[PATCH net] selftests: net: make busywait timeout clock portable

[Nirmoy Das]

loopy_wait() expects millisecond timestamps. However, Ubuntu Resolute
can use uutils date, where `date -u +%s%3N` returns seconds plus full
nanoseconds instead of a 3-digit millisecond field. This makes
busywait expire too early and can make vlan_bridge_binding.sh read a
stale operstate.

Fixes: 25ae948b4478 ("selftests/net: add lib.sh")
Cc: sta...@vger.kernel.org # 6.8+
Link: https://github.com/uutils/coreutils/issues/11658
Signed-off-by: Nirmoy Das <nirm...@nvidia.com>
---
 tools/testing/selftests/net/lib.sh | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/lib.sh 
b/tools/testing/selftests/net/lib.sh
index b40694573f4c7..fcaec058be6d0 100644
--- a/tools/testing/selftests/net/lib.sh
+++ b/tools/testing/selftests/net/lib.sh
@@ -70,12 +70,27 @@ ksft_exit_status_merge()
                $ksft_xfail $ksft_pass $ksft_skip $ksft_fail
 }
 
+timestamp_ms()
+{
+       local now=$(date -u +%s:%N)
+       local seconds=${now%:*}
+       local nanoseconds=${now#*:}
+
+       if [[ $nanoseconds =~ ^[0-9]+$ ]]; then
+               nanoseconds=${nanoseconds:0:9}
+       else
+               nanoseconds=0
+       fi
+
+       echo $((seconds * 1000 + 10#$nanoseconds / 1000000))
+}
+
 loopy_wait()
 {
        local sleep_cmd=$1; shift
        local timeout_ms=$1; shift
 
-       local start_time="$(date -u +%s%3N)"
+       local start_time=$(timestamp_ms)
        while true
        do
                local out
@@ -84,7 +99,7 @@ loopy_wait()
                        return 0
                fi
 
-               local current_time="$(date -u +%s%3N)"
+               local current_time=$(timestamp_ms)
                if ((current_time - start_time > timeout_ms)); then
                        echo -n "$out"
                        return 1
-- 
2.43.0