[PATCH v2 5/6] rust: error: Add a helper to convert a C ERR_PTR to a `Result`

From: Asahi Lina
Date: Wed Mar 29 2023 - 08:05:21 EST


From: Sven Van Asbroeck <thesven73@xxxxxxxxx>

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>`.

Lina: Imported from rust-for-linux/linux, with subsequent refactoring
and contributions squashed in and attributed below. Replaced usage of
from_kernel_errno_unchecked() with an open-coded constructor, since this
is the only user anyway. Renamed the function to from_err_ptr().

Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Co-developed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Co-developed-by: Fox Chen <foxhlchen@xxxxxxxxx>
Signed-off-by: Fox Chen <foxhlchen@xxxxxxxxx>
Co-developed-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
rust/helpers.c | 12 ++++++++++++
rust/kernel/error.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 61 insertions(+)

diff --git a/rust/helpers.c b/rust/helpers.c
index 89f4cd1e0df3..04b9be46e887 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -53,6 +53,18 @@ __force void *rust_helper_ERR_PTR(long err)
}
EXPORT_SYMBOL_GPL(rust_helper_ERR_PTR);

+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);
+
/*
* We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
* as the Rust `usize` type, so we can use it in contexts where Rust
diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 4f599c4d1752..6b10129075a7 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -177,3 +177,52 @@ pub fn to_result(err: core::ffi::c_int) -> Result {
Ok(())
}
}
+
+/// 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
+///
+/// ```ignore
+/// # use kernel::from_err_ptr;
+/// # use kernel::bindings;
+/// fn devm_platform_ioremap_resource(
+/// pdev: &mut PlatformDevice,
+/// index: u32,
+/// ) -> Result<*mut core::ffi::c_void> {
+/// // SAFETY: FFI call.
+/// unsafe {
+/// from_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_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
+ // CAST: Casting a pointer to `*const core::ffi::c_void` is always valid.
+ let const_ptr: *const core::ffi::c_void = ptr.cast();
+ // SAFETY: The FFI function does not deref the pointer.
+ if unsafe { bindings::IS_ERR(const_ptr) } {
+ // SAFETY: The FFI function does not deref the pointer.
+ let err = unsafe { bindings::PTR_ERR(const_ptr) };
+ // CAST: If `IS_ERR()` returns `true`,
+ // then `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.
+ //
+ // SAFETY: `IS_ERR()` ensures `err` is a
+ // negative value greater-or-equal to `-bindings::MAX_ERRNO`.
+ #[cfg_attr(CONFIG_ARM, allow(clippy::unnecessary_cast))]
+ return Err(Error(err as i32));
+ }
+ Ok(ptr)
+}

--
2.40.0