Direct access user memory from kernel on Linux

Question

I've got a user-mode process and kernel module. Now I want to read certain regions of usermode process from kernel, but there's one catch: no copying of usermode memory and simple access by VA. So what we have: task_struct for target process, other related structs (like mm_struct, vma_struct) and virtual address like 0x0070abcd that I want to read or rather map somehow to my kernel module.

I can get page list using get_user_pages for desired memory regions, but what next? Should I map pages somehow into kernel and then try to read them as continuous memory region or there are better solutions?

Answer 1

The problem is that "looking" at user-space requires locking a ton of stuff. So it's better that you do a short copy than leave everything locked for arbitrary amounts of time. Your user-space process may not be VM-mapped into the current CPU. In fact, it may be entirely swapped out to disk, running on another CPU, in the middle of it's own kernel call, etc.

Linux Kernel: copy_from_user - struct with pointers