[PATCH bpf-next 5/5] selftests/bpf: Test trampoline args of a BPF function at an odd slot

[Yonghong Song <yonghong.song@linux.dev>]

Attach fentry and fexit programs to two functions of another BPF
program, each taking a struct holding an __int128 at an odd argument
slot:

  - callee_odd(): the struct after a single u64, where arm64 would
    leave a register empty.
  - callee_stack(): the struct after seven u64s, where x86-64 would
    leave a stack slot empty and arm64 a register.

BPF code calls them with the arguments packed in the BPF convention, so
that is where the trampoline has to take them from. The fentry programs
read each argument slot from ctx[], and the fexit programs check the
return value, weighted by slot, which is only right if the function got
its arguments back where its caller put them. Both take a struct by
value, which GCC passes by reference instead, so they are only built
with clang, and callee_stack() only with a clang that supports stack
arguments.

Placing a BPF function's arguments by the kernel convention instead, the
x86-64 trampoline fails the test with:

  test_int128_tgt_prog:FAIL:stack:h.lo unexpected stack:h.lo: actual 9 != expected 8
  ...
  test_int128_tgt_prog:FAIL:stack fexit ret unexpected stack fexit ret: actual 34359738697 != expected 385

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 .../selftests/bpf/prog_tests/tracing_struct.c | 74 +++++++++++++++++++
 .../bpf/progs/tracing_struct_int128_tgt.c     | 68 +++++++++++++++++
 .../bpf/progs/tracing_struct_int128_tracer.c  | 44 +++++++++++
 3 files changed, 186 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/tracing_struct_int128_tgt.c
 create mode 100644 tools/testing/selftests/bpf/progs/tracing_struct_int128_tracer.c

diff --git a/tools/testing/selftests/bpf/prog_tests/tracing_struct.c b/tools/testing/selftests/bpf/prog_tests/tracing_struct.c
index eced07a61dde..d0bdd2a72b0b 100644
--- a/tools/testing/selftests/bpf/prog_tests/tracing_struct.c
+++ b/tools/testing/selftests/bpf/prog_tests/tracing_struct.c
@@ -5,6 +5,8 @@
 #include "tracing_struct.skel.h"
 #include "tracing_struct_many_args.skel.h"
 #include "tracing_struct_int128.skel.h"
+#include "tracing_struct_int128_tgt.skel.h"
+#include "tracing_struct_int128_tracer.skel.h"
 
 static void test_struct_args(void)
 {
@@ -162,6 +164,76 @@ static void test_int128_args(void)
 #endif
 }
 
+static void test_int128_tgt_prog(void)
+{
+#if defined(__x86_64__) || defined(__aarch64__)
+	struct tracing_struct_int128_tracer *tracer = NULL;
+	struct tracing_struct_int128_tgt *tgt;
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	int err, fd;
+
+	tgt = tracing_struct_int128_tgt__open_and_load();
+	if (!ASSERT_OK_PTR(tgt, "tgt__open_and_load"))
+		return;
+	if (!tgt->rodata->has_struct_arg) {
+		test__skip();
+		goto out;
+	}
+	fd = bpf_program__fd(tgt->progs.run);
+
+	tracer = tracing_struct_int128_tracer__open();
+	if (!ASSERT_OK_PTR(tracer, "tracer__open"))
+		goto out;
+
+	err = bpf_program__set_attach_target(tracer->progs.odd_entry, fd, "callee_odd");
+	err = err ?: bpf_program__set_attach_target(tracer->progs.odd_exit, fd, "callee_odd");
+	if (tgt->rodata->has_stack_arg) {
+		err = err ?: bpf_program__set_attach_target(tracer->progs.stack_entry, fd,
+							     "callee_stack");
+		err = err ?: bpf_program__set_attach_target(tracer->progs.stack_exit, fd,
+							     "callee_stack");
+	} else {
+		bpf_program__set_autoload(tracer->progs.stack_entry, false);
+		bpf_program__set_autoload(tracer->progs.stack_exit, false);
+	}
+	if (!ASSERT_OK(err, "set_attach_target"))
+		goto out;
+
+	err = tracing_struct_int128_tracer__load(tracer);
+	if (!ASSERT_OK(err, "tracer__load"))
+		goto out;
+	err = tracing_struct_int128_tracer__attach(tracer);
+	if (!ASSERT_OK(err, "tracer__attach"))
+		goto out;
+
+	err = bpf_prog_test_run_opts(fd, &topts);
+	if (!ASSERT_OK(err, "test_run") || !ASSERT_EQ(topts.retval, 0, "retval"))
+		goto out;
+
+	ASSERT_EQ(tracer->bss->odd_a, 1, "odd:a");
+	ASSERT_EQ(tracer->bss->odd_lo, 8, "odd:v.lo");
+	ASSERT_EQ(tracer->bss->odd_hi, 9, "odd:v.hi");
+	ASSERT_EQ(tracer->bss->odd_b, 2, "odd:b");
+	ASSERT_EQ(tgt->bss->ret_odd, 52, "odd ret");
+	ASSERT_EQ(tracer->bss->odd_ret, 52, "odd fexit ret");
+
+	if (tgt->rodata->has_stack_arg) {
+		ASSERT_EQ(tracer->bss->stack_g, 7, "stack:g");
+		ASSERT_EQ(tracer->bss->stack_lo, 8, "stack:h.lo");
+		ASSERT_EQ(tracer->bss->stack_hi, 9, "stack:h.hi");
+		ASSERT_EQ(tracer->bss->stack_i, 10, "stack:i");
+		ASSERT_EQ(tgt->bss->ret_stack, 385, "stack ret");
+		ASSERT_EQ(tracer->bss->stack_ret, 385, "stack fexit ret");
+	}
+
+out:
+	tracing_struct_int128_tracer__destroy(tracer);
+	tracing_struct_int128_tgt__destroy(tgt);
+#else
+	test__skip();
+#endif
+}
+
 static void test_union_args(void)
 {
 	struct tracing_struct *skel;
@@ -197,6 +269,8 @@ void test_tracing_struct(void)
 		test_struct_many_args();
 	if (test__start_subtest("int128_args"))
 		test_int128_args();
+	if (test__start_subtest("int128_tgt_prog"))
+		test_int128_tgt_prog();
 	if (test__start_subtest("union_args"))
 		test_union_args();
 }
diff --git a/tools/testing/selftests/bpf/progs/tracing_struct_int128_tgt.c b/tools/testing/selftests/bpf/progs/tracing_struct_int128_tgt.c
new file mode 100644
index 000000000000..2ec8f741d48b
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/tracing_struct_int128_tgt.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+struct int128_arg {
+	__int128 v;
+};
+
+__u64 base;
+__u64 ret_odd, ret_stack;
+
+#if defined(__clang__)
+
+const volatile bool has_struct_arg = true;
+
+/* arm64 would want v in x2 and x3, and b in x4. */
+__noinline __u64 callee_odd(__u64 a, struct int128_arg v, __u64 b)
+{
+	return a + (__u64)v.v * 2 + (__u64)((unsigned __int128)v.v >> 64) * 3 +
+	       b * 4;
+}
+
+#else
+
+const volatile bool has_struct_arg = false;
+
+#endif
+
+#if defined(__clang__) && defined(__BPF_FEATURE_STACK_ARGUMENT)
+
+const volatile bool has_stack_arg = true;
+
+/*
+ * x86-64 would want h in its third and fourth stack slots, and arm64 in
+ * its first and second, leaving x7 empty.
+ */
+static __noinline __u64 callee_stack(__u64 a, __u64 b, __u64 c, __u64 d,
+				     __u64 e, __u64 f, __u64 g,
+				     struct int128_arg h, __u64 i)
+{
+	return a + b * 2 + c * 3 + d * 4 + e * 5 + f * 6 + g * 7 +
+	       (__u64)h.v * 8 + (__u64)((unsigned __int128)h.v >> 64) * 9 +
+	       i * 10;
+}
+
+#else
+
+const volatile bool has_stack_arg = false;
+
+#endif
+
+SEC("syscall")
+int run(void *ctx)
+{
+#if defined(__clang__)
+	struct int128_arg v = { .v = ((__int128)(base + 9) << 64) | (base + 8) };
+
+	ret_odd = callee_odd(base + 1, v, base + 2);
+#if defined(__BPF_FEATURE_STACK_ARGUMENT)
+	ret_stack = callee_stack(base + 1, base + 2, base + 3, base + 4,
+				 base + 5, base + 6, base + 7, v, base + 10);
+#endif
+#endif
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/progs/tracing_struct_int128_tracer.c b/tools/testing/selftests/bpf/progs/tracing_struct_int128_tracer.c
new file mode 100644
index 000000000000..133f24fc9158
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/tracing_struct_int128_tracer.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+long odd_a, odd_lo, odd_hi, odd_b, odd_ret;
+
+SEC("fentry/callee_odd")
+int odd_entry(unsigned long long *ctx)
+{
+	odd_a = ctx[0];
+	odd_lo = ctx[1];
+	odd_hi = ctx[2];
+	odd_b = ctx[3];
+	return 0;
+}
+
+SEC("fexit/callee_odd")
+int odd_exit(unsigned long long *ctx)
+{
+	odd_ret = ctx[4];
+	return 0;
+}
+
+long stack_g, stack_lo, stack_hi, stack_i, stack_ret;
+
+SEC("fentry/callee_stack")
+int stack_entry(unsigned long long *ctx)
+{
+	stack_g = ctx[6];
+	stack_lo = ctx[7];
+	stack_hi = ctx[8];
+	stack_i = ctx[9];
+	return 0;
+}
+
+SEC("fexit/callee_stack")
+int stack_exit(unsigned long long *ctx)
+{
+	stack_ret = ctx[10];
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.53.0-Meta