Re: [PATCH v4 04/28] rust: alloc: implement `Allocator` for `Kmalloc`

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

On 05.08.24 17:19, Danilo Krummrich wrote:
> Implement `Allocator` for `Kmalloc`, the kernel's default allocator,
> typically used for objects smaller than page size.
> 
> All memory allocations made with `Kmalloc` end up in `krealloc()`.
> 
> It serves as allocator for the subsequently introduced types `KBox` and
> `KVec`.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/helpers.c                 |  3 +-
>  rust/kernel/alloc.rs           |  2 +-
>  rust/kernel/alloc/allocator.rs | 68 +++++++++++++++++++++++++++++++++-
>  3 files changed, 69 insertions(+), 4 deletions(-)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 92d3c03ae1bd..9f7275493365 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -193,8 +193,7 @@ void rust_helper_init_work_with_key(struct work_struct *work, work_func_t func,
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_init_work_with_key);
> 
> -void * __must_check __realloc_size(2)
> -rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
> +void *rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
>  {
>  	return krealloc(objp, new_size, flags);
>  }
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index 8a71a589469d..bc01a17df5e0 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -4,7 +4,7 @@
> 
>  #[cfg(not(test))]
>  #[cfg(not(testlib))]
> -mod allocator;
> +pub mod allocator;
>  pub mod box_ext;
>  pub mod vec_ext;
> 
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index 2c1eae25da84..c6ad1dd59dd0 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -5,8 +5,16 @@
>  use super::{flags::*, Flags};
>  use core::alloc::{GlobalAlloc, Layout};
>  use core::ptr;
> +use core::ptr::NonNull;
> 
> -struct Kmalloc;
> +use crate::alloc::{AllocError, Allocator};
> +use crate::bindings;
> +
> +/// The contiguous kernel allocator.
> +///
> +/// The contiguous kernel allocator only ever allocates physically contiguous memory through
> +/// `bindings::krealloc`.
> +pub struct Kmalloc;
> 
>  /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
>  fn aligned_size(new_layout: Layout) -> usize {
> @@ -40,6 +48,64 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
>      }
>  }
> 
> +/// # 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.

This INVARIANT comment should be moved one line downwards.

> +    fn krealloc() -> Self {
> +        Self(bindings::krealloc)
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// This method has the exact same safety requirements as `Allocator::realloc`.

I would remove "exact", I don't think we want to mean "almost the same"
when we write just "same".

> +    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: `ptr` is valid by the safety requirements of this function.

"`ptr` is either NULL or 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()

If we call `realloc(Some(ptr), <layout with size = 0>, ...)`, then this
leaks the pointer returned by the call to `self.0` above. I don't know
what the return value of the different functions are that can appear in
`self.0`, do they return NULL?

What about the following sequence:

    let ptr = realloc(None, <layout with size = 0>, ...);
    let ptr = realloc(Some(ptr), <layout with size = 0>, ...);

Then the above call to `self.0` is done with a dangling pointer, can the
functions that appear in `self.0` handle that?

> +        } 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`.

What about the other requirements? (they should be satisfied, since they
are also requirements for calling this function)

> +        unsafe { realloc.call(ptr, layout, flags) }

If you make `ReallocFunc::krealloc()` into a constant
`ReallocFunc::KREALLOC`, then we could avoid the let binding above.

---
Cheers,
Benno

> +    }
> +}
> +
>  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
>