Re: [PATCH v3 04/25] rust: alloc: implement `Allocator` for `Kmalloc`

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Aug 1, 2024 at 2:07 AM Danilo Krummrich <dakr@kernel.org> wrote:
> +/// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
>  fn aligned_size(new_layout: Layout) -> usize {

This comment could potentially be moved to the previous patch that
defined the function.

> +struct ReallocFunc(
> +    // INVARIANT: One of the following `krealloc`, `vrealloc`, `kvrealloc`.
> +    unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
> +);

In this case, the comment would usually be formatted with markdown.

/// # Invariants
///
/// Must contain one of the following: `krealloc`, `vrealloc`, `kvrealloc`.

The // INVARIANT: syntax is used when constructing an instance to
argue why the documentented invariants are satisfied.

> +impl ReallocFunc {
> +    fn krealloc() -> Self {
> +        Self(bindings::krealloc)
> +    }

Technically this should have an // INVARIANT: explaining why the
invariants are satisfied by this new value.

> +
> +    // SAFETY: `call` has the exact same safety requirements as `Allocator::realloc`.
> +    unsafe fn call(

Similarly to the above, the // SAFETY: syntax is used when arguing why
the preconditions are satisfied, but when explaining what the
preconditions are, we usually use this syntax instead:

/// # Safety
///
/// This method has the same safety requirements as `Allocator::realloc`.

> +        &self,
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        let size = aligned_size(layout);
> +        let ptr = match ptr {
> +            Some(ptr) => ptr.as_ptr(),
> +            None => ptr::null(),
> +        };
> +
> +        // SAFETY: `ptr` is valid by the safety requirements of this function.
> +        let raw_ptr = unsafe {
> +            // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
> +            self.0(ptr.cast(), size, flags.0).cast()
> +        };
> +
> +        let ptr = if size == 0 {
> +            NonNull::dangling()
> +        } else {
> +            NonNull::new(raw_ptr).ok_or(AllocError)?
> +        };
> +
> +        Ok(NonNull::slice_from_raw_parts(ptr, size))
> +    }
> +}
> +
> +unsafe impl Allocator for Kmalloc {
> +    unsafe fn realloc(
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        let realloc = ReallocFunc::krealloc();
> +
> +        // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
> +        // allocated with this `Allocator`.
> +        unsafe { realloc.call(ptr, layout, flags) }
> +    }
> +}
> +
>  unsafe impl GlobalAlloc for Kmalloc {
>      unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
>          // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
> --
> 2.45.2
>