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Fix zero-sized reference to deallocated memory #91797

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Dec 12, 2021
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Fix zero-sized reference to deallocated memory #91797

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10 changes: 6 additions & 4 deletions library/alloc/src/vec/drain.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -128,10 +128,6 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {

let iter = mem::replace(&mut self.iter, (&mut []).iter());
let drop_len = iter.len();
let drop_ptr = iter.as_slice().as_ptr();

// forget iter so there's no aliasing reference
drop(iter);
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@RalfJung RalfJung Dec 11, 2021

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Does iter have an actual impl Drop that used to be run here?
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@the8472 the8472 Dec 12, 2021 •
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No, it wasn't actually necessary. I thought slice::Iter internally holds a slice reference that would be invalidated by mutating the vec through other pointers. But it actually contains pointers.
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@RalfJung RalfJung Dec 12, 2021

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Also, at least under Stacked Borrows, droping a reference doesn't help with anything -- it only marks the reference as active the moment it gets dropped.

let mut vec = self.vec;

Expand All @@ -155,6 +151,12 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
return;
}

// as_slice() must only be called when iter.len() is > 0 because
// vec::Splice modifies vec::Drain fields and may grow the vec which would invalidate
// the iterator's internal pointers. Creating a reference to deallocated memory
// is invalid even when it is zero-length
let drop_ptr = iter.as_slice().as_ptr();

unsafe {
// drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
// a pointer with mutable provenance is necessary. Therefore we must reconstruct
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