rust/error: add helper function to convert a C error pointer to a Result

Some kernel C API functions return a pointer which embeds an optional
`errno`. Callers are supposed to check the returned pointer with
`IS_ERR()` and if this returns `true`, retrieve the `errno` using
`PTR_ERR()`.

Create a Rust helper function to implement the Rust equivalent:
transform a `*mut T` to `Result<*mut T>`.

Co-Created-By: Boqun Feng <boqun.feng@gmail.com>
Co-Created-By: Miguel Ojeda <ojeda@users.noreply.github.com>
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>

Author: Sven Van Asbroeck

rust/helpers.c

@@ -105,6 +105,18 @@ size_t rust_helper_copy_to_iter(const void *addr, size_t bytes, struct iov_iter
 }
 EXPORT_SYMBOL_GPL(rust_helper_copy_to_iter);
 
+bool rust_helper_is_err(__force const void *ptr)
+{
+	return IS_ERR(ptr);
+}
+EXPORT_SYMBOL_GPL(rust_helper_is_err);
+
+long rust_helper_ptr_err(__force const void *ptr)
+{
+	return PTR_ERR(ptr);
+}
+EXPORT_SYMBOL_GPL(rust_helper_ptr_err);
+
 #if !defined(CONFIG_ARM)
 // See https://github.com/rust-lang/rust-bindgen/issues/1671
 static_assert(__builtin_types_compatible_p(size_t, uintptr_t),

rust/kernel/error.rs

@@ -152,3 +152,54 @@ macro_rules! from_kernel_result {
         })())
     }};
 }
+
+/// Transform a kernel "error pointer" to a normal pointer.
+///
+/// Some kernel C API functions return an "error pointer" which optionally
+/// embeds an `errno`. Callers are supposed to check the returned pointer
+/// for errors. This function performs the check and converts the "error pointer"
+/// to a normal pointer in an idiomatic fashion.
+///
+/// # Examples
+///
+/// ```rust,no_run
+/// fn devm_platform_ioremap_resource(
+///     pdev: &mut PlatformDevice,
+///     index: u32,
+/// ) -> Result<*mut c_types::c_void> {
+///     // SAFETY: FFI call.
+///     unsafe {
+///         from_kernel_err_ptr(bindings::devm_platform_ioremap_resource(
+///             pdev.to_ptr(),
+///             index,
+///         ))
+///     }
+/// }
+/// ```
+// TODO: remove `dead_code` marker once an in-kernel client is available.
+#[allow(dead_code)]
+pub(crate) fn from_kernel_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
+    extern "C" {
+        #[allow(improper_ctypes)]
+        fn rust_helper_is_err(ptr: *const c_types::c_void) -> bool;
+
+        #[allow(improper_ctypes)]
+        fn rust_helper_ptr_err(ptr: *const c_types::c_void) -> c_types::c_long;
+    }
+
+    // CAST: casting a pointer to `*const c_types::c_void` is always valid.
+    let const_ptr: *const c_types::c_void = ptr.cast();
+    // SAFETY: the FFI function does not deref the pointer.
+    if unsafe { rust_helper_is_err(const_ptr) } {
+        // SAFETY: the FFI function does not deref the pointer.
+        let err = unsafe { rust_helper_ptr_err(const_ptr) };
+        // CAST: if `rust_helper_is_err()` returns `true`,
+        // then `rust_helper_ptr_err()` is guaranteed to return a
+        // negative value greater-or-equal to `-bindings::MAX_ERRNO`,
+        // which always fits in an `i16`, as per the invariant above.
+        // And an `i16` always fits in an `i32`. So casting `err` to
+        // an `i32` can never overflow, and is always valid.
+        return Err(Error::from_kernel_errno(err as i32));
+    }
+    Ok(ptr)
+}