[PATCH v5 22/26] rust: alloc: implement `Cmalloc` in module allocator_test

Danilo Krummrich
2024-08-12T18:26:07Z

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>
---
I know, having an `old_size` parameter would indeed help implementing `Cmalloc`.

However, I really don't want test infrastructure to influence the design of
kernel internal APIs.

It's on the test infrastructure to find a way to deal with it, even if it's more
complicated.
---
rust/kernel/alloc/allocator_test.rs | 175 ++++++++++++++++++++++++++--
1 file changed, 168 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
index 1b2642c547ec..7a20e64431ea 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -2,20 +2,181 @@

#![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.
+ ///
+ /// 0 16 32 96
+ /// |----------------|----------------|------------------------------------------------|
+ /// empty CmallocData data
+ ///
+ /// 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.
+ 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`.
+ 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`.
+ 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()) };
+ }
+
+ 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()
+ } else {
+ return Self::alloc(layout, flags);
+ };
+
+ if layout.size() == 0 {
+ // SAFETY: `src` has been created by `Self::alloc_store_data`.
+ 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) };
+
+ // 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),
+ )
+ };
+
+ Ok(dst)
}
}
--
2.45.2