Bug description

OCaml versions 4.02.3 and earlier have a runtime bug that, on 64-bit platforms, causes sizes arguments to an internal memmove call to be sign-extended from 32 to 64-bits before being passed to the memmove function.

This leads arguments between 2GiB and 4GiB to be interpreted as larger than they are (specifically, a bit below 2^64), causing a buffer overflow.

Arguments between 4GiB and 6GiB are interpreted as 4GiB smaller than they should be, causing a possible information leak.

This commit fixes the bug: https://github.com/ocaml/ocaml/commit/659615c7b100a89eafe6253e7a5b9d84d0e8df74#diff-a97df53e3ebc59bb457191b496c90762 The function camlbitstring is called indirectly from such functions as String.copy. String.copy for instance is supposed to be a "safe" function for which OCaml's memory safety guarantees apply.