[RFC bpf-next 15/15] selftests/bpf: Add test tracing inline site using SEC("kloc")

[Alan Maguire <alan.maguire@oracle.com>]

Add a test tracing a vmlinux inline function called from
__sys_bpf() and ensure one of its arguments - if available -
is as expected.

A simple test as a starting point but it does demonstrate the
viability of the approach.

Ideally we would add a bunch of inlines to bpf_testmod, but
need to have BTF distillation/relocation working for .BTF.extra
sections; that is not yet implemented.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/testing/selftests/bpf/prog_tests/kloc.c | 51 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/kloc.c      | 36 +++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/kloc.c
 create mode 100644 tools/testing/selftests/bpf/progs/kloc.c

diff --git a/tools/testing/selftests/bpf/prog_tests/kloc.c b/tools/testing/selftests/bpf/prog_tests/kloc.c
new file mode 100644
index 000000000000..a88a43f25909
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/kloc.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025, Oracle and/or its affiliates. */
+
+#include <test_progs.h>
+#include <sys/stat.h>
+
+#include "kloc.skel.h"
+
+void test_kloc(void)
+{
+	int err = 0, duration = 0;
+	struct kloc *skel;
+	struct stat sb;
+
+	/* If CONFIG_DEBUG_INFO_BTF_EXTRA=m , ensure vmlinux BTF extra is
+	 * loaded.
+	 */
+	system("modprobe btf_extra");
+
+	/* Kernel may not have been compiled with extra BTF info or pahole
+	 * may not have support.
+	 */
+	if (stat("/sys/kernel/btf_extra/vmlinux", &sb) != 0)
+		test__skip();
+
+	skel = kloc__open_and_load();
+	if (CHECK(!skel, "skel_load", "skeleton failed: %d\n", err))
+		goto cleanup;
+
+	skel->bss->test_pid = getpid();
+
+	err = kloc__attach(skel);
+	if (!ASSERT_OK(err, "attach"))
+		goto cleanup;
+	/* trigger bpf syscall to trigger kloc */
+	(void) bpf_obj_get("/sys/fs/bpf/noexist");
+
+	ASSERT_GT(skel->bss->kloc_triggered, 0, "verify kloc was triggered");
+
+	/* this is a conditional since it is possible the size parameter
+	 * is not available at the inline site.
+	 *
+	 * Expected size here is that from bpf_obj_get_opts(); see
+	 * tools/lib/bpf/bpf.c.
+	 */
+	if (skel->bss->kloc_size > 0)
+		ASSERT_EQ(skel->bss->kloc_size, offsetofend(union bpf_attr, path_fd), "verify kloc size set");
+
+cleanup:
+	kloc__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/kloc.c b/tools/testing/selftests/bpf/progs/kloc.c
new file mode 100644
index 000000000000..8007e53f3210
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/kloc.c
@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025, Oracle and/or its affiliates. */
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+#include <bpf/loc.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+int kloc_triggered;
+size_t kloc_size;
+int test_pid;
+
+/* This function is inlined to __sys_bpf() and we trigger a call to
+ * it via bpf_obj_get_opts().
+ */
+SEC("kloc/vmlinux:copy_from_bpfptr_offset")
+int BPF_KLOC(trace_copy_from_bpfptr_offset, void *dst, void *uattr, size_t offset, size_t size)
+{
+	int pid = bpf_get_current_pid_tgid() >> 32;
+	long s;
+
+	if (test_pid != pid)
+		return 0;
+
+	kloc_triggered++;
+
+	/* is arg available? */
+	if (bpf_loc_arg(ctx, 3, &s) == 0)
+		kloc_size = size;
+
+	return 0;
+}
-- 
2.39.3