[PATCH v5 3/3] rust: kunit: allow to know if we are in a test

David Gow
2024-12-13T08:10:57Z

From: José Expósito <jose.exposito89@gmail.com>

In some cases, we need to call test-only code from outside the test
case, for example, to mock a function or a module.

In order to check whether we are in a test or not, we need to test if
`CONFIG_KUNIT` is set.
Unfortunately, we cannot rely only on this condition because:
- a test could be running in another thread,
- some distros compile KUnit in production kernels, so checking at runtime
that `current->kunit_test != NULL` is required.

Forturately, KUnit provides an optimised check in
`kunit_get_current_test()`, which checks CONFIG_KUNIT, a global static
key, and then the current thread's running KUnit test.

Add a safe wrapper function around this to know whether or not we are in
a KUnit test and examples showing how to mock a function and a module.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Co-developed-by: David Gow <davidgow@google.com>
Signed-off-by: David Gow <davidgow@google.com>
---
Changes since v4:
https://lore.kernel.org/linux-kselftest/20241101064505.3820737-4-davidgow@google.com/
- Rebased against 6.13-rc1
- Fix some missing safety comments, and remove some unneeded 'unsafe'
blocks. (Thanks Boqun)

Changes since v3:
https://lore.kernel.org/linux-kselftest/20241030045719.3085147-8-davidgow@google.com/
- The example test has been updated to no longer use assert_eq!() with
a constant bool argument (fixes a clippy warning).

No changes since v2:
https://lore.kernel.org/linux-kselftest/20241029092422.2884505-4-davidgow@google.com/

Changes since v1:
https://lore.kernel.org/lkml/20230720-rustbind-v1-3-c80db349e3b5@google.com/
- Rebased on top of rust-next.
- Use the `kunit_get_current_test()` C function, which wasn't previously
available, instead of rolling our own.
- (Thanks also to Boqun for suggesting a nicer way of implementing this,
which I tried, but the `kunit_get_current_test()` version obsoleted.)

---
rust/kernel/kunit.rs | 75 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 75 insertions(+)

diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index a92f12da77d5..2196e35e5d75 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -285,11 +285,86 @@ macro_rules! kunit_unsafe_test_suite {
};
}

+/// In some cases, you need to call test-only code from outside the test case, for example, to
+/// create a function mock. This function can be invoked to know whether we are currently running a
+/// KUnit test or not.
+///
+/// # Examples
+///
+/// This example shows how a function can be mocked to return a well-known value while testing:
+///
+/// ```
+/// # use kernel::kunit::in_kunit_test;
+/// #
+/// fn fn_mock_example(n: i32) -> i32 {
+/// if in_kunit_test() {
+/// 100
+/// } else {
+/// n + 1
+/// }
+/// }
+///
+/// let mock_res = fn_mock_example(5);
+/// assert_eq!(mock_res, 100);
+/// ```
+///
+/// Sometimes, you don't control the code that needs to be mocked. This example shows how the
+/// `bindings` module can be mocked:
+///
+/// ```
+/// // Import our mock naming it as the real module.
+/// #[cfg(CONFIG_KUNIT)]
+/// use bindings_mock_example as bindings;
+///
+/// // This module mocks `bindings`.
+/// mod bindings_mock_example {
+/// use kernel::kunit::in_kunit_test;
+/// use kernel::bindings::u64_;
+///
+/// // Make the other binding functions available.
+/// pub(crate) use kernel::bindings::*;
+///
+/// /// Mock `ktime_get_boot_fast_ns` to return a well-known value when running a KUnit test.
+/// pub(crate) fn ktime_get_boot_fast_ns() -> u64_ {
+/// if in_kunit_test() {
+/// 1234
+/// } else {
+/// // SAFETY: ktime_get_boot_fast_ns() is safe to call, and just returns a u64.
+/// // Additionally, this is never actually called in this example, as we're in a test
+/// // and it's mocked out.
+/// unsafe { kernel::bindings::ktime_get_boot_fast_ns() }
+/// }
+/// }
+/// }
+///
+/// // This is the function we want to test. Since `bindings` has been mocked, we can use its
+/// // functions seamlessly.
+/// fn get_boot_ns() -> u64 {
+/// bindings::ktime_get_boot_fast_ns()
+/// }
+///
+/// let time = get_boot_ns();
+/// assert_eq!(time, 1234);
+/// ```
+pub fn in_kunit_test() -> bool {
+ // SAFETY: kunit_get_current_test() is always safe to call from C (it has fallbacks for
+ // when KUnit is not enabled), and we're only comparing the result to NULL.
+ unsafe { !bindings::kunit_get_current_test().is_null() }
+}
+
#[kunit_tests(rust_kernel_kunit)]
mod tests {
+ use super::*;
+
#[test]
fn rust_test_kunit_example_test() {
#![expect(clippy::eq_op)]
assert_eq!(1 + 1, 2);
}
+
+ #[test]
+ fn rust_test_kunit_in_kunit_test() {
+ let in_kunit = in_kunit_test();
+ assert!(in_kunit);
+ }
}
--
2.47.1.613.gc27f4b7a9f-goog