Re: [PATCH] libpcapng: fix timestamp wrapping in output files

[Stephen Hemminger <stephen@networkplumber.org>]

On Sat,  7 May 2022 17:12:36 +0100
Quentin Armitage <quentin@armitage.org.uk> wrote:

> In pcap_tsc_to_ns(), delta * NSEC_PER_SEC will overflow approx 8
> seconds after pcap_init is called when using a TSC with a frequency
> of 2.5GHz.
> 
> To avoid the overflow, reread the time and TSC once
> delta * NSEC_PER_SEC > (1 << 63). In order to ensure that there
> is no overflow if there is a several second gap between calls to
> pcapng_tsc_to_ns() the actual check to reread the clock is:
>   delta > ((1ULL << 63) / NSEC_PER_SEC)
> 
> Fixes: 8d23ce8f5ee ("pcapng: add new library for writing pcapng files")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>

What about something like this instead.

diff --git a/lib/pcapng/rte_pcapng.c b/lib/pcapng/rte_pcapng.c
index 90b2f5bc6905..c5534301bf2c 100644
--- a/lib/pcapng/rte_pcapng.c
+++ b/lib/pcapng/rte_pcapng.c
@@ -19,6 +19,7 @@
 #include <rte_ether.h>
 #include <rte_mbuf.h>
 #include <rte_pcapng.h>
+#include <rte_reciprocal.h>
 #include <rte_time.h>
 
 #include "pcapng_proto.h"
@@ -34,27 +35,39 @@ struct rte_pcapng {
 };
 
 /* For converting TSC cycles to PCAPNG ns format */
-struct pcapng_time {
+#define TICK_SCALE 16u
+static struct {
 	uint64_t ns;
 	uint64_t cycles;
+	struct rte_reciprocal_u64 inverse;
 } pcapng_time;
 
 RTE_INIT(pcapng_init)
 {
 	struct timespec ts;
+	uint64_t scale_tick_per_ns;
 
 	pcapng_time.cycles = rte_get_tsc_cycles();
 	clock_gettime(CLOCK_REALTIME, &ts);
 	pcapng_time.ns = rte_timespec_to_ns(&ts);
+
+	scale_tick_per_ns = (rte_get_tsc_hz() * TICK_SCALE) / NSEC_PER_SEC;
+	pcapng_time.inverse = rte_reciprocal_value_u64(scale_tick_per_ns);
 }
 
 /* PCAPNG timestamps are in nanoseconds */
 static uint64_t pcapng_tsc_to_ns(uint64_t cycles)
 {
-	uint64_t delta;
+	uint64_t delta, elapsed;
 
 	delta = cycles - pcapng_time.cycles;
-	return pcapng_time.ns + (delta * NSEC_PER_SEC) / rte_get_tsc_hz();
+
+	/* Compute elapsed time in nanoseconds scaled by TICK_SCALE
+	 * since the start of the capture.
+	 * With scale of 4 this will roll over in 36 years.
+	 */
+	elapsed = rte_reciprocal_divide_u64(delta, &pcapng_time.inverse);
+	return pcapng_time.ns + elapsed / TICK_SCALE;
 }
 
 /* length of option including padding */