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

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Aug 01, 2024 at 10:28:09AM +0200, Alice Ryhl wrote:
> 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`.

Agreed, I will change this one and the above.

> 
> > +        &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
> >
>