[daanx]

Interesting article -- thanks! I see they used the `key ^ P` encoding where the `key` is `L >> PAGESHIFT` to use the ASLR randomized bits from the free list position `L`.

In [mimalloc](https://github.com/microsoft/mimalloc) we use a similar strategy to protect the free list in secure mode. However, there are some weaknesses associated with using a plain _xor_ -- if the attacker can guess `P` that reveals the key immediately (`P^key^P == key`) or even if there is a read overflow, and the attacker can read multiple encodings, they can xor with each other, say `(P1^key)^(P2^key)` and then we have`(P1^P2)` which may reveal information about the pointers (like alignment).

What we use in _mimalloc_ instead is two keys `key1` and `key2` and encode as `((P^key2)<<<key1)+key1`. Since these operations are not associative, the above approaches do not work so well any more even if the `P` can be guesstimated. For example, for the read case we can subtract two entries to discard the `+key1` term, but that leads to `((P1^key2)<<<key1) - ((P2^key2)<<<key1)` at best. (We include the left-rotation since xor and addition are otherwise linear in the lowest bit).

Just some thoughts. Of course, this may be too much for the use-case. However, we found just little extra overhead for the extra operations (as most programs are dominated by memory access) so it may be of benefit.