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>
---

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 | 72 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index ac296467a552..d5926f7ef405 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -272,11 +272,83 @@ 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) unsafe fn ktime_get_boot_fast_ns() -> u64_ {
+///         if in_kunit_test() {
+///             1234
+///         } else {
+///             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 {
+///     unsafe { 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_kunit_tests() {
         let running = true;
         assert_eq!(running, true);
     }
+
+    #[test]
+    fn rust_test_kunit_in_kunit_test() {
+        let in_kunit = in_kunit_test();
+        assert_eq!(in_kunit, true);
+    }
 }
-- 
2.47.0.163.g1226f6d8fa-goog