[PATCH net v3 2/2] selftests: tc-testing: add u32 node ID pool exhaustion test

[Jamal Hadi Salim <jhs@mojatatu.com>]

Add a tdc test case that fills the u32 node ID space with 4095
auto-generated handles, then attempts to add a 4096th. On the fixed
kernel the 4096th filter is rejected with ENOSPC (exit 2). On the
unfixed kernel it silently succeeds with a duplicate handle.

The setup pipes the 4095 add commands directly into `tc -b -` inside a
single bash -c (matching the existing test id 1234 pattern), avoiding
any temp file.

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
v2 -> v3:
- Dropped the /tmp/u32_batch.txt temp file. The setup now pipes the
  4095 add commands directly into `$TC -b -` inside a single bash -c
  (matching the existing test id 1234 pattern), eliminating the
  symlink/TOCTOU risk and the leftover-file cleanup gap flagged by
  both Sashikos (gemini Medium + nipa gpt-5-6-sol-6-9, gpt-5-6-sol-4-7).
  New auto-generated tdc id: 70fd.

v1 -> v2:
- Renewed tdc test (v1 tdc failed in long-running sweep, prompting v2).
---
 .../tc-testing/tc-tests/filters/u32.json      | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json b/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
index b2ca9d4e991b..e2b03f2b5e89 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
+++ b/tools/testing/selftests/tc-testing/tc-tests/filters/u32.json
@@ -353,5 +353,28 @@
         "teardown": [
             "$TC qdisc del dev $DEV1 parent root drr"
         ]
+    },
+    {
+        "id": "70fd",
+        "name": "Add u32 filter when node ID pool is exhausted (4096th filter rejected)",
+        "category": [
+            "filter",
+            "u32"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$TC qdisc add dev $DUMMY clsact",
+            "bash -c 'for i in {1..4095}; do echo filter add dev $DUMMY ingress prio 1 protocol ip u32 match u8 0 0 at 0; done | $TC -b -'"
+        ],
+        "cmdUnderTest": "$TC filter add dev $DUMMY ingress prio 1 protocol ip u32 match u8 0 0 at 0",
+        "expExitCode": "2",
+        "verifyCmd": "$TC -d filter show dev $DUMMY ingress",
+        "matchPattern": "fh 800::",
+        "matchCount": "4095",
+        "teardown": [
+            "$TC qdisc del dev $DUMMY clsact"
+        ]
     }
 ]
-- 
2.43.0