selftests/tc-testing: test that taprio can only be attached as root

Check that the "Can only be attached as root qdisc" error message from
taprio is effective by attempting to attach it to a class of another
taprio qdisc. That operation should fail.

In the bug that was squashed by change "net/sched: taprio: try again to
report q->qdiscs[] to qdisc_leaf()", grafting a child taprio to a root
software taprio would be misinterpreted as a change() to the root
taprio. Catch this by looking at whether the base-time of the root
taprio has changed to follow the base-time of the child taprio,
something which should have absolutely never happened assuming correct
semantics.

Vinicius points out that looking at "base_time" in the tc qdisc show
output is unreliable because user space is in a race with the kernel
applying the setting. So we create a helper bash script which waits
while there is any pending schedule.

Link: https://lore.kernel.org/netdev/87il9w0xx7.fsf@intel.com/Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-11-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/tc-testing/taprio_wait_for_admin.sh

+#!/bin/bash
+
+TC="$1"; shift
+ETH="$1"; shift
+
+# The taprio architecture changes the admin schedule from a hrtimer and not
+# from process context, so we need to wait in order to make sure that any
+# schedule change actually took place.
+while :; do
+	has_admin="$($TC -j qdisc show dev $ETH root | jq '.[].options | has("admin")')"
+	if [ "$has_admin" = "false" ]; then
+		break;
+	fi
+
+	sleep 1
+done

tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json

@@ -156,5 +156,55 @@
         "teardown": [
             "echo \"1\" > /sys/bus/netdevsim/del_device"
         ]
+    },
+    {
+        "id": "39b4",
+        "name": "Reject grafting taprio as child qdisc of software taprio",
+        "category": [
+            "qdisc",
+            "taprio"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "echo \"1 1 8\" > /sys/bus/netdevsim/new_device",
+            "$TC qdisc replace dev $ETH handle 8001: parent root stab overhead 24 taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0 sched-entry S ff 20000000 clockid CLOCK_TAI",
+            "./taprio_wait_for_admin.sh $TC $ETH"
+        ],
+        "cmdUnderTest": "$TC qdisc replace dev $ETH parent 8001:7 taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 200 sched-entry S ff 20000000 clockid CLOCK_TAI",
+        "expExitCode": "2",
+        "verifyCmd": "bash -c \"./taprio_wait_for_admin.sh $TC $ETH && $TC -j qdisc show dev $ETH root | jq '.[].options.base_time'\"",
+        "matchPattern": "0",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $ETH root",
+            "echo \"1\" > /sys/bus/netdevsim/del_device"
+        ]
+    },
+    {
+        "id": "e8a1",
+        "name": "Reject grafting taprio as child qdisc of offloaded taprio",
+        "category": [
+            "qdisc",
+            "taprio"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "echo \"1 1 8\" > /sys/bus/netdevsim/new_device",
+            "$TC qdisc replace dev $ETH handle 8001: parent root stab overhead 24 taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0 sched-entry S ff 20000000 flags 0x2",
+            "./taprio_wait_for_admin.sh $TC $ETH"
+        ],
+        "cmdUnderTest": "$TC qdisc replace dev $ETH parent 8001:7 taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 200 sched-entry S ff 20000000 flags 0x2",
+        "expExitCode": "2",
+        "verifyCmd": "bash -c \"./taprio_wait_for_admin.sh $TC $ETH && $TC -j qdisc show dev $ETH root | jq '.[].options.base_time'\"",
+        "matchPattern": "0",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $ETH root",
+            "echo \"1\" > /sys/bus/netdevsim/del_device"
+        ]
     }
 ]