[PATCH] libpcapng: fix timestamp wrapping in output files

[Quentin Armitage]

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: sta...@dpdk.org

Signed-off-by: Quentin Armitage <quen...@armitage.org.uk>
---
 lib/pcapng/rte_pcapng.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/lib/pcapng/rte_pcapng.c b/lib/pcapng/rte_pcapng.c
index 90b2f5bc69..7770be725f 100644
--- a/lib/pcapng/rte_pcapng.c
+++ b/lib/pcapng/rte_pcapng.c
@@ -34,7 +34,7 @@ struct rte_pcapng {
 };
 
 /* For converting TSC cycles to PCAPNG ns format */
-struct pcapng_time {
+static struct pcapng_time {
        uint64_t ns;
        uint64_t cycles;
 } pcapng_time;
@@ -53,7 +53,21 @@ static uint64_t pcapng_tsc_to_ns(uint64_t cycles)
 {
        uint64_t delta;
 
+       /* With a TSC frequency of 2.5GHz, delta * NSEC_PER_SEC will
+        * wrap in under 8 seconds. Once half that time has elapsed
+        * reread the system clock and TSC to ensure wrapping does not
+        * occur.
+        */
        delta = cycles - pcapng_time.cycles;
+       if (delta > ((1ULL << 63) / NSEC_PER_SEC)) {
+               pcapng_init();
+               if (cycles > pcapng_time.cycles)
+                       delta = cycles - pcapng_time.cycles;
+               else {
+                       delta = pcapng_time.cycles - cycles;
+                       return pcapng_time.ns - (delta * NSEC_PER_SEC) / 
rte_get_tsc_hz();
+               }
+       }
        return pcapng_time.ns + (delta * NSEC_PER_SEC) / rte_get_tsc_hz();
 }
 
-- 
2.34.1