[PATCH bpf v2] selftests/bpf: Use both hrtimer enqueue helpers in vmlinux test

[Ihor Solodrai <ihor.solodrai@linux.dev>]

The vmlinux selftest triggers nanosleep and checks that both kprobe
and fentry programs observe the hrtimer enqueue path.

After the hrtimer_start_expires_user() conversion [1], nanosleep
reaches hrtimer_start_range_ns_user() instead of
hrtimer_start_range_ns(). Hard-coding either symbol makes the test
fail either on bpf tree or on linux-next [2].

Update the test to resolve the target symbol at runtime via
libbpf_find_vmlinux_btf_id(). This is a nice example of how to modify
a BPF program to work on both older and newer kernel revision.

[1] https://lore.kernel.org/all/20260408114952.062400833@kernel.org/
[2] https://github.com/kernel-patches/bpf/actions/runs/25485909958/job/74782902203

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>

---

v1->v2:
  * Use bpf_program__set_attach_target() and bpf_program__attach_kprobe()
    to avoid duplicating BPF programs (Amery)
v1: https://lore.kernel.org/all/20260507113915.24988-1-ihor.solodrai@linux.dev/

---
 .../selftests/bpf/prog_tests/vmlinux.c        | 45 ++++++++++++++++++-
 .../selftests/bpf/progs/test_vmlinux.c        |  4 +-
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/vmlinux.c b/tools/testing/selftests/bpf/prog_tests/vmlinux.c
index 6fb2217d940b..b5fdd593910d 100644
--- a/tools/testing/selftests/bpf/prog_tests/vmlinux.c
+++ b/tools/testing/selftests/bpf/prog_tests/vmlinux.c
@@ -14,21 +14,61 @@ static void nsleep()
 	(void)syscall(__NR_nanosleep, &ts, NULL);
 }
 
+static const char *hrtimer_func = "hrtimer_start_range_ns";
+
+static int setup_hrtimer_progs(struct test_vmlinux *skel)
+{
+	int err;
+
+	if (libbpf_find_vmlinux_btf_id("hrtimer_start_range_ns_user", BPF_TRACE_FENTRY) > 0)
+		hrtimer_func = "hrtimer_start_range_ns_user";
+
+	err = bpf_program__set_attach_target(skel->progs.handle__fentry, 0, hrtimer_func);
+	if (err)
+		return err;
+
+	/*
+	 * Bare SEC("kprobe") has no target function, so attach it manually
+	 * later after selecting the hrtimer function to probe.
+	 */
+	bpf_program__set_autoattach(skel->progs.handle__kprobe, false);
+
+	return 0;
+}
+
 void test_vmlinux(void)
 {
 	int err;
 	struct test_vmlinux* skel;
 	struct test_vmlinux__bss *bss;
+	struct bpf_link *kprobe_link = NULL;
 
-	skel = test_vmlinux__open_and_load();
-	if (!ASSERT_OK_PTR(skel, "test_vmlinux__open_and_load"))
+	skel = test_vmlinux__open();
+	if (!ASSERT_OK_PTR(skel, "test_vmlinux__open"))
 		return;
+
+	err = setup_hrtimer_progs(skel);
+	if (!ASSERT_OK(err, "setup_hrtimer_progs"))
+		goto cleanup;
+
+	err = test_vmlinux__load(skel);
+	if (!ASSERT_OK(err, "test_vmlinux__load"))
+		goto cleanup;
+
 	bss = skel->bss;
 
 	err = test_vmlinux__attach(skel);
 	if (!ASSERT_OK(err, "test_vmlinux__attach"))
 		goto cleanup;
 
+	/* manually attach kprobe with the selected function */
+	if (hrtimer_func) {
+		kprobe_link = bpf_program__attach_kprobe(skel->progs.handle__kprobe,
+							 false /* retprobe */, hrtimer_func);
+		if (!ASSERT_OK_PTR(kprobe_link, "bpf_program__attach_kprobe"))
+			goto cleanup;
+	}
+
 	/* trigger everything */
 	nsleep();
 
@@ -39,5 +79,6 @@ void test_vmlinux(void)
 	ASSERT_TRUE(bss->fentry_called, "fentry");
 
 cleanup:
+	bpf_link__destroy(kprobe_link);
 	test_vmlinux__destroy(skel);
 }
diff --git a/tools/testing/selftests/bpf/progs/test_vmlinux.c b/tools/testing/selftests/bpf/progs/test_vmlinux.c
index 78b23934d9f8..eea556940df6 100644
--- a/tools/testing/selftests/bpf/progs/test_vmlinux.c
+++ b/tools/testing/selftests/bpf/progs/test_vmlinux.c
@@ -69,7 +69,7 @@ int BPF_PROG(handle__tp_btf, struct pt_regs *regs, long id)
 	return 0;
 }
 
-SEC("kprobe/hrtimer_start_range_ns")
+SEC("kprobe")
 int BPF_KPROBE(handle__kprobe, struct hrtimer *timer, ktime_t tim, u64 delta_ns,
 	       const enum hrtimer_mode mode)
 {
@@ -78,7 +78,7 @@ int BPF_KPROBE(handle__kprobe, struct hrtimer *timer, ktime_t tim, u64 delta_ns,
 	return 0;
 }
 
-SEC("fentry/hrtimer_start_range_ns")
+SEC("fentry")
 int BPF_PROG(handle__fentry, struct hrtimer *timer, ktime_t tim, u64 delta_ns,
 	     const enum hrtimer_mode mode)
 {
-- 
2.54.0