diff mbox series

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

Message ID 20240812182355.11641-23-dakr@kernel.org (mailing list archive)
State New
Headers show
Series Generic `Allocator` support for Rust | expand

Commit Message

Danilo Krummrich Aug. 12, 2024, 6:23 p.m. UTC
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(-)

Comments

Heghedus Razvan Aug. 13, 2024, 7:07 a.m. UTC | #1
On Mon Aug 12, 2024 at 9:23 PM EEST, 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>
> ---
> 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),
> +            )
> +        };
> +
At this point should we free the src? Or is the original pointer expected to remain valid?

> +        Ok(dst)
>      }
>  }
> --
> 2.45.2
Danilo Krummrich Aug. 13, 2024, 12:34 p.m. UTC | #2
On Tue, Aug 13, 2024 at 07:07:46AM +0000, Heghedus Razvan wrote:
> On Mon Aug 12, 2024 at 9:23 PM EEST, 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>
> > ---
> > 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),
> > +            )
> > +        };
> > +
> At this point should we free the src? Or is the original pointer expected to remain valid?

Yes, we're indeed missing `Self::free_read_data(src)` here.

> 
> > +        Ok(dst)
> >      }
> >  }
> > --
> > 2.45.2
> 
>
diff mbox series

Patch

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)
     }
 }