Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

David Miller
2010-09-14T03:17:14Z

From: Shirley Ma <mashirle@us.ibm.com>
Date: Mon, 13 Sep 2010 13:48:03 -0700

> + base = (unsigned long)from->iov_base + offset1;
> + size = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
> + num_pages = get_user_pages_fast(base, size, 0, &page[i]);
> + if ((num_pages != size) ||
> + (num_pages > MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags))
> + /* put_page is in skb free */
> + return -EFAULT;

What keeps the user from writing to these pages in it's address space
after the write call returns?

A write() return of success means:

"I wrote what you gave to me"

not

"I wrote what you gave to me, oh and BTW don't touch these
pages for a while."

In fact "a while" isn't even defined in any way, as there is no way
for the write() invoker to know when the networking card is done with
those pages.