mmap breaks easily #2010
Description
This looks like an ok fix for the issue at hand to me. However, I think mmap has a bunch of problems and this is more of a band-aid solution. For example, this implementaiton breaks if the affected area contains both pages mapped with PROT_NONE and pages with some access allowed. From my reading of the man pages, it is perfectly fine to use linux mmap like that.
For a proper solution, we should decide on which level mmap should operate on:
If it is a high level api, we should track the state of all pages mapped via mmap. munmap should refuse to unmap pages which have not been allocated via mmap. This could be done via a hashtable or using the extra bits available in the pagetable. As an example of why this is needed, you could use munmap to unmap the identity mappings with the current implementation.
If it is a low level api, it should interact directly with the free lists and the page table. In that case, munmap should just walk the page table and add anything it finds to the physical free list. We should specify incorrect use of mmap to be UB, as we can not really detect it reliably. This would make our mmap incompatible with the linux one.
I would prefer mmap to be an optional high level feature and to have an additional set of syscalls that allows direct access to the free lists.
Originally posted by @m-mueller678 in #2008 (comment)