selftests/bpf: Add case to test bpf_in_interrupt()

Add a timer test case to test 'bpf_in_interrupt()'.

cd tools/testing/selftests/bpf
./test_progs -t timer_interrupt
462     timer_interrupt:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20250903140438.59517-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 48 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/timer_interrupt.c b/tools/testing/selftests/bpf/progs/timer_interrupt.c
new file mode 100644
index 000000000000..19180a455f40
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/timer_interrupt.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_experimental.h"
+
+char _license[] SEC("license") = "GPL";
+
+#define CLOCK_MONOTONIC		1
+
+int preempt_count;
+int in_interrupt;
+int in_interrupt_cb;
+
+struct elem {
+	struct bpf_timer t;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} array SEC(".maps");
+
+static int timer_in_interrupt(void *map, int *key, struct bpf_timer *timer)
+{
+	preempt_count = get_preempt_count();
+	in_interrupt_cb = bpf_in_interrupt();
+	return 0;
+}
+
+SEC("fentry/bpf_fentry_test1")
+int BPF_PROG(test_timer_interrupt)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&array, &key);
+	if (!timer)
+		return 0;
+
+	in_interrupt = bpf_in_interrupt();
+	bpf_timer_init(timer, &array, CLOCK_MONOTONIC);
+	bpf_timer_set_callback(timer, timer_in_interrupt);
+	bpf_timer_start(timer, 0, 0);
+	return 0;
+}