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rust: types: add Opaque::cast_from

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Since commit b20fbbc08a ("rust: check type of `$ptr` in
`container_of!`") we have enforced that the field pointer passed to
container_of! must match the declared field. This caused mismatches when
using a pointer to bindings::x for fields of type Opaque<bindings::x>.

This situation encourages the user to simply pass field.cast() to the
container_of! macro, but this is not great because you might
accidentally pass a *mut bindings::y when the field type is
Opaque<bindings::x>, which would be wrong.

To help catch this kind of mistake, add a new Opaque::cast_from that
wraps a raw pointer in Opaque without changing the inner type. Also
update the docs to reflect this as well as some existing users.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250624-opaque-from-raw-v2-1-e4da40bdc59c@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Alice Ryhl 2025-06-24 15:27:55 +00:00 committed by Miguel Ojeda
parent a68a6bef0e
commit 8802e16843
4 changed files with 14 additions and 6 deletions
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4
rust/kernel/drm/device.rs
View File

@ -135,11 +135,9 @@ impl<T: drm::Driver> Device<T> {
///
/// `ptr` must be a valid pointer to a `struct device` embedded in `Self`.
unsafe fn from_drm_device(ptr: *const bindings::drm_device) -> *mut Self {
let ptr: *const Opaque<bindings::drm_device> = ptr.cast();
// SAFETY: By the safety requirements of this function `ptr` is a valid pointer to a
// `struct drm_device` embedded in `Self`.
unsafe { crate::container_of!(ptr, Self, dev) }.cast_mut()
unsafe { crate::container_of!(Opaque::cast_from(ptr), Self, dev) }.cast_mut()
}
/// Not intended to be called externally, except via declare_drm_ioctls!()

4
rust/kernel/drm/gem/mod.rs
View File

@ -125,11 +125,9 @@ impl<T: DriverObject> IntoGEMObject for Object<T> {
}
unsafe fn as_ref<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self {
let self_ptr: *mut Opaque<bindings::drm_gem_object> = self_ptr.cast();
// SAFETY: `obj` is guaranteed to be in an `Object<T>` via the safety contract of this
// function
unsafe { &*crate::container_of!(self_ptr, Object<T>, obj) }
unsafe { &*crate::container_of!(Opaque::cast_from(self_ptr), Object<T>, obj) }
}
}

7
rust/kernel/lib.rs
View File

@ -204,6 +204,13 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
/// Produces a pointer to an object from a pointer to one of its fields.
///
/// If you encounter a type mismatch due to the [`Opaque`] type, then use [`Opaque::raw_get`] or
/// [`Opaque::cast_from`] to resolve the mismatch.
///
/// [`Opaque`]: crate::types::Opaque
/// [`Opaque::raw_get`]: crate::types::Opaque::raw_get
/// [`Opaque::cast_from`]: crate::types::Opaque::cast_from
///
/// # Safety
///
/// The pointer passed to this macro, and the pointer returned by this macro, must both be in

5
rust/kernel/types.rs
View File

@ -413,6 +413,11 @@ impl<T> Opaque<T> {
pub const fn raw_get(this: *const Self) -> *mut T {
UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
}
/// The opposite operation of [`Opaque::raw_get`].
pub const fn cast_from(this: *const T) -> *const Self {
this.cast()
}
}
/// Types that are _always_ reference counted.