[PATCH v2] rust: alloc: Add realloc and alloc_zeroed to the GlobalAlloc impl

[Boqun Feng <boqun.feng@gmail.com>]

While there are default impls for these methods, using the respective C
api's is faster. Currently neither the existing nor these new
GlobalAlloc method implementations are actually called. Instead the
__rust_* function defined below the GlobalAlloc impl are used. With
rustc 1.71 these functions will be gone and all allocation calls will go
through the GlobalAlloc implementation.

Link: https://github.com/Rust-for-Linux/linux/issues/68
Signed-off-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
[boqun: add size adjustment for alignment requirement]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
Miguel, I fold my diff into Björn's original patch and make a v2, it
relies on the other patch:

	https://lore.kernel.org/rust-for-linux/6e61f06f-2411-0bcb-926b-0a6927096f20@gmail.com/

So this v2 (if all goes well) is targeted for v6.6, JFYI.

v1 -> v2:

*	Add size adjustment for align requirement.

 rust/kernel/allocator.rs | 59 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 54 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/allocator.rs b/rust/kernel/allocator.rs
index 66575cf87ce2..af723c2924dc 100644
--- a/rust/kernel/allocator.rs
+++ b/rust/kernel/allocator.rs
@@ -9,8 +9,17 @@
 
 struct KernelAllocator;
 
-unsafe impl GlobalAlloc for KernelAllocator {
-    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+impl KernelAllocator {
+    /// # Safety
+    ///
+    /// * `ptr` can be either null or a pointer which has been allocated by this allocator.
+    /// * `layout` must have a non-zero size.
+    unsafe fn krealloc_with_flags(
+        &self,
+        ptr: *mut u8,
+        layout: Layout,
+        flags: bindings::gfp_t,
+    ) -> *mut u8 {
         // Customized layouts from `Layout::from_size_align()` can have size < align, so pads first.
         let layout = layout.pad_to_align();
 
@@ -26,9 +35,22 @@ unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
             size = size.next_power_of_two();
         }
 
-        // `krealloc()` is used instead of `kmalloc()` because the latter is
-        // an inline function and cannot be bound to as a result.
-        unsafe { bindings::krealloc(ptr::null(), size, bindings::GFP_KERNEL) as *mut u8 }
+        // SAFETY:
+        //
+        // * `ptr` is either null or a pointer returned from a previous k{re}alloc() by the function
+        //   safety requirement.
+        //
+        // * `size` is greater than 0 since it's either a `layout.size()` (which cannot be zero
+        //    according to the function safety requirement) or a result from `next_power_of_two()`.
+        unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags) as *mut u8 }
+    }
+}
+
+unsafe impl GlobalAlloc for KernelAllocator {
+    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
+        // requirement.
+        unsafe { self.krealloc_with_flags(ptr::null_mut(), layout, bindings::GFP_KERNEL) }
     }
 
     unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
@@ -36,6 +58,33 @@ unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
             bindings::kfree(ptr as *const core::ffi::c_void);
         }
     }
+
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        // SAFETY:
+        // * `new_size` when rounded up to the nearest multiple of `layout.align()`, will not
+        //   overflow `isize` by the function safety requirement.
+        // * `layout.align()` is a proper alignment (i.e. not zero and must be a power of two).
+        let layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
+
+        // SAFETY:
+        // * `ptr` is either null or a pointer allocated by this allocator by function safety
+        //   requirement.
+        // * the size of `layout` is not zero because `new_size` is not zero by function safety
+        //   requirement.
+        unsafe { self.krealloc_with_flags(ptr, layout, bindings::GFP_KERNEL) }
+    }
+
+    unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
+        // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
+        // requirement.
+        unsafe {
+            self.krealloc_with_flags(
+                ptr::null_mut(),
+                layout,
+                bindings::GFP_KERNEL | bindings::__GFP_ZERO,
+            )
+        }
+    }
 }
 
 #[global_allocator]
-- 
2.39.2