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Auto merge of #79814 - lcnr:deque-f, r=Mark-Simulacrum
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fix soundness issue in `make_contiguous`

fixes #79808
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bors committed Dec 10, 2020
2 parents 80cc2ec + 4fb9f1d commit d32c320
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Showing 2 changed files with 27 additions and 5 deletions.
18 changes: 13 additions & 5 deletions library/alloc/src/collections/vec_deque/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1469,6 +1469,8 @@ impl<T> VecDeque<T> {

#[inline]
fn is_contiguous(&self) -> bool {
// FIXME: Should we consider `head == 0` to mean
// that `self` is contiguous?
self.tail <= self.head
}

Expand Down Expand Up @@ -2198,7 +2200,7 @@ impl<T> VecDeque<T> {
if self.is_contiguous() {
let tail = self.tail;
let head = self.head;
return unsafe { &mut self.buffer_as_mut_slice()[tail..head] };
return unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 };
}

let buf = self.buf.ptr();
Expand All @@ -2224,7 +2226,13 @@ impl<T> VecDeque<T> {
self.tail = 0;
self.head = len;
}
} else if free >= self.head {
} else if free > self.head {
// FIXME: We currently do not consider ....ABCDEFGH
// to be contiguous because `head` would be `0` in this
// case. While we probably want to change this it
// isn't trivial as a few places expect `is_contiguous`
// to mean that we can just slice using `buf[tail..head]`.

// there is enough free space to copy the head in one go,
// this means that we first shift the tail forwards, and then
// copy the head to the correct position.
Expand All @@ -2238,7 +2246,7 @@ impl<T> VecDeque<T> {
// ...ABCDEFGH.

self.tail = self.head;
self.head = self.tail + len;
self.head = self.wrap_add(self.tail, len);
}
} else {
// free is smaller than both head and tail,
Expand Down Expand Up @@ -2278,7 +2286,7 @@ impl<T> VecDeque<T> {

let tail = self.tail;
let head = self.head;
unsafe { &mut self.buffer_as_mut_slice()[tail..head] }
unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 }
}

/// Rotates the double-ended queue `mid` places to the left.
Expand Down Expand Up @@ -2839,7 +2847,7 @@ impl<T> From<VecDeque<T>> for Vec<T> {
let len = other.len();
let cap = other.cap();

if other.head != 0 {
if other.tail != 0 {
ptr::copy(buf.add(other.tail), buf, len);
}
Vec::from_raw_parts(buf, len, cap)
Expand Down
14 changes: 14 additions & 0 deletions library/alloc/src/collections/vec_deque/tests.rs
Original file line number Diff line number Diff line change
Expand Up @@ -210,6 +210,20 @@ fn make_contiguous_small_free() {
);
}

#[test]
fn make_contiguous_head_to_end() {
let mut dq = VecDeque::with_capacity(3);
dq.push_front('B');
dq.push_front('A');
dq.push_back('C');
dq.make_contiguous();
let expected_tail = 0;
let expected_head = 3;
assert_eq!(expected_tail, dq.tail);
assert_eq!(expected_head, dq.head);
assert_eq!((&['A', 'B', 'C'] as &[_], &[] as &[_]), dq.as_slices());
}

#[test]
fn test_remove() {
// This test checks that every single combination of tail position, length, and
Expand Down

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