why copy_to_user() is required?

Question

As we all know that kernel has access to user space also. Thus, it doesn't need the special function - copy_to_user(). Why is it required then? The kernel can access the user space address directly. Isn't it?

Answer 1

Because kernel does not want userspace to modify the passed data while it process the same data.

Copy from/to user

1. Validates buffer

2. Copy buffer to/from user so it does not have worry about data being updated in user space.

Answer 2

Note that on some architectures, it's not implemented with a simple memcpy():

• http://lxr.free-electrons.com/source/arch/alpha/include/asm/uaccess.h#L358