Issue
I'm writing a linux kernel driver and for every function that sends data up to userspace or reads data from userspace, I am using copy_to_user() and copy_from_user(). My question is: do I need to use these calls if I am just copying a basic data type such as a u32 or an int?
Solution
If the function receives a pointer to user-space data, you have to use copy_from_user() to copy the pointed-to data from user space into kernel space (and vice versa).
Note that the pointer value itself is passed by value (like all C parameters), so you don't have to do a copy_from_user() to obtain the pointer value before you can copy_from_user() the data it points to.
Numeric arguments work the same way as pointer arguments; in C terms, they're both scalars. You don't have to use copy_from_user() to copy the value of the parameter; that's already been copied. You only have to use it to copy data that's pointed to by a passed pointer.
So if you have a parameter of type int, you can use it directly. If your parameter points to an int, then the int object will be in user space, and you need to use copy_to_user to copy the value of that object into kernel space.
Answered By - Keith Thompson
