Re: [PATCH v5 1/4] rust: sync: change `<Arc<T> as ForeignOwnable>::PointedTo` to `T`

[Andreas Hindborg <a.hindborg@kernel.org>]

"Tamir Duberstein" <tamird@gmail.com> writes:

> On Thu, Feb 27, 2025 at 7:37 AM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> Using `ArcInner` as `PoinedTo` in the `ForeignOwnable` implementation for
>> `Arc` is a bit unfortunate. Using `T` as `PointedTo` does not remove any
>> functionality, but allows `ArcInner` to be private. Further, it allows
>> downstream users to write code that is generic over `Box` and `Arc`, when
>> downstream users need access to `T` after calling `into_foreign`.
>>
>> Reviewed-by: Fiona Behrens <me@kloenk.dev>
>> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
>> Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> ---
>>
>> This patch is a dependency for Rust `configfs` abstractions. It allows both
>> `Box` and `Arc` to be used as pointer types in the `configfs` hierarchy.
>> ---
>>  rust/kernel/sync/arc.rs | 21 ++++++++++++++++-----
>>  1 file changed, 16 insertions(+), 5 deletions(-)
>>
>> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
>> index dfe4abf82c25..3d77a31e116f 100644
>> --- a/rust/kernel/sync/arc.rs
>> +++ b/rust/kernel/sync/arc.rs
>> @@ -143,7 +143,7 @@ pub struct Arc<T: ?Sized> {
>>  #[doc(hidden)]
>>  #[pin_data]
>>  #[repr(C)]
>> -pub struct ArcInner<T: ?Sized> {
>> +struct ArcInner<T: ?Sized> {
>>      refcount: Opaque<bindings::refcount_t>,
>>      data: T,
>>  }
>> @@ -345,18 +345,25 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
>>
>>  // SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
>>  unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
>> -    type PointedTo = ArcInner<T>;
>> +    type PointedTo = T;
>>      type Borrowed<'a> = ArcBorrow<'a, T>;
>>      type BorrowedMut<'a> = Self::Borrowed<'a>;
>>
>>      fn into_foreign(self) -> *mut Self::PointedTo {
>> -        ManuallyDrop::new(self).ptr.as_ptr()
>> +        let this = ManuallyDrop::new(self).ptr.as_ptr();
>> +        // SAFETY: `x` is a valid pointer to `Self` so the projection below is
>> +        // in bounds of the allocation.
>
> Isn't the unsafe bit `*this`, which is what this comment should
> justify?

Yes, not sure what the origin of that name is. Good catch.

> In Rust 1.82+ `addr_of_mut!` isn't unsafe I believe. Also `x`
> is likely meant to be `this`.

The field projection (*this).data is still unsafe, as far as I know.

Actually the macro is not required any longer, the underlying raw
reference syntax has been stabilized. But since our minimum rust version
is 1.78, we have to keep the macro.

>
>> +        unsafe { core::ptr::addr_of_mut!((*this).data) }
>>      }
>>
>>      unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
>> +        // SAFETY: We did the reverse offset calculation in `into_foreign`, so
>> +        // the offset calculation below is in bounds of the allocation.
>> +        let inner_ptr = unsafe { kernel::container_of!(ptr, ArcInner<T>, data).cast_mut() };
>> +
>>          // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
>>          // call to `Self::into_foreign`.
>> -        let inner = unsafe { NonNull::new_unchecked(ptr) };
>> +        let inner = unsafe { NonNull::new_unchecked(inner_ptr) };
>>
>>          // SAFETY: By the safety requirement of this function, we know that `ptr` came from
>>          // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
>> @@ -365,9 +372,13 @@ unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
>>      }
>>
>>      unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
>> +        // SAFETY: We did the reverse offset calculation in `into_foreign`, so
>> +        // the offset calculation below is in bounds of the allocation.
>> +        let inner_ptr = unsafe { kernel::container_of!(ptr, ArcInner<T>, data).cast_mut() };
>> +
>>          // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
>>          // call to `Self::into_foreign`.
>> -        let inner = unsafe { NonNull::new_unchecked(ptr) };
>> +        let inner = unsafe { NonNull::new_unchecked(inner_ptr) };
>>
>>          // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
>>          // for the lifetime of the returned value.
>
> It might be good to extract a shared helper from `from_foreign` and
> `borrow`, though the duplication isn't new in this patch. Either way:

I would encourage a follow-up patch :)

>
> Reviewed-by: Tamir Duberstein <tamird@gmail.com>

Thanks!


Best regards,
Andreas Hindborg