[PATCH bpf-next 2/2] selftests/sched_ext: Cover scx_bpf_cpu_rq() null return

[Nuoqi Gui]

Add a sched_ext negative-load test that calls scx_bpf_cpu_rq(-1) and then
immediately reads rq->nr_running. The verifier should reject the program
because scx_bpf_cpu_rq() may return NULL.

This guards the KF_RET_NULL metadata for scx_bpf_cpu_rq().

Signed-off-by: Nuoqi Gui <gnq25@xxxxxxxxxxxxxxxxxxxxx>
---
 tools/testing/selftests/sched_ext/Makefile         |  1 +
 .../selftests/sched_ext/cpu_rq_ret_null.bpf.c      | 27 +++++++++++++++
 .../testing/selftests/sched_ext/cpu_rq_ret_null.c  | 38 ++++++++++++++++++++++
 3 files changed, 66 insertions(+)

diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
index 5d2dffca0e91..15cedcf56a49 100644
--- a/tools/testing/selftests/sched_ext/Makefile
+++ b/tools/testing/selftests/sched_ext/Makefile
@@ -174,6 +174,7 @@ auto-test-targets :=			\
 	init_enable_count		\
 	maximal				\
 	maybe_null			\
+	cpu_rq_ret_null		\
 	minimal				\
 	non_scx_kfunc_deny		\
 	numa				\
diff --git a/tools/testing/selftests/sched_ext/cpu_rq_ret_null.bpf.c b/tools/testing/selftests/sched_ext/cpu_rq_ret_null.bpf.c
new file mode 100644
index 000000000000..d08f0b582542
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/cpu_rq_ret_null.bpf.c
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 Nuoqi Gui
+ */
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+u32 nr_running;
+
+void BPF_STRUCT_OPS(cpu_rq_ret_null_enable, struct task_struct *p)
+{}
+
+void BPF_STRUCT_OPS(cpu_rq_ret_null_dispatch, s32 cpu, struct task_struct *p)
+{
+	struct rq *rq;
+
+	rq = scx_bpf_cpu_rq(-1);
+	nr_running = rq->nr_running;
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops cpu_rq_ret_null = {
+	.dispatch		= (void *)cpu_rq_ret_null_dispatch,
+	.enable			= (void *)cpu_rq_ret_null_enable,
+	.name			= "cpu_rq_ret_null",
+};
diff --git a/tools/testing/selftests/sched_ext/cpu_rq_ret_null.c b/tools/testing/selftests/sched_ext/cpu_rq_ret_null.c
new file mode 100644
index 000000000000..bd82c310ad7a
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/cpu_rq_ret_null.c
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 Nuoqi Gui
+ */
+#include <bpf/bpf.h>
+#include <scx/common.h>
+#include "cpu_rq_ret_null.bpf.skel.h"
+#include "scx_test.h"
+
+static enum scx_test_status run(void *ctx)
+{
+	struct cpu_rq_ret_null *skel;
+	int err;
+
+	skel = cpu_rq_ret_null__open();
+	if (!skel) {
+		SCX_ERR("Failed to open skel");
+		return SCX_TEST_FAIL;
+	}
+
+	err = cpu_rq_ret_null__load(skel);
+	cpu_rq_ret_null__destroy(skel);
+
+	if (!err) {
+		SCX_ERR("scx_bpf_cpu_rq() direct dereference loaded");
+		return SCX_TEST_FAIL;
+	}
+
+	return SCX_TEST_PASS;
+}
+
+struct scx_test cpu_rq_ret_null = {
+	.name = "cpu_rq_ret_null",
+	.description = "Verify that scx_bpf_cpu_rq() returns a nullable pointer",
+	.run = run,
+};
+
+REGISTER_SCX_TEST(&cpu_rq_ret_null)

-- 
2.34.1