How to use mmap to share user-space and kernel threads

Question

I am having some trouble finding some suitable examples to solve my problem. I want to share 4K (4096) byte of data between user and kernel space. I found many ideas which says I have to allocate memory from kernel and mmap it in user-space. Can someone provide an example on how to do it in Linux 2.6.38. Is there any good document which explains it?

Thanks in advance.

Answer 1

Your proposed way is one way, but as userspace is not within your control (meaning any userspace program have a possibility of poking into the kernel), you are opening up the opportunities for malicious attack from userspace. This kernel-based memory-sharing-with-userspace is also described here:

https://web.archive.org/web/20130217131239/http://www.scs.ch/~frey/linux/memorymap.html

Instead, how about allocating memory in userspace, and then from kernel use the API copy_from_user() and copy_to_user() to copy to/from userspace memory? If u want to share the memory among the different processes, then u can always use IPC related API to allocate and define the memory, eg shmget() etc. And in this case there are lots of sample codes within the kernel source itself.

eg.

fs/checksum.c: missing = __copy_from_user(dst, src, len);