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>
---
v3->v4:
introduce (and use) taprio_wait_for_admin.sh. The test depends upon this
taprio patch to work properly:
https://patchwork.kernel.org/project/netdevbpf/patch/20230807160827.4087483-2-vladimir.oltean@nxp.com/
Note: I couldn't find a way around the horrible tcmd.safe_substitute(NAMES)
and the string quoting (no possibility to perform complex shell operations)
than to put the logic I needed in a separate shell script and execute that.

v2->v3: none
v1->v2: patch is new

 .../tc-testing/taprio_wait_for_admin.sh       | 16 ++++++
 .../tc-testing/tc-tests/qdiscs/taprio.json    | 50 +++++++++++++++++++
 2 files changed, 66 insertions(+)
 create mode 100755 tools/testing/selftests/tc-testing/taprio_wait_for_admin.sh

diff --git a/tools/testing/selftests/tc-testing/taprio_wait_for_admin.sh b/tools/testing/selftests/tc-testing/taprio_wait_for_admin.sh
new file mode 100755
index 000000000000..f5335e8ad6b4
--- /dev/null
+++ b/tools/testing/selftests/tc-testing/taprio_wait_for_admin.sh
@@ -0,0 +1,16 @@
+#!/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
diff --git a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
index 860b55df0d6d..f475967c509f 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
+++ b/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"
+        ]
     }
 ]
-- 
2.34.1