rust: error: Add a helper to convert a C ERR_PTR 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>`.

Lina: Imported from rust-for-linux/linux, with subsequent refactoring
and contributions squashed in and attributed below. Renamed the function
to from_err_ptr().

Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Co-developed-by: Fox Chen <foxhlchen@gmail.com>
Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Asahi Lina <lina@asahilina.net>
Link: https://lore.kernel.org/r/20230224-rust-error-v3-5-03779bddc02b@asahilina.net
[ Add a removal of `#[allow(dead_code)]`. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Sven Van Asbroeck 2023-04-03 18:48:14 +09:00 committed by Miguel Ojeda
parent 086fbfa3b3
commit 752417b3f0
2 changed files with 61 additions and 1 deletions

View File

@ -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

View File

@ -96,7 +96,6 @@ impl Error {
/// # Safety
///
/// `errno` must be within error code range (i.e. `>= -MAX_ERRNO && < 0`).
#[allow(dead_code)]
unsafe fn from_errno_unchecked(errno: core::ffi::c_int) -> Error {
// INVARIANT: The contract ensures the type invariant
// will hold.
@ -189,3 +188,52 @@ 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`.
#[allow(clippy::unnecessary_cast)]
return Err(unsafe { Error::from_errno_unchecked(err as core::ffi::c_int) });
}
Ok(ptr)
}