[PATCH v3 3/6] rust: error: Add Error::from_errno{_unchecked}()

[Asahi Lina]

From: Miguel Ojeda <ojeda@xxxxxxxxxx>

Add a function to create `Error` values out of a kernel error return,
which safely upholds the invariant that the error code is well-formed
(negative and greater than -MAX_ERRNO). If a malformed code is passed
in, it will be converted to EINVAL.

Lina: Imported from rust-for-linux/rust as authored by Miguel and Fox
with refactoring from Wedson, renamed from_kernel_errno() to
from_errno().

Co-developed-by: Fox Chen <foxhlchen@xxxxxxxxx>
Signed-off-by: Fox Chen <foxhlchen@xxxxxxxxx>
Co-developed-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
 rust/kernel/error.rs | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 154d0ca6e2dc..1af0d75d3a73 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -72,6 +72,36 @@ pub mod code {
 pub struct Error(core::ffi::c_int);
 
 impl Error {
+    /// Creates an [`Error`] from a kernel error code.
+    ///
+    /// It is a bug to pass an out-of-range `errno`. `EINVAL` would
+    /// be returned in such a case.
+    pub(crate) fn from_errno(errno: core::ffi::c_int) -> Error {
+        if errno < -(bindings::MAX_ERRNO as i32) || errno >= 0 {
+            // TODO: Make it a `WARN_ONCE` once available.
+            crate::pr_warn!(
+                "attempted to create `Error` with out of range `errno`: {}",
+                errno
+            );
+            return code::EINVAL;
+        }
+
+        // INVARIANT: The check above ensures the type invariant
+        // will hold.
+        Error(errno)
+    }
+
+    /// Creates an [`Error`] from a kernel error code.
+    ///
+    /// # Safety
+    ///
+    /// `errno` must be within error code range (i.e. `>= -MAX_ERRNO && < 0`).
+    unsafe fn from_errno_unchecked(errno: core::ffi::c_int) -> Error {
+        // INVARIANT: The contract ensures the type invariant
+        // will hold.
+        Error(errno)
+    }
+
     /// Returns the kernel error code.
     pub fn to_errno(self) -> core::ffi::c_int {
         self.0

-- 
2.40.0