[PATCH net v4 5/5] ptp: vclock: preserve missing hardware timestamps

[James Hilliard <james.hilliard1@gmail.com>]

Socket timestamp delivery converts hardware timestamps to a bound
virtual clock before checking whether a hardware timestamp is present.
A zero timestamp means that the packet has no hardware timestamp, but
timecounter_cyc2time() can turn that zero into a nonzero virtual time.
The socket then reports a hardware timestamp which was never supplied.

Return zero without conversion when the input timestamp is absent.
This also covers skb_get_tx_timestamp(), which checks for an absent
timestamp after the same conversion.

This was identified during review of virtual clock sampling changes.

Fixes: d7c088265588 ("net: socket: support hardware timestamp conversion to PHC bound")
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/ptp/ptp_vclock.c         | 2 ++
 include/linux/ptp_clock_kernel.h | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c
index 88161be5eabc..39c3276ded77 100644
--- a/drivers/ptp/ptp_vclock.c
+++ b/drivers/ptp/ptp_vclock.c
@@ -312,6 +312,8 @@ ktime_t ptp_convert_timestamp(const ktime_t *hwtstamp, int vclock_index)
 	u64 ns;
 
 	ns = ktime_to_ns(*hwtstamp);
+	if (!ns)
+		return 0;
 
 	srcu_idx = srcu_read_lock(&vclock_srcu);
 
diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index 36a27a910595..56f6ab1855d3 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -496,7 +496,7 @@ int ptp_get_vclocks_index(int pclock_index, int **vclock_index);
  * @hwtstamp:     timestamp
  * @vclock_index: phc index of ptp vclock.
  *
- * Returns: converted timestamp, or 0 on error.
+ * Returns: converted timestamp, or 0 if @hwtstamp is zero or on error.
  */
 ktime_t ptp_convert_timestamp(const ktime_t *hwtstamp, int vclock_index);
 #else

-- 
2.53.0