Wrong ptr dereferencing in Alloc::alloc_one

[lseugene]

    fn alloc_one<T>(&mut self) -> Result<Unique<T>, AllocErr>
        where Self: Sized
    {
        let k = Layout::new::<T>();
        if k.size() > 0 {
            unsafe { self.alloc(k).map(|p|Unique::new(*p as *mut T)) }
        } else {
            Err(AllocErr::invalid_input("zero-sized type invalid for alloc_one"))
        }
    }

@pnkfelix

[Mark-Simulacrum]

For context:

rust/src/liballoc/allocator.rs

Lines 871 to 880 in 74fa279

	fn alloc_one<T>(&mut self) -> Result<Unique<T>, AllocErr>
	where Self: Sized
	{
	let k = Layout::new::<T>();
	if k.size() > 0 {
	unsafe { self.alloc(k).map(|p|Unique::new(*p as *mut T)) }
	} else {
	Err(AllocErr::invalid_input("zero-sized type invalid for alloc_one"))
	}
	}

.

[sfackler]

Could you clarify what specifically is the problem with that implementation?

[lseugene]

I guess it's just typo.
We don't need to deref raw pointer p before cast it in Unique::new(*p as *mut T)

[sfackler]

p isn't a raw pointer - it's a Unique<T>.

[lseugene]

Underlying alloc:

rust/src/liballoc/allocator.rs

Line 481 in 74fa279

unsafe fn alloc(&mut self, layout: Layout) -> Result<*mut u8, AllocErr>;

returns raw pointer *mut u8. alloc_one should just cast it to *mut T and wrap in Unique.
Please see correct implementation in alloc_array:

rust/src/liballoc/allocator.rs

Line 947 in 74fa279

Unique::new(p as *mut T)

the diff and bug is in Unique::new(*p as *mut T) vs Unique::new(p as *mut T)

[sfackler]

Oh no, casts u8 to *mut T strike again! :(