Question

As per the linux design on x86 and ppc, the 4g virtual address space is divided into 3:1.
User virtual address’s are till 3g.

Now if user app does an ioctl passing a pointer to buffer, the kernel module, can directly do a memcpy, I tried and it worked.
=> Why do we then need a copy_to/copy_from user.

Note: If the page is swapped out, then the kernel pagefault handler would bring that back, and it is invisible to kernel module.

need yr ideas … comments

Answer 1

There are several good reasons that copy_to_user / copy_from_user are the correct functions to use:

• On some architectures a simple memcpy() does not work, so using those functions allows your code to work there. I believe even x86 with the HIGHMEM config option selected is in this boat.

• Those functions do an access_ok() check to ensure that the user space addresses referenced really are genuine user space addresses. If you just do a memcpy(), the caller of the ioctl() can supply an address range that overlaps kernel addresses, which is a security hole.

• However, the major reason is to properly handle bad user addresses. If you just use a bare memcpy(), the unhandled fault will result in a kernel oops. The user access functions use the “fixup” mechanism, which allows the fault to be handled (the read or write is short, and usually EFAULT is returned to userspace in this case).