Messages in this thread | | | From | Andreas Hindborg <> | Subject | Re: [PATCH 4/5] rust: error: Add a helper to convert a C ERR_PTR to a `Result` | Date | Mon, 27 Feb 2023 14:41:25 +0100 |
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Asahi Lina <lina@asahilina.net> writes:
> From: Sven Van Asbroeck <thesven73@gmail.com> > > 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. > > 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> > Signed-off-by: Asahi Lina <lina@asahilina.net> > --- > 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 1e8371f28746..cf3d089477d2 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_kernel_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_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> {
For consistency, if `from_kernel_errno()` should be `from_errno()` this should be `from_err_ptr()` as well?
BR Andreas
> + // 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) > +}
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