User space access from Kernel space - get_user_pages

Question

Hello,

I'd like pass a pointer from a user space memory into a function in my kernel module. I don't want to use copy_from_user. I've read that I should use get_user_pages function.

For example one page.

struct page **pages;
pages = kmalloc(1 * sizeof(*pages), GFP_KERNEL);

down_read(&current->mm->mmap_sem);
get_user_pages(current,current->mm,uaddr, 1, 1, 0,pages,NULL);
up_read(&current->mm->mmap_sem);

uaddr is an address in User Space.

1. After doing this, am I allowed to cast and pass uaddr into my kernel module function? Or maybe I have to use these struct pages in some way?
2. Why do I have to use down/up read?
3. After everything do I have to use SetPageDirty() and page_cache_release() functions ?

Answer 1

This is not what get_user_pages is for (and no - you can't then just cast and pass uaddr into your kernel module function).

If you don't want to call copy_from_user in the calling function, then just pass a void __user * to your module function and have it do the copy_from_user.

Answer 2

You can only use the user pages for page type activity, for example setting up Scatter/Gather DMA into userspace memory. You cannot use it to directly access user space from kernel mode code. Hence the copy_to/from functions that are there for that reason. Unless your moving large amounts of data why not use these functions?