From: Willem de Bruijn The blamed commit updated a tc replace command by adding a handle. - tc(f"qdisc replace dev {ifname} root {qdisc} {optargs}") + tc(f"qdisc replace dev {ifname} root handle 1: {qdisc} {optargs}") This breaks the test if the root qdisc already has that handle and is of different kind, with "Invalid qdisc name: must match existing qdisc." If no handle is asked, or the kind differs, tc replace removes the old qdisc and grafts a new one. If a handle is asked and exists, tc replace tries to change the qdisc in place, for which the kind must be the same. It does not trigger in all setups, like netdevsim or debian 13, which do not have root handle 1:. But it is a common root handle. Solve the bug by first deleting the existing root qdisc if one exists. Wrap that command in a try block, because it will fail for default qdiscs with handle 0: with "Error: Cannot delete qdisc with handle of zero." Reported-by: Jakub Kicinski Closes: https://lore.kernel.org/netdev/20260810183118.32d5c06a@kernel.org/ Fixes: ef3d6cca02c8 ("selftests: drv-net: so_txtime: only send test traffic to sch_etf") Signed-off-by: Willem de Bruijn --- tools/testing/selftests/drivers/net/so_txtime.py | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/tools/testing/selftests/drivers/net/so_txtime.py b/tools/testing/selftests/drivers/net/so_txtime.py index 9fbc0278d28b..a097fae0b335 100755 --- a/tools/testing/selftests/drivers/net/so_txtime.py +++ b/tools/testing/selftests/drivers/net/so_txtime.py @@ -12,6 +12,7 @@ import time from lib.py import ksft_exit, ksft_run, ksft_variants from lib.py import KsftNamedVariant, KsftSkipEx from lib.py import NetDrvEpEnv, bkg, cmd, defer, tc +from lib.py import CmdExitFailure def test_so_txtime(cfg, clockid, ipver, args_tx, args_rx, expect_success): @@ -45,6 +46,10 @@ def _qdisc_setup(ifname, qdisc, optargs=""): """ orig = tc(f"qdisc show dev {ifname} root", json=True)[0].get("kind", None) defer(tc, f"qdisc replace dev {ifname} root {orig}") + try: + tc(f"qdisc del dev {ifname} root") + except CmdExitFailure: + pass tc(f"qdisc replace dev {ifname} root handle 1: {qdisc} {optargs}") -- 2.55.0.679.g6767b8d81c-goog