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

Benno Lossin
2024-08-06T16:51:36Z

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
>