[PATCH 5/5] rust: error: Add from_kernel_result!() macro

[Asahi Lina <lina@asahilina.net>]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

Add a helper macro to easily return C result codes from a Rust function
that calls functions which return a Result<T>.

Lina: Imported from rust-for-linux/rust, originally developed by Wedson
as part of file_operations.rs. Added the allow() flags since there is no
user in the kernel crate yet and fixed a typo in a comment.

Co-developed-by: Fox Chen <foxhlchen@gmail.com>
Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Asahi Lina <lina@asahilina.net>
---
 rust/kernel/error.rs | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index cf3d089477d2..8a9222595cd1 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -226,3 +226,55 @@ pub(crate) fn from_kernel_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
     }
     Ok(ptr)
 }
+
+// TODO: Remove `dead_code` marker once an in-kernel client is available.
+#[allow(dead_code)]
+pub(crate) fn from_kernel_result_helper<T>(r: Result<T>) -> T
+where
+    T: From<i16>,
+{
+    match r {
+        Ok(v) => v,
+        // NO-OVERFLOW: negative `errno`s are no smaller than `-bindings::MAX_ERRNO`,
+        // `-bindings::MAX_ERRNO` fits in an `i16` as per invariant above,
+        // therefore a negative `errno` always fits in an `i16` and will not overflow.
+        Err(e) => T::from(e.to_kernel_errno() as i16),
+    }
+}
+
+/// Transforms a [`crate::error::Result<T>`] to a kernel C integer result.
+///
+/// This is useful when calling Rust functions that return [`crate::error::Result<T>`]
+/// from inside `extern "C"` functions that need to return an integer
+/// error result.
+///
+/// `T` should be convertible from an `i16` via `From<i16>`.
+///
+/// # Examples
+///
+/// ```ignore
+/// # use kernel::from_kernel_result;
+/// # use kernel::bindings;
+/// unsafe extern "C" fn probe_callback(
+///     pdev: *mut bindings::platform_device,
+/// ) -> core::ffi::c_int {
+///     from_kernel_result! {
+///         let ptr = devm_alloc(pdev)?;
+///         bindings::platform_set_drvdata(pdev, ptr);
+///         Ok(0)
+///     }
+/// }
+/// ```
+// TODO: Remove `unused_macros` marker once an in-kernel client is available.
+#[allow(unused_macros)]
+macro_rules! from_kernel_result {
+    ($($tt:tt)*) => {{
+        $crate::error::from_kernel_result_helper((|| {
+            $($tt)*
+        })())
+    }};
+}
+
+// TODO: Remove `unused_imports` marker once an in-kernel client is available.
+#[allow(unused_imports)]
+pub(crate) use from_kernel_result;

-- 
2.35.1