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Message-ID: <20230706220131.428276e4.gary@garyguo.net>
Date: Thu, 6 Jul 2023 22:01:31 +0100
From: Gary Guo <gary@...yguo.net>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
Wedson Almeida Filho <wedsonaf@...il.com>,
Alex Gaynor <alex.gaynor@...il.com>,
Boqun Feng <boqun.feng@...il.com>,
"Björn Roy Baron" <bjorn3_gh@...tonmail.com>,
Benno Lossin <benno.lossin@...ton.me>,
linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] rust: delete `ForeignOwnable::borrow_mut`
On Thu, 6 Jul 2023 09:46:15 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:
> We discovered that the current design of `borrow_mut` is problematic.
> This patch removes it until a better solution can be found.
>
> Specifically, the current design gives you access to a `&mut T`, which
> lets you change where the `ForeignOwnable` points (e.g., with
> `core::mem::swap`). No upcoming user of this API intended to make that
> possible, making all of them unsound.
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
Reviewed-by: Gary Guo <gary@...yguo.net>
> ---
> rust/kernel/sync/arc.rs | 3 +--
> rust/kernel/types.rs | 22 ++--------------------
> 2 files changed, 3 insertions(+), 22 deletions(-)
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..172f563976a9 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -243,8 +243,7 @@ unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
> let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
>
> // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
> - // for the lifetime of the returned value. Additionally, the safety requirements of
> - // `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
> + // for the lifetime of the returned value.
> unsafe { ArcBorrow::new(inner) }
> }
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 1e5380b16ed5..d479f8da8f38 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -35,34 +35,16 @@ pub trait ForeignOwnable: Sized {
> ///
> /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> - /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
> - /// for this object must have been dropped.
> unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
>
> - /// Mutably borrows a foreign-owned object.
> - ///
> - /// # Safety
> - ///
> - /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> - /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> - /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> - /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> - unsafe fn borrow_mut(ptr: *const core::ffi::c_void) -> ScopeGuard<Self, fn(Self)> {
> - // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
> - // `into_foreign`.
> - ScopeGuard::new_with_data(unsafe { Self::from_foreign(ptr) }, |d| {
> - d.into_foreign();
> - })
> - }
> -
> /// Converts a foreign-owned object back to a Rust-owned one.
> ///
> /// # Safety
> ///
> /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> - /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> - /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
> + /// this object must have been dropped.
> unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
> }
>
>
> base-commit: d2e3115d717197cb2bc020dd1f06b06538474ac3
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