Memory management for closures

[0009]Another aspect of this description relates to defenses against viruses and other malicious code. In order to prevent viruses and other malicious code from harming a computation it is necessary to not allow writeable data to be used as executable instructions. There is a common need and use of compiler generated thunks to recover or initialize extra data before calling specific functions. A thunk as used herein broadly refers to a piece of code to perform some delayed computation. This allows the functions to appear to take fewer arguments than are necessary yet be supplied with the correct and complete amount of data. The prior art often requires that each such thunk be allocated on its own page of memory such that it can start with writeable permission, be written upon with instructions to recover specific data, and then having the page marked as not writeable and executable, and depending on the architecture of the processor, the processor's instruction cache is flushed; these operations in the prior art are computationally expensive. Since each thunk requires a page of memory, these thunks need to be tracked and deallocated in many implementations. Uses of such thunks include “inner functions” in GCC and certain implementations of closures. At least certain embodiments of the invention provide an efficient way to use thunks by allocating paired chunks of memory and on one chunk of the pair write a series of small thunks that dereference a data area counterpart on the other chunk in the pair. Once written, the memory is protected as execute only and each thunk-data pair in the series is separately allocated and restored. Each thunk-data pair is provided as part of a thunk allocation which also takes the extra data that needs to be recovered. The extra data is stored in the appropriate location in the writeable chunk of memory. The instructions that are written vary according to the needs of the compiler or client and can be as simple as pushing the extra data as an extra stack argument.

The prior art often requires that each such thunk be allocated on its own page of memory such that it can start with writeable permission, be written upon with instructions to recover specific data, and then having the page marked as not writeable and executable, and depending on the architecture of the processor, the processor's instruction cache is flushed; these operations in the prior art are computationally expensive.