How to map two virtual adresses on the same physic

Question

I'm facing a quite tricky problem. I'm trying to get 2 virtual memory areas pointing to the same physical memory. The point is to have different page protection parameters on different memory areas.

On this forum, the user seems to have a solution, but it seems kinda hacky and it's pretty clear that something better can be done performance-wise : http://www.linuxforums.org/forum/programming-scripting/19491-map-two-virtual-memory-addres-same-physical-page.html

As I'm facing the same problem, I want to give a shot here to know if somebody has a better idea. Don't be afraid to mention the dirty details behind the hood, this is what this question is about.

Thank by advance.

Answer 1

I'm trying to get 2 virtual memory area pointing on the same physical memory.

mmap the same region in the same file, twice, or use System V shared memory (which does not require mapping a file in memory).

Answer 2

I suppose if you dislike Sys V shared memrory you could use POSIX shared memory objects. They're not very popular but available on Linux and BSDs at least.

Once you get an fd with shm_open you could immediately call shm_unlink. Then no other process can attach to the same shared memory, and you can mmap it multiple times. Still a small race period available though.

Answer 3

If you are root, you can mmap("/dev/mem", ...) but there are caveats in the newer kernels, see accessing mmaped /dev/mem?