[PATCH net v4 2/2] selftests/tc-testing: add taprio test for class dump after child delete

[Weiming Shi <bestswngs@gmail.com>]

Add a regression test for the NULL pointer dereference fixed in the
previous commit. Before the fix, taprio_graft() stored NULL into
q->qdiscs[cl - 1] when an explicitly grafted child qdisc was deleted
via RTM_DELQDISC; the next RTM_GETTCLASS dump then crashed the kernel
in taprio_dump_class() while reading child->handle.

The test installs a taprio root qdisc on a multi-queue netdevsim
device, grafts a pfifo child onto class 8001:1, deletes that child,
and then performs a class dump. On a fixed kernel the dump succeeds
and all eight taprio classes are listed; on an unpatched kernel the
class dump crashes, which surfaces as a test failure.

Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-6
---
 .../tc-testing/tc-tests/qdiscs/taprio.json    | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

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 557fb074acf0c..cd19d05925e40 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
+++ b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
@@ -302,5 +302,31 @@
             "$TC qdisc del dev $ETH root",
             "echo \"1\" > /sys/bus/netdevsim/del_device"
         ]
+    },
+    {
+        "id": "c7e1",
+        "name": "Class dump after graft and delete of explicit child qdisc",
+        "category": [
+            "qdisc",
+            "taprio"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "echo \"1 1 8\" > /sys/bus/netdevsim/new_device",
+            "$TC qdisc replace dev $ETH handle 8001: parent root taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0 sched-entry S ff 20000000 clockid CLOCK_TAI",
+            "$TC qdisc add dev $ETH parent 8001:1 handle 8002: pfifo",
+            "$TC qdisc del dev $ETH parent 8001:1 handle 8002:"
+        ],
+        "cmdUnderTest": "$TC class show dev $ETH",
+        "expExitCode": "0",
+        "verifyCmd": "$TC class show dev $ETH",
+        "matchPattern": "class taprio 8001:[0-9]+ root",
+        "matchCount": "8",
+        "teardown": [
+            "$TC qdisc del dev $ETH root",
+            "echo \"1\" > /sys/bus/netdevsim/del_device"
+        ]
     }
 ]
-- 
2.43.0