So let's I have a struct that I want to read from user-level space that is defined in the kernel-space, but the user-level space has multiple processes.
Example:
In a kernel module, I have a global struct. struct { int a; int b; } test;
In a user-level module, I have "externed" that global struct
extern struct { int a; int b; } test;
Compiler doesn't complain, and linkage editor doesn't complain. However, if the user has multiple processes, then is that struct cloned for each process? If I use shared memory along with extern, then I could access the kernel's struct, and if I have n processes, then there's only 1 struct since its shared. I can access a kernel-level variable with 1 user-level process, but if I have more processes, then I get clones for each struct that is "externed"
My question is, Can multiple user-level processes read a kernel-level variable?
Userspace cannot see kernel ram directly in any case - and mmap'ing /dev/kmem isn't a good solution either (it is really ugly in my opinion and should only be used for kernel debugging).
I think the nicest way is to expose it either through a file in /proc (which is pretty easy) or a character-device with an IOCTL (which is only slightly more complicated).
(NB: this is Linux / Unix specific)
On most operating systems you cannot access kernel space variables from user space.
You will need to expose your data via the mechanisms that your os provides. This could be a custom system call, a file exposed via the vfs or any other form of IPC.
On Unix this is usually done by mmap-ing some special device file like /dev/kmem.
来源：https://stackoverflow.com/questions/2202160/accessing-variable-in-kernel-space-from-user-level-space