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From:   Asahi Lina <lina@asahilina.net>
Date:   Wed, 29 Mar 2023 21:04:38 +0900
Subject: [PATCH v2 6/6] rust: error: Add from_result() helper
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Message-Id: <20230224-rust-error-v2-6-3900319812da@asahilina.net>
References: <20230224-rust-error-v2-0-3900319812da@asahilina.net>
In-Reply-To: <20230224-rust-error-v2-0-3900319812da@asahilina.net>
To:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Sven Van Asbroeck <thesven73@gmail.com>
Cc:     Fox Chen <foxhlchen@gmail.com>,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        asahi@lists.linux.dev, Asahi Lina <lina@asahilina.net>
Precedence: bulk

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

Add a helper function 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. Replaced the
macro with a function taking a closure, per discussion on the ML.

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 | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 6b10129075a7..4d8b19d5967d 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -226,3 +226,42 @@ pub(crate) fn from_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
     }
     Ok(ptr)
 }
+
+/// Calls a closure returning a [`crate::error::Result<T>`] and converts the result to
+/// a 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_result;
+/// # use kernel::bindings;
+/// unsafe extern "C" fn probe_callback(
+///     pdev: *mut bindings::platform_device,
+/// ) -> core::ffi::c_int {
+///     from_result(|| {
+///         let ptr = devm_alloc(pdev)?;
+///         bindings::platform_set_drvdata(pdev, ptr);
+///         Ok(0)
+///     })
+/// }
+/// ```
+// TODO: Remove `dead_code` marker once an in-kernel client is available.
+#[allow(dead_code)]
+pub(crate) fn from_result<T, F>(f: F) -> T
+where
+    T: From<i16>,
+    F: FnOnce() -> Result<T>,
+{
+    match f() {
+        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_errno() as i16),
+    }
+}

-- 
2.40.0