Re: [PATCH 06/10] rust: alloc: introduce the `BoxExt` trait

[Benno Lossin]

On 25.03.24 20:54, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
> 
> Make fallible versions of `new` and `new_uninit` methods available in
> `Box` even though it doesn't implement them because we build `alloc`
> with the `no_global_oom_handling` config.
> 
> They also have an extra `flags` parameter that allows callers to pass
> flags to the allocator.
> 
> Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
> ---
>   rust/kernel/alloc.rs           |  1 +
>   rust/kernel/alloc/allocator.rs |  6 +++-
>   rust/kernel/alloc/boxext.rs    | 61 ++++++++++++++++++++++++++++++++++
>   rust/kernel/init.rs            | 13 ++++----
>   rust/kernel/prelude.rs         |  2 +-
>   rust/kernel/sync/arc.rs        |  3 +-
>   6 files changed, 77 insertions(+), 9 deletions(-)
>   create mode 100644 rust/kernel/alloc/boxext.rs
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index ad48ac8dc13d..5712c81b1308 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -5,6 +5,7 @@
>   #[cfg(not(test))]
>   #[cfg(not(testlib))]
>   mod allocator;
> +pub mod boxext;
>   pub mod vecext;
> 
>   /// Flags to be used when allocating memory.
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocatorrs
> index 01ad139e19bc..fc0439455faa 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -15,7 +15,11 @@
>   ///
>   /// - `ptr` can be either null or a pointer which has been allocated by this allocator.
>   /// - `new_layout` must have a non-zero size.
> -unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: bindings::gfp_t) -> *mut u8 {
> +pub(crate) unsafe fn krealloc_aligned(
> +    ptr: *mut u8,
> +    new_layout: Layout,
> +    flags: bindings::gfp_t,
> +) -> *mut u8 {
>       // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
>       let layout = new_layout.pad_to_align();
> 
> diff --git a/rust/kernel/alloc/boxext.rs b/rust/kernel/alloc/boxext.rs
> new file mode 100644
> index 000000000000..26a918df7acf
> --- /dev/null
> +++ b/rust/kernel/alloc/boxext.rs
> @@ -0,0 +1,61 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Extensions to [`Box`] for fallible allocations.
> +
> +use super::Flags;
> +use alloc::boxed::Box;
> +use core::alloc::AllocError;
> +use core::mem::MaybeUninit;
> +use core::result::Result;
> +
> +/// Extensions to [`Box`].
> +pub trait BoxExt<T>: Sized {
> +    /// Allocates a new box.
> +    ///
> +    /// The allocation may fail, in which case an error is returned.
> +    fn new(x: T, flags: Flags) -> Result<Self, AllocError>;
> +
> +    /// Allocates a new uninitialised box.
> +    ///
> +    /// The allocation may fail, in which case an error is returned.
> +    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
> +}
> +
> +impl<T> BoxExt<T> for Box<T> {
> +    #[cfg(any(test, testlib))]
> +    fn new(x: T, _flags: Flags) -> Result<Self, AllocError> {
> +        Ok(Box::new(x))
> +    }

When running under `cfg(test)`, are we using the normal standard
library? Or why is this needed?

> +
> +    #[cfg(not(any(test, testlib)))]
> +    fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
> +        let ptr = if core::mem::size_of::<T>() == 0 {
> +            core::ptr::NonNull::<T>::dangling().as_ptr()
> +        } else {
> +            let layout = core::alloc::Layout::new::<T>();
> +
> +            // SAFETY: Memory is being allocated (first arg is null). The only other source of
> +            // safety issues is sleeping on atomic context, which is addressed by klint.

The `krealloc_aligned` function states:

/// # Safety
///
/// - `ptr` can be either null or a pointer which has been allocated by this allocator.
/// - `new_layout` must have a non-zero size.

So it should also mention that you checked for `layout.size() > 0`
above.

> +            let ptr = unsafe {
> +                super::allocator::krealloc_aligned(core::ptr::null_mut(), layout, flags.0)
> +            };
> +            if ptr.is_null() {
> +                return Err(AllocError);
> +            }
> +
> +            let ptr = ptr.cast::<T>();
> +
> +            // SAFETY: We just allocated the memory above, it is valid for write.
> +            unsafe { ptr.write(x) };
> +            ptr
> +        };
> +
> +        // SAFETY: For non-zero-sized types, we allocate above using the global allocator. For
> +        // zero-sized types, we use `NonNull::dangling`.
> +        Ok(unsafe { Box::from_raw(ptr) })
> +    }
> +
> +    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
> +        <Box<_> as BoxExt<_>>::new(MaybeUninit::<T>::uninit(), flags)

Why do you use the extended syntax? I tried to use `Box::new` and it
compiled.

-- 
Cheers,
Benno