;
>
> len = rte_pktmbuf_pkt_len(mbuf);
> caplen = RTE_MIN(len, RTE_ETH_PCAP_SNAPSHOT_LEN);
> @@ -415,15 +416,16 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
> calculate_timestamp(&header.ts);
> header.len = len;
> header.caplen = caplen;
> - /* rte_pktmbuf_read() returns a pointer to the data directly
> - * in the mbuf (when the mbuf is contiguous) or, otherwise,
> - * a pointer to temp_data after copying into it.
> - */
> - pcap_dump((u_char *)dumper, &header,
> - rte_pktmbuf_read(mbuf, 0, caplen, temp_data));
> +
> + data = rte_pktmbuf_read(mbuf, 0, caplen, temp_data);
> +
> + /* This could only happen if mbuf is bogus pkt_len > data_len */
> + RTE_ASSERT(data != NULL);
> + pcap_dump((u_char *)dumper, &header, data);
>
> num_tx++;
> tx_bytes += caplen;
> +
> rte_pktmbuf_free(mbuf);
> }
>
> @@ -435,9 +437,8 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
> pcap_dump_flush(dumper);
> dumper_q->tx_stat.pkts += num_tx;
> dumper_q->tx_stat.bytes += tx_bytes;
> - dumper_q->tx_stat.err_pkts += nb_pkts - num_tx;
>
> - return nb_pkts;
> + return i;
> }

While not wrong or problematic, the changes here are unnecessary IMHO,
since there is no error that I can see in this function. The changes I see
are:
* change datatype from size_t to uint32_t - ok but the original code wasn't
really wrong for this.
* having an RTE_ASSERT for the data being NULL - I suppose this is useful
for certain debug builds, but only if the user knows to define
RTE_ENABLE_ASSERT.
* extra whitespace line - unnecessary
* drop status increment - the num_tx value would always equal nb_pkts, so
this was an increment of zero. Removing it is good, as it saves a store.
* change return value from nb_pkts to i - this is equivalent since the loop
always completes so that i == nb_pkts, so change is not needed.

The change I would keep is the unnecessary error increment since it was
always zero - however, it doesn't go far enough, as num_tx can be removed
in the function completely. By function end, i == num_tx == nb_pkts, so
let's just use nb_pkts directly, and we can also make "i" a loop-local
variable.

>
> /*
> @@ -462,7 +463,17 @@ eth_tx_drop(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
> }
>
> /*
> - * Callback to handle sending packets through a real NIC.
> + * Send a burst of packets to a pcap device.
> + *
> + * On Linux, pcap_sendpacket() calls send() on a blocking PF_PACKET
> + * socket with default kernel buffer sizes and no TX ring (PACKET_TX_RING).
> + * The send() call only blocks when the kernel socket send buffer is full,
> + * providing limited backpressure.
> + *
> + * On error, pcap_sendpacket() returns non-zero and the loop breaks,
> + * leaving remaining packets unsent.
> + *
> + * Bottom line: backpressure is not an error.
> */
> static uint16_t
> eth_pcap_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
> @@ -484,34 +495,37 @@ eth_pcap_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>
> for (i = 0; i < nb_pkts; i++) {
> struct rte_mbuf *mbuf = bufs[i];
> - size_t len = rte_pktmbuf_pkt_len(mbuf);
> - int ret;
> + uint32_t len = rte_pktmbuf_pkt_len(mbuf);
> + const uint8_t *data;
>
> - if (unlikely(!rte_pktmbuf_is_contiguous(mbuf) &&
> - len > RTE_ETH_PCAP_SNAPSHOT_LEN)) {
> + if (unlikely(!rte_pktmbuf_is_contiguous(mbuf) && len > RTE_ETH_PCAP_SNAPSHOT_LEN)) {
> PMD_LOG(ERR,
> - "Dropping multi segment PCAP packet. Size (%zd) > max size (%u).",
> + "Dropping multi segment PCAP packet. Size (%u) > max size (%u).",
> len, RTE_ETH_PCAP_SNAPSHOT_LEN);
> + tx_queue->tx_stat.err_pkts++;
> rte_pktmbuf_free(mbuf);
> continue;
> }
>
> - /* rte_pktmbuf_read() returns a pointer to the data directly
> - * in the mbuf (when the mbuf is contiguous) or, otherwise,
> - * a pointer to temp_data after copying into it.
> + data = rte_pktmbuf_read(mbuf, 0, len, temp_data);
> + RTE_ASSERT(data != NULL);
> +
> + /*
> + * No good way to separate back pressure from failure here
> + * Assume it is EBUSY, ENOMEM, or EINTR, something that can be retried.
> */
> - ret = pcap_sendpacket(pcap,
> - rte_pktmbuf_read(mbuf, 0, len, temp_data), len);
> - if (unlikely(ret != 0))
> + if (pcap_sendpacket(pcap, data, len) != 0) {
> + PMD_LOG(ERR, "pcap_sendpacket() failed: %s", pcap_geterr(pcap));
> break;
> + }
> num_tx++;
> tx_bytes += len;
> +
> rte_pktmbuf_free(mbuf);
> }
>
> tx_queue->tx_stat.pkts += num_tx;
> tx_queue->tx_stat.bytes += tx_bytes;
> - tx_queue->tx_stat.err_pkts += i - num_tx;
>
> return i;
> }
> --
> 2.51.0
>