Re: [PATCH v6 22/26] rust: alloc: implement `Cmalloc` in module allocator_test

[Benno Lossin <benno.lossin@proton.me>]

On 16.08.24 02:11, Danilo Krummrich wrote:
> So far the kernel's `Box` and `Vec` types can't be used by userspace
> test cases, since all users of those types (e.g. `CString`) use kernel
> allocators for instantiation.
> 
> In order to allow userspace test cases to make use of such types as
> well, implement the `Cmalloc` allocator within the allocator_test module
> and type alias all kernel allocators to `Cmalloc`. The `Cmalloc`
> allocator uses libc's realloc() function as allocator backend.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/alloc/allocator_test.rs | 178 ++++++++++++++++++++++++++--
>  1 file changed, 171 insertions(+), 7 deletions(-)
> 
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index 1b2642c547ec..7fff308d02dc 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -2,20 +2,184 @@
> 

Could add a short paragraph as the module description why this module
exists? Would probably be enough to paste the commit message.

>  #![allow(missing_docs)]
> 
> -use super::{AllocError, Allocator, Flags};
> +use super::{flags::*, AllocError, Allocator, Flags};
>  use core::alloc::Layout;
> +use core::cmp;
> +use core::mem;
> +use core::ptr;
>  use core::ptr::NonNull;
> 
> -pub struct Kmalloc;
> +pub struct Cmalloc;
> +pub type Kmalloc = Cmalloc;
>  pub type Vmalloc = Kmalloc;
>  pub type KVmalloc = Kmalloc;
> 
> -unsafe impl Allocator for Kmalloc {
> +extern "C" {
> +    #[link_name = "aligned_alloc"]
> +    fn libc_aligned_alloc(align: usize, size: usize) -> *mut core::ffi::c_void;
> +
> +    #[link_name = "free"]
> +    fn libc_free(ptr: *mut core::ffi::c_void);
> +}
> +
> +struct CmallocData {
> +    // The actual size as requested through `Cmalloc::alloc` or `Cmalloc::realloc`.
> +    size: usize,
> +    // The offset from the pointer returned to the caller of `Cmalloc::alloc` or `Cmalloc::realloc`
> +    // to the actual base address of the allocation.
> +    offset: usize,
> +}
> +
> +impl Cmalloc {
> +    /// Adjust the size and alignment such that we can additionally store `CmallocData` right
> +    /// before the actual data described by `layout`.
> +    ///
> +    /// Example:
> +    ///
> +    /// For `CmallocData` assume an alignment of 8 and a size of 16.
> +    /// For `layout` assume and alignment of 16 and a size of 64.

This looks like you want it rendered as bulletpoints (but it won't).

> +    ///
> +    /// 0                16               32                                               96
> +    /// |----------------|----------------|------------------------------------------------|
> +    ///        empty         CmallocData                         data

Can you put this inside of '```'? Then it will render nicely in markdown
(don't forget to specify the type 'text')

> +    ///
> +    /// For this example the returned `Layout` has an alignment of 32 and a size of 96.
> +    fn layout_adjust(layout: Layout) -> Result<Layout, AllocError> {
> +        let layout = layout.pad_to_align();
> +
> +        // Ensure that `CmallocData` fits into half the alignment. Additionally, this guarantees
> +        // that advancing a pointer aligned to `align` by `align / 2` we still satisfy or exceed
> +        // the alignment requested through `layout`.
> +        let align = cmp::max(
> +            layout.align(),
> +            mem::size_of::<CmallocData>().next_power_of_two(),
> +        ) * 2;
> +
> +        // Add the additional space required for `CmallocData`.
> +        let size = layout.size() + mem::size_of::<CmallocData>();
> +
> +        Ok(Layout::from_size_align(size, align)
> +            .map_err(|_| AllocError)?
> +            .pad_to_align())
> +    }
> +
> +    fn alloc_store_data(layout: Layout) -> Result<NonNull<u8>, AllocError> {
> +        let requested_size = layout.size();
> +
> +        let layout = Self::layout_adjust(layout)?;
> +        let min_align = layout.align() / 2;
> +
> +        // SAFETY: Returns either NULL or a pointer to a memory allocation that satisfies or
> +        // exceeds the given size and alignment requirements.
> +        let raw_ptr = unsafe { libc_aligned_alloc(layout.align(), layout.size()) } as *mut u8;
> +
> +        let priv_ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
> +
> +        // SAFETY: Advance the pointer by `min_align`. The adjustments from `Self::layout_adjust`
> +        // ensure that after this operation the original size and alignment requirements are still
> +        // satisfied or exceeded.

This SAFETY comment should address why it's OK to call `add`. You
justify something different, namely why the allocation still satisfies
the requirements of `layout`. That is something that this function
should probably guarantee.

> +        let ptr = unsafe { priv_ptr.as_ptr().add(min_align) };
> +
> +        // SAFETY: `min_align` is greater than or equal to the size of `CmallocData`, hence we
> +        // don't exceed the allocation boundaries.
> +        let data_ptr: *mut CmallocData = unsafe { ptr.sub(mem::size_of::<CmallocData>()) }.cast();
> +
> +        let data = CmallocData {
> +            size: requested_size,
> +            offset: min_align,
> +        };
> +
> +        // SAFETY: `data_ptr` is properly aligned and within the allocation boundaries reserved for
> +        // `CmallocData`.
> +        unsafe { data_ptr.write(data) };
> +
> +        NonNull::new(ptr).ok_or(AllocError)
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `ptr` must have been previously allocated with `Self::alloc_store_data`.

You additionally need that you have shared access to the pointee.

> +    unsafe fn data<'a>(ptr: NonNull<u8>) -> &'a CmallocData {
> +        // SAFETY: `Self::alloc_store_data` stores the `CmallocData` right before the address
> +        // returned to callers of `Self::alloc_store_data`.
> +        let data_ptr: *mut CmallocData =
> +            unsafe { ptr.as_ptr().sub(mem::size_of::<CmallocData>()) }.cast();
> +
> +        // SAFETY: The `CmallocData` has been previously stored at this offset with
> +        // `Self::alloc_store_data`.
> +        unsafe { &*data_ptr }
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// This function must not be called more than once for the same allocation.
> +    ///
> +    /// `ptr` must have been previously allocated with `Self::alloc_store_data`.

You additionally need that you have exclusive access to the pointee.

> +    unsafe fn free_read_data(ptr: NonNull<u8>) {
> +        // SAFETY: `ptr` has been created by `Self::alloc_store_data`.
> +        let data = unsafe { Self::data(ptr) };
> +
> +        // SAFETY: `ptr` has been created by `Self::alloc_store_data`.
> +        let priv_ptr = unsafe { ptr.as_ptr().sub(data.offset) };
> +
> +        // SAFETY: `priv_ptr` has previously been allocatored with this `Allocator`.
> +        unsafe { libc_free(priv_ptr.cast()) };
> +    }
> +}
> +
> +unsafe impl Allocator for Cmalloc {
> +    fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
> +        if layout.size() == 0 {
> +            return Ok(NonNull::slice_from_raw_parts(NonNull::dangling(), 0));
> +        }
> +
> +        let ptr = Self::alloc_store_data(layout)?;
> +
> +        if flags.contains(__GFP_ZERO) {
> +            // SAFETY: `Self::alloc_store_data` guarantees that `ptr` points to memory of at least
> +            // `layout.size()` bytes.
> +            unsafe { ptr.as_ptr().write_bytes(0, layout.size()) };
> +        }

This makes me wonder, what other flags should we handle for this
allocator?

> +
> +        Ok(NonNull::slice_from_raw_parts(ptr, layout.size()))
> +    }
> +
>      unsafe fn realloc(
> -        _ptr: Option<NonNull<u8>>,
> -        _layout: Layout,
> -        _flags: Flags,
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        flags: Flags,
>      ) -> Result<NonNull<[u8]>, AllocError> {
> -        panic!();
> +        let src: NonNull<u8> = if let Some(src) = ptr {
> +            src.cast()

Why the cast?

> +        } else {
> +            return Self::alloc(layout, flags);
> +        };

You should be able to write this instead:

    let Some(src) = ptr else {
        return Self::alloc(layout, flags);
    };

> +
> +        if layout.size() == 0 {
> +            // SAFETY: `src` has been created by `Self::alloc_store_data`.

This is not true, consider:

    let ptr = alloc(size = 0);
    free(ptr)

Alloc will return a dangling pointer due to the first if statement and
then this function will pass it to `free_read_data`, even though it
wasn't created by `alloc_store_data`.
This isn't forbidden by the `Allocator` trait function's safety
requirements.

> +            unsafe { Self::free_read_data(src) };
> +
> +            return Ok(NonNull::slice_from_raw_parts(NonNull::dangling(), 0));
> +        }
> +
> +        let dst = Self::alloc(layout, flags)?;
> +
> +        // SAFETY: `src` has been created by `Self::alloc_store_data`.
> +        let data = unsafe { Self::data(src) };

Same issue here, if the allocation passed in is zero size. I think you
have no other choice than to allocate even for zero size requests...
Otherwise how would you know that they are zero-sized.

---
Cheers,
Benno

> +
> +        // SAFETY: `src` has previously been allocated with this `Allocator`; `dst` has just been
> +        // newly allocated. Copy up to the smaller of both sizes.
> +        unsafe {
> +            ptr::copy_nonoverlapping(
> +                src.as_ptr(),
> +                dst.as_ptr().cast(),
> +                cmp::min(layout.size(), data.size),
> +            )
> +        };
> +
> +        // SAFETY: `src` has been created by `Self::alloc_store_data`.
> +        unsafe { Self::free_read_data(src) };
> +
> +        Ok(dst)
>      }
>  }
> --
> 2.46.0
>