[PATCH v1] rust: error: integrate Rust error type with `errname`

[Alice Ryhl]

From: Gary Guo <gary@xxxxxxxxxxx>

This integrates the `Error` type with the `errname` by making it
accessible via the `name` method or via the `Debug` trait.

Co-Developed-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
Co-Developed-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
Signed-off-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/kernel/error.rs            | 39 +++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 50e7a76d5455..dc8691f83253 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -10,6 +10,7 @@
 #include <linux/refcount.h>
 #include <linux/wait.h>
 #include <linux/sched.h>
+#include <linux/errname.h>
 
 /* `bindgen` gets confused at certain things. */
 const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;
diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 5f4114b30b94..59ff362028d0 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -4,12 +4,15 @@
 //!
 //! C header: [`include/uapi/asm-generic/errno-base.h`](../../../include/uapi/asm-generic/errno-base.h)
 
+use crate::str::CStr;
+
 use alloc::{
     alloc::{AllocError, LayoutError},
     collections::TryReserveError,
 };
 
 use core::convert::From;
+use core::fmt;
 use core::num::TryFromIntError;
 use core::str::Utf8Error;
 
@@ -113,6 +116,42 @@ impl Error {
         // SAFETY: self.0 is a valid error due to its invariant.
         unsafe { bindings::ERR_PTR(self.0.into()) as *mut _ }
     }
+
+    /// Returns a string representing the error, if one exists.
+    #[cfg(not(testlib))]
+    pub fn name(&self) -> Option<&'static CStr> {
+        // SAFETY: Just an FFI call, there are no extra safety requirements.
+        let ptr = unsafe { bindings::errname(-self.0) };
+        if ptr.is_null() {
+            None
+        } else {
+            // SAFETY: The string returned by `errname` is static and `NUL`-terminated.
+            Some(unsafe { CStr::from_char_ptr(ptr) })
+        }
+    }
+
+    /// Returns a string representing the error, if one exists.
+    ///
+    /// When `testlib` is configured, this always returns `None` to avoid the dependency on a
+    /// kernel function so that tests that use this (e.g., by calling [`Result::unwrap`]) can still
+    /// run in userspace.
+    #[cfg(testlib)]
+    pub fn name(&self) -> Option<&'static CStr> {
+        None
+    }
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.name() {
+            // Print out number if no name can be found.
+            None => f.debug_tuple("Error").field(&-self.0).finish(),
+            // SAFETY: These strings are ASCII-only.
+            Some(name) => f
+                .debug_tuple(unsafe { core::str::from_utf8_unchecked(name) })
+                .finish(),
+        }
+    }
 }
 
 impl From<AllocError> for Error {

base-commit: ac9a78681b921877518763ba0e89202254349d1b
-- 
2.41.0.rc0.172.g3f132b7071-goog