Commit 701608bd authored by Benno Lossin's avatar Benno Lossin Committed by Miguel Ojeda
Browse files

rust: sync: reduce stack usage of `UniqueArc::try_new_uninit` 



`UniqueArc::try_new_uninit` calls `Arc::try_new(MaybeUninit::uninit())`.
This results in the uninitialized memory being placed on the stack,
which may be arbitrarily large due to the generic `T` and thus could
cause a stack overflow for large types.

Change the implementation to use the pin-init API which enables in-place
initialization. In particular it avoids having to first construct and
then move the uninitialized memory from the stack into the final location.

Signed-off-by: default avatarBenno Lossin <benno.lossin@proton.me>
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Reviewed-by: default avatarGary Guo <gary@garyguo.net>
Reviewed-by: default avatarAndreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-15-y86-dev@protonmail.com


Signed-off-by: default avatarMiguel Ojeda <ojeda@kernel.org>
parent 692e8935

rust/kernel/lib.rs

+0 −1
Original line number Diff line number Diff line
@@ -28,7 +28,6 @@ 
#[cfg(not(CONFIG_RUST))]

compile_error!("Missing kernel configuration for conditional compilation");



#[allow(unused_extern_crates)]

// Allow proc-macros to refer to `::kernel` inside the `kernel` crate (this crate).

extern crate self as kernel;

rust/kernel/sync/arc.rs

+13 −3
Original line number Diff line number Diff line
@@ -18,7 +18,8 @@ 
use crate::{

    bindings,

    error::{self, Error},

    init::{InPlaceInit, Init, PinInit},

    init::{self, InPlaceInit, Init, PinInit},

    try_init,

    types::{ForeignOwnable, Opaque},

};

use alloc::boxed::Box;
@@ -31,6 +32,7 @@ 
    pin::Pin,

    ptr::NonNull,

};

use macros::pin_data;



mod std_vendor;
@@ -125,6 +127,7 @@ pub struct Arc<T: ?Sized> { 
    _p: PhantomData<ArcInner<T>>,

}



#[pin_data]

#[repr(C)]

struct ArcInner<T: ?Sized> {

    refcount: Opaque<bindings::refcount_t>,
@@ -505,9 +508,16 @@ pub fn try_new(value: T) -> Result<Self, AllocError> { 


    /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.

    pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {

        Ok(UniqueArc::<MaybeUninit<T>> {

        // INVARIANT: The refcount is initialised to a non-zero value.

        let inner = Box::try_init::<AllocError>(try_init!(ArcInner {

            // SAFETY: There are no safety requirements for this FFI call.

            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),

            data <- init::uninit::<T, AllocError>(),

        }? AllocError))?;

        Ok(UniqueArc {

            // INVARIANT: The newly-created object has a ref-count of 1.

            inner: Arc::try_new(MaybeUninit::uninit())?,

            // SAFETY: The pointer from the `Box` is valid.

            inner: unsafe { Arc::from_inner(Box::leak(inner).into()) },

        })

    }

}