Question

I’m curious, because I got a kernel panic after trying to access memory directly (then I found these functions).

Answer 1

These functions do a few things:

• They check if the supplied userspace block is entirely within the user portion of the address space (access_ok()) – this prevents userspace applications from asking the kernel to read/write kernel addresses;
• They return an error if any of the addresses are inaccessible, allowing the error to be returned to userspace (EFAULT) instead of crashing the kernel (this is implemented by special co-operation with the page fault handler, which specifically can detect when a fault occurs in one of the user memory access functions);
• They allow architecture-specific magic, for example to ensure consistency on architectures with virtually-tagged caches, to disable protections like SMAP or to switch address spaces on architectures with separate user/kernel address spaces like S/390.