selftests/tc-testing: add test for HFSC divide-by-zero in rtsc_min()

Add a regression test for the divide-by-zero in rtsc_min() triggered
when m2sm() converts a large m1 value (e.g. 32gbit) to a u64 scaled
slope reaching 2^32. rtsc_min() stores the difference of two such u64
values (sm1 - sm2) in a u32 variable `dsm`, truncating 2^32 to zero
and causing a divide-by-zero oops in the concave-curve intersection
path. The test configures an HFSC class with m1=32gbit d=1ms m2=0bit,
sends a packet to activate the class, waits for it to drain and go
idle, then sends another packet to trigger reactivation through
rtsc_min().

Signed-off-by: Xiang Mei <xmei5@asu.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260326204310.1549327-2-xmei5@asu.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -1111,5 +1111,30 @@
         "teardown": [
             "$TC qdisc del dev $DUMMY root handle 1:"
         ]
+    },
+    {
+        "id": "a3d7",
+        "name": "HFSC with large m1 - no divide-by-zero on class reactivation",
+        "category": [
+            "qdisc",
+            "hfsc"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$TC qdisc replace dev $DUMMY root handle 1: hfsc default 1",
+            "$TC class replace dev $DUMMY parent 1: classid 1:1 hfsc rt m1 32gbit d 1ms m2 0bit ls m1 32gbit d 1ms m2 0bit",
+            "ping -I$DUMMY -f -c1 -s64 -W1 10.10.10.1 || true",
+            "sleep 1"
+        ],
+        "cmdUnderTest": "ping -I$DUMMY -f -c1 -s64 -W1 10.10.10.1 || true",
+        "expExitCode": "0",
+        "verifyCmd": "$TC qdisc show dev $DUMMY",
+        "matchPattern": "qdisc hfsc 1: root",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $DUMMY handle 1: root"
+        ]
     }
 ]