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I don't really understand why we need to use 'put_user' macro
to access the user space memory.
Why can't we just dereference the pointer to this user space
memory and access it directly?
On the x86 platform (others?) memory is actually "segmented" in hardware. Thus, you have to copy mem user <-> kernel. Ring 0 (like being root) is where the kernel resides, Ring 3 is user space. That's my simplified understanding at least.
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