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Fix undefined behavior in Rc/Arc allocation #54922

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Merged
merged 1 commit into from
Nov 6, 2018
Merged

Fix undefined behavior in Rc/Arc allocation #54922

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12 changes: 7 additions & 5 deletions src/liballoc/rc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -665,15 +665,17 @@ impl Rc<dyn Any> {
impl<T: ?Sized> Rc<T> {
// Allocates an `RcBox<T>` with sufficient space for an unsized value
unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
// Create a fake RcBox to find allocation size and alignment
let fake_ptr = ptr as *mut RcBox<T>;

let layout = Layout::for_value(&*fake_ptr);
// Calculate layout using the given value.
// Previously, layout was calculated on the expression
// `&*(ptr as *const RcBox<T>)`, but this created a misaligned
// reference (see #54908).
let (layout, _) = Layout::new::<RcBox<()>>()
.extend(Layout::for_value(&*ptr)).unwrap();

let mem = Global.alloc(layout)
.unwrap_or_else(|_| handle_alloc_error(layout));

// Initialize the real RcBox
// Initialize the RcBox
let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut RcBox<T>;

ptr::write(&mut (*inner).strong, Cell::new(1));
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12 changes: 7 additions & 5 deletions src/liballoc/sync.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -566,15 +566,17 @@ impl<T: ?Sized> Arc<T> {
impl<T: ?Sized> Arc<T> {
// Allocates an `ArcInner<T>` with sufficient space for an unsized value
unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
// Create a fake ArcInner to find allocation size and alignment
let fake_ptr = ptr as *mut ArcInner<T>;

let layout = Layout::for_value(&*fake_ptr);
// Calculate layout using the given value.
// Previously, layout was calculated on the expression
// `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
// reference (see #54908).
let (layout, _) = Layout::new::<ArcInner<()>>()
.extend(Layout::for_value(&*ptr)).unwrap();

let mem = Global.alloc(layout)
.unwrap_or_else(|_| handle_alloc_error(layout));

// Initialize the real ArcInner
// Initialize the ArcInner
let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;

ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
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