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    SubjectRe: [PATCH 5/5] tun: vringfd xmit support.
    On Fri, 18 Apr 2008 14:43:24 +1000 Rusty Russell <rusty@rustcorp.com.au> wrote:

    > This patch modifies tun to allow a vringfd to specify the send
    > buffer. The user does a write to push out packets from the buffer.
    >
    > Again we use the 'struct virtio_net_hdr' to allow userspace to send
    > GSO packets. In this case, it can hint how much to copy, and the
    > other pages will be made into skb fragments.
    >
    >
    > ...
    >
    > +/* We don't consolidate consecutive iovecs, so huge iovecs can break here.
    > + * Users will learn not to do that. */
    > +static int get_user_skb_frags(const struct iovec *iv, size_t len,
    > + struct skb_frag_struct *f)
    > +{
    > + unsigned int i, j, num_pg = 0;
    > + int err;
    > + struct page *pages[MAX_SKB_FRAGS];
    > +
    > + down_read(&current->mm->mmap_sem);
    > + while (len) {
    > + int n, npages;
    > + unsigned long base, len;
    > + base = (unsigned long)iv->iov_base;
    > + len = (unsigned long)iv->iov_len;
    > +
    > + if (len == 0) {
    > + iv++;
    > + continue;
    > + }
    > +
    > + /* How many pages will this take? */
    > + npages = 1 + (base + len - 1)/PAGE_SIZE - base/PAGE_SIZE;

    Brain hurts. I hope you got that right.

    > + if (unlikely(num_pg + npages > MAX_SKB_FRAGS)) {
    > + err = -ENOSPC;
    > + goto fail;
    > + }
    > + n = get_user_pages(current, current->mm, base, npages,
    > + 0, 0, pages, NULL);

    What is the maximum numbet of pages which an unpriviliged user can
    concurrently pin with this code?

    > + if (unlikely(n < 0)) {
    > + err = n;
    > + goto fail;
    > + }
    > +
    > + /* Transfer pages to the frag array */
    > + for (j = 0; j < n; j++) {
    > + f[num_pg].page = pages[j];
    > + if (j == 0) {
    > + f[num_pg].page_offset = offset_in_page(base);
    > + f[num_pg].size = min(len, PAGE_SIZE -
    > + f[num_pg].page_offset);
    > + } else {
    > + f[num_pg].page_offset = 0;
    > + f[num_pg].size = min(len, PAGE_SIZE);
    > + }
    > + len -= f[num_pg].size;
    > + base += f[num_pg].size;
    > + num_pg++;
    > + }

    This loop is a fancy way of doing

    num_pg = n;

    > + if (unlikely(n != npages)) {
    > + err = -EFAULT;
    > + goto fail;
    > + }

    why not do this immediately after running get_user_pages()?

    > + }
    > + up_read(&current->mm->mmap_sem);
    > + return num_pg;
    > +
    > +fail:
    > + for (i = 0; i < num_pg; i++)
    > + put_page(f[i].page);

    release_pages() could be a tad more efficient, but it's only error-path.

    > + up_read(&current->mm->mmap_sem);
    > + return err;
    > +}
    > +
    >



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