[RFC PATCH 2/2] kunit: expose ftrace-based API for stubbing out functions during tests

[David Gow <davidgow@google.com>]

From: Daniel Latypov <dlatypov@google.com>

Allow function redirection using ftrace and kernel livepatch. This is
basically equivalent to the static_stub support in the previous patch,
but does not require the function being replaced to be modified (save
for the addition of KUNIT_STUBBABLE/noinline).

This is hidden behind the CONFIG_KUNIT_FTRACE_STUBS option, and has a
number of dependencies, including ftrace, livepatch and
CONFIG_KALLSYMS_ALL. As a result, it only works on architectures where
these are available.

You can run the KUnit example tests with the following:
$ ./tools/testing/kunit/kunit.py run --kunitconfig lib/kunit/stubs_example.kunitconfig --arch=x86_64

To the end user, replacing a function is very simple, e.g.
  KUNIT_STUBBABLE void real_func(int n);
  void replacement_func(int n);

  /* in tests */
  kunit_activate_ftrace_stub(test, real_func, replacement_func);

The implementation is inspired by Steven's snippet here [1].

Some more details:
* stubbing is automatically undone at the end of tests
* it can also be manually undone with kunit_deactive_ftrace_stub()
* stubbing only applies when current->kunit_test == test
  * note: currently can't have more than one test running at a time
* KUNIT_STUBBABLE marks tests as noinline when CONFIG_KUNIT_STUBS is set
  * this ensures we can actually stub all calls
* KUNIT_STUBBABLE_TRAMPOLINE is a version that evaluates to
  __always_inline when stubbing is not enabled
  * This may need to be used with a wrapper function.
  * See the doc comment for more details.

Sharp-edges:
* kernel livepatch only works on some arches (not UML)
* if you don't use noinline/KUNIT_STUBBABLE, functions might be inlined
  and thus none of this works:
  * if it's always inlined, at least the attempt to stub will fail
  * if it's sometimes inlined, then the stub silently won't work

[1] https://lore.kernel.org/lkml/20220224091550.2b7e8784@gandalf.local.home

Co-developed-by: David Gow <davidgow@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
---
 include/kunit/ftrace_stub.h         |  84 +++++++++++++++++
 lib/kunit/Kconfig                   |  11 +++
 lib/kunit/Makefile                  |   4 +
 lib/kunit/ftrace_stub.c             | 138 ++++++++++++++++++++++++++++
 lib/kunit/kunit-example-test.c      |  27 +++++-
 lib/kunit/stubs_example.kunitconfig |  11 +++
 6 files changed, 274 insertions(+), 1 deletion(-)
 create mode 100644 include/kunit/ftrace_stub.h
 create mode 100644 lib/kunit/ftrace_stub.c
 create mode 100644 lib/kunit/stubs_example.kunitconfig

diff --git a/include/kunit/ftrace_stub.h b/include/kunit/ftrace_stub.h
new file mode 100644
index 000000000000..54c053b7e9c1
--- /dev/null
+++ b/include/kunit/ftrace_stub.h
@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _KUNIT_FTRACE_STUB_H
+#define _KUNIT_FTRACE_STUB_H
+
+/** KUNIT_STUBBABLE - marks a function as stubbable when stubbing support is
+ * enabled.
+ *
+ * Stubbing uses ftrace internally, so we can only stub out functions when they
+ * are not inlined. This macro eavlautes to noinline when stubbing support is
+ * enabled to thus make it safe.
+ *
+ * If you cannot add this annotation to the function, you can instead use
+ * KUNIT_STUBBABLE_TRAMPOLINE, which is the same, but evaluates to
+ * __always_inline when stubbing is not enabled.
+ *
+ * Consider copy_to_user, which is marked as __always_inline:
+ *
+ * .. code-block:: c
+ *	static KUNIT_STUBBABLE_TRAMPOLINE unsigned long
+ *	copy_to_user_trampoline(void __user *to, const void *from, unsigned long n)
+ *	{
+ *		return copy_to_user(to, from, n);
+ *	}
+ *
+ * Then we simply need to update our code to go through this function instead
+ * (in the places where we want to stub it out).
+ */
+#if IS_ENABLED(CONFIG_KUNIT_FTRACE_STUBS)
+#define KUNIT_STUBBABLE noinline
+#define KUNIT_STUBBABLE_TRAMPOLINE noinline
+#else
+#define KUNIT_STUBBABLE
+#define KUNIT_STUBBABLE_TRAMPOLINE __always_inline
+#endif
+
+struct kunit;
+
+/**
+ * kunit_activate_ftrace_stub() - makes all calls to @func go to @replacement during @test.
+ * @test: The test context object.
+ * @func: The function to stub out, must be annotated with KUNIT_STUBBABLE.
+ * @replacement: The function to replace @func with.
+ *
+ * All calls to @func will instead call @replacement for the duration of the
+ * current test. If called from outside the test's thread, the function will
+ * not be redirected.
+ *
+ * The redirection can be disabled again with kunit_deactivate_ftrace_stub().
+ *
+ * Example:
+ *
+ * .. code-block:: c
+ *	KUNIT_STUBBABLE int real_func(int n)
+ *	{
+ *		pr_info("real_func() called with %d", n);
+ *		return 0;
+ *	}
+ *
+ *	void replacement_func(int n)
+ *	{
+ *		pr_info("replacement_func() called with %d", n);
+ *		return 42;
+ *	}
+ *
+ *	void example_test(struct kunit *test)
+ *	{
+ *		kunit_active_ftrace_stub(test, real_func, replacement_func);
+ *		KUNIT_EXPECT_EQ(test, real_func(1), 42);
+ *	}
+ *
+ */
+#define kunit_activate_ftrace_stub(test, func, replacement) do { \
+	typecheck(typeof(func), replacement); \
+	__kunit_activate_ftrace_stub(test, #func, func, replacement); \
+} while (0)
+
+void __kunit_activate_ftrace_stub(struct kunit *test,
+				  const char *name,
+				  void *real_fn_addr,
+				  void *replacement_addr);
+
+
+void kunit_deactivate_ftrace_stub(struct kunit *test, void *real_fn_addr);
+#endif  /* _KUNIT_STUB_H */
diff --git a/lib/kunit/Kconfig b/lib/kunit/Kconfig
index 0b5dfb001bac..978e4f72bae0 100644
--- a/lib/kunit/Kconfig
+++ b/lib/kunit/Kconfig
@@ -59,4 +59,15 @@ config KUNIT_ALL_TESTS
 
 	  If unsure, say N.
 
+config KUNIT_FTRACE_STUBS
+	bool "Support for stubbing out functions in KUnit tests with ftrace and kernel livepatch"
+	depends on FTRACE=y && FUNCTION_TRACER=y && MODULES=y && DEBUG_KERNEL=y && KALLSYMS_ALL=y && LIVEPATCH=y
+	help
+	  Builds support for stubbing out functions for the duration of KUnit
+	  test cases or suites using ftrace and kernel livepatch.
+	  See KUNIT_EXAMPLE_TEST for an example.
+
+	  NOTE: this does not work on all architectures (like UML, arm64) and
+	  relies on a lot of magic (see the dependencies list).
+
 endif # KUNIT
diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index f9e929700782..75092c12c3d0 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -19,3 +19,7 @@ obj-$(CONFIG_KUNIT_TEST) +=		string-stream-test.o
 endif
 
 obj-$(CONFIG_KUNIT_EXAMPLE_TEST) +=	kunit-example-test.o
+
+ifeq ($(CONFIG_KUNIT_FTRACE_STUBS),y)
+kunit-objs +=				ftrace_stub.o
+endif
diff --git a/lib/kunit/ftrace_stub.c b/lib/kunit/ftrace_stub.c
new file mode 100644
index 000000000000..13207e1c7aff
--- /dev/null
+++ b/lib/kunit/ftrace_stub.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <kunit/test.h>
+
+#include <linux/typecheck.h>
+
+#include <linux/ftrace.h>
+#include <linux/livepatch.h>
+#include <linux/sched.h>
+
+struct kunit_ftrace_stub_ctx {
+	struct kunit *test;
+	unsigned long real_fn_addr; /* used as a key to lookup the stub */
+	unsigned long replacement_addr;
+	struct ftrace_ops ops; /* a copy of kunit_stub_base_ops with .private set */
+};
+
+static void kunit_stub_trampoline(unsigned long ip, unsigned long parent_ip,
+				  struct ftrace_ops *ops,
+				  struct ftrace_regs *fregs)
+{
+	struct kunit_ftrace_stub_ctx *ctx = ops->private;
+	int lock_bit;
+
+	if (current->kunit_test != ctx->test)
+		return;
+
+	lock_bit = ftrace_test_recursion_trylock(ip, parent_ip);
+	KUNIT_ASSERT_GE(ctx->test, lock_bit, 0);
+
+	klp_arch_set_pc(fregs, ctx->replacement_addr);
+
+	ftrace_test_recursion_unlock(lock_bit);
+}
+
+static struct ftrace_ops kunit_stub_base_ops = {
+	.func = &kunit_stub_trampoline,
+	.flags = FTRACE_OPS_FL_IPMODIFY |
+#ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
+		FTRACE_OPS_FL_SAVE_REGS |
+#endif
+		FTRACE_OPS_FL_DYNAMIC
+};
+
+static void __kunit_ftrace_stub_resource_free(struct kunit_resource *res)
+{
+	struct kunit_ftrace_stub_ctx *ctx = res->data;
+
+	unregister_ftrace_function(&ctx->ops);
+	kfree(ctx);
+}
+
+/* Matching function for kunit_find_resource(). match_data is real_fn_addr. */
+static bool __kunit_static_stub_resource_match(struct kunit *test,
+						struct kunit_resource *res,
+						void *match_real_fn_addr)
+{
+	/* This pointer is only valid if res is a static stub resource. */
+	struct kunit_ftrace_stub_ctx *ctx = res->data;
+
+	/* Make sure the resource is a static stub resource. */
+	if (res->free != &__kunit_ftrace_stub_resource_free)
+		return false;
+
+	return ctx->real_fn_addr == (unsigned long)match_real_fn_addr;
+}
+
+void kunit_deactivate_ftrace_stub(struct kunit *test, void *real_fn_addr)
+{
+	struct kunit_resource *res;
+
+	KUNIT_ASSERT_PTR_NE_MSG(test, real_fn_addr, NULL,
+				"Tried to deactivate a NULL stub.");
+
+	/* Look up the existing stub for this function. */
+	res = kunit_find_resource(test,
+				  __kunit_static_stub_resource_match,
+				  real_fn_addr);
+
+	/* Error out if the stub doesn't exist. */
+	KUNIT_ASSERT_PTR_NE_MSG(test, res, NULL,
+				"Tried to deactivate a nonexistent stub.");
+
+	/* Free the stub. We 'put' twice, as we got a reference
+	 * from kunit_find_resource(). The free function will deactivate the
+	 * ftrace stub.
+	 */
+	kunit_remove_resource(test, res);
+	kunit_put_resource(res);
+}
+EXPORT_SYMBOL_GPL(kunit_deactivate_ftrace_stub);
+
+void __kunit_activate_ftrace_stub(struct kunit *test,
+				  const char *name,
+				  void *real_fn_addr,
+				  void *replacement_addr)
+{
+	unsigned long ftrace_ip;
+	struct kunit_ftrace_stub_ctx *ctx;
+	int ret;
+
+	ftrace_ip = ftrace_location((unsigned long)real_fn_addr);
+	if (!ftrace_ip)
+		KUNIT_ASSERT_FAILURE(test, "%s ip is invalid: not a function, or is marked notrace or inline", name);
+
+	/* Allocate the stub context, which contains pointers to the replacement
+	 * function and the test object. It's also registered as a KUnit
+	 * resource which can be looked up by address (to deactivate manually)
+	 * and is destroyed automatically on test exit.
+	 */
+	ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL_MSG(test, ctx, "failed to allocate kunit stub for %s", name);
+
+	ctx->test = test;
+	ctx->ops = kunit_stub_base_ops;
+	ctx->ops.private = ctx;
+	ctx->real_fn_addr = (unsigned long)real_fn_addr;
+	ctx->replacement_addr = (unsigned long)replacement_addr;
+
+	ret = ftrace_set_filter_ip(&ctx->ops, ftrace_ip, 0, 0);
+	if (ret) {
+		kfree(ctx);
+		KUNIT_ASSERT_FAILURE(test, "failed to set filter ip for %s: %d", name, ret);
+	}
+
+	ret = register_ftrace_function(&ctx->ops);
+	if (ret) {
+		kfree(ctx);
+		if (ret == -EBUSY)
+			KUNIT_ASSERT_FAILURE(test, "failed to register stub (-EBUSY) for %s, likely due to already stubbing it?", name);
+		KUNIT_ASSERT_FAILURE(test, "failed to register stub for %s: %d", name, ret);
+	}
+
+	/* Register the stub as a resource with a cleanup function */
+	kunit_alloc_resource(test, NULL,
+			     __kunit_ftrace_stub_resource_free,
+			     GFP_KERNEL, ctx);
+}
+EXPORT_SYMBOL_GPL(__kunit_activate_ftrace_stub);
diff --git a/lib/kunit/kunit-example-test.c b/lib/kunit/kunit-example-test.c
index 670c21e74446..7f20b132343b 100644
--- a/lib/kunit/kunit-example-test.c
+++ b/lib/kunit/kunit-example-test.c
@@ -8,6 +8,7 @@
 
 #include <kunit/test.h>
 #include <kunit/static_stub.h>
+#include <kunit/ftrace_stub.h>
 
 /*
  * This is the most fundamental element of KUnit, the test case. A test case
@@ -72,7 +73,7 @@ static void example_mark_skipped_test(struct kunit *test)
 }
 
 /* This is a function we'll replace with static stubs. */
-static int add_one(int i)
+static KUNIT_STUBBABLE int add_one(int i)
 {
 	/* This will trigger the stub if active. */
 	KUNIT_STATIC_STUB_REDIRECT(add_one, i);
@@ -107,6 +108,29 @@ static void example_static_stub_test(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, add_one(1), 2);
 }
 
+/*
+ * This test shows the use of static stubs.
+ */
+static void example_ftrace_stub_test(struct kunit *test)
+{
+#if !IS_ENABLED(CONFIG_KUNIT_FTRACE_STUBS)
+	kunit_skip(test, "KUNIT_FTRACE_STUBS not enabled");
+#else
+	/* By default, function is not stubbed. */
+	KUNIT_EXPECT_EQ(test, add_one(1), 2);
+
+	/* Replace add_one() with subtract_one(). */
+	kunit_activate_ftrace_stub(test, add_one, subtract_one);
+
+	/* add_one() is now replaced. */
+	KUNIT_EXPECT_EQ(test, add_one(1), 0);
+
+	/* Return add_one() to normal. */
+	kunit_deactivate_ftrace_stub(test, add_one);
+	KUNIT_EXPECT_EQ(test, add_one(1), 2);
+#endif
+}
+
 /*
  * Here we make a list of all the test cases we want to add to the test suite
  * below.
@@ -122,6 +146,7 @@ static struct kunit_case example_test_cases[] = {
 	KUNIT_CASE(example_skip_test),
 	KUNIT_CASE(example_mark_skipped_test),
 	KUNIT_CASE(example_static_stub_test),
+	KUNIT_CASE(example_ftrace_stub_test),
 	{}
 };
 
diff --git a/lib/kunit/stubs_example.kunitconfig b/lib/kunit/stubs_example.kunitconfig
new file mode 100644
index 000000000000..a47369199fb9
--- /dev/null
+++ b/lib/kunit/stubs_example.kunitconfig
@@ -0,0 +1,11 @@
+CONFIG_KUNIT=y
+CONFIG_KUNIT_FTRACE_STUBS=y
+CONFIG_KUNIT_EXAMPLE_TEST=y
+
+# Depedencies
+CONFIG_FTRACE=y
+CONFIG_FUNCTION_TRACER=y
+CONFIG_MODULES=y
+CONFIG_DEBUG_KERNEL=y
+CONFIG_KALLSYMS_ALL=y
+CONFIG_LIVEPATCH=y
-- 
2.35.1.894.gb6a874cedc-goog