Re: [PATCH v7 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

[Benno Lossin]

On 12.09.24 00:52, Danilo Krummrich wrote:
> +/// # Invariants
> +///
> +/// One of the following `krealloc`, `vrealloc`, `kvrealloc`.
> +struct ReallocFunc(
> +    unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
> +);
> +
> +impl ReallocFunc {
> +    // INVARIANT: `krealloc` satisfies the type invariants.
> +    const KREALLOC: Self = Self(bindings::krealloc);
> +
> +    /// # Safety
> +    ///
> +    /// This method has the same safety requirements as [`Allocator::realloc`].
> +    ///
> +    /// # Guarantees
> +    ///
> +    /// This method has the same guarantees as `Allocator::realloc`. Additionally
> +    /// - it accepts any pointer to a valid memory allocation allocated by this function.
> +    /// - memory allocated by this function remains valid until it is passed to this function.
> +    unsafe fn call(
> +        &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:
> +        // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc` and thus only requires that
> +        //   `ptr` is NULL or valid.
> +        // - `ptr` is either NULL or valid by the safety requirements of this function.
> +        //
> +        // GUARANTEE:
> +        // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc`.
> +        // - Those functions provide the guarantees 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))
> +    }
> +}

I remember asking you to split this into a different commit. I think you
argued that it would be better to keep it in the same commit when
bisecting. I don't think that applies in this case, are there any other
disadvantages?

---
Cheers,
Benno

> +
> +// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
> +// - memory remains valid until it is explicitly freed,
> +// - passing a pointer to a valid memory allocation is OK,
> +// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
> +unsafe impl Allocator for Kmalloc {
> +    #[inline]
> +    unsafe fn realloc(
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
> +        unsafe { ReallocFunc::KREALLOC.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
> --
> 2.46.0
>