// SPDX-License-Identifier: GPL-2.0
/*
 *      SUCS NET3:
 *
 *      Generic datagram handling routines. These are generic for all
 *      protocols. Possibly a generic IP version on top of these would
 *      make sense. Not tonight however 8-).
 *      This is used because UDP, RAW, PACKET, DDP, IPX, AX.25 and
 *      NetROM layer all have identical poll code and mostly
 *      identical recvmsg() code. So we share it here. The poll was
 *      shared before but buried in udp.c so I moved it.
 *
 *      Authors:        Alan Cox <alan@lxorguk.ukuu.org.uk>. (datagram_poll() from old
 *                                                   udp.c code)
 *
 *      Fixes:
 *              Alan Cox        :       NULL return from skb_peek_copy()
 *                                      understood
 *              Alan Cox        :       Rewrote skb_read_datagram to avoid the
 *                                      skb_peek_copy stuff.
 *              Alan Cox        :       Added support for SOCK_SEQPACKET.
 *                                      IPX can no longer use the SO_TYPE hack
 *                                      but AX.25 now works right, and SPX is
 *                                      feasible.
 *              Alan Cox        :       Fixed write poll of non IP protocol
 *                                      crash.
 *              Florian  La Roche:      Changed for my new skbuff handling.
 *              Darryl Miles    :       Fixed non-blocking SOCK_SEQPACKET.
 *              Linus Torvalds  :       BSD semantic fixes.
 *              Alan Cox        :       Datagram iovec handling
 *              Darryl Miles    :       Fixed non-blocking SOCK_STREAM.
 *              Alan Cox        :       POSIXisms
 *              Pete Wyckoff    :       Unconnected accept() fix.
 *
 */

#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/mm.h>
#include <linux/interrupt.h>
#include <linux/errno.h>
#include <linux/sched.h>
#include <linux/inet.h>
#include <linux/netdevice.h>
#include <linux/rtnetlink.h>
#include <linux/poll.h>
#include <linux/highmem.h>
#include <linux/spinlock.h>
#include <linux/slab.h>
#include <linux/pagemap.h>
#include <linux/uio.h>

#include <net/protocol.h>
#include <linux/skbuff.h>

#include <net/checksum.h>
#include <net/sock.h>
#include <net/tcp_states.h>
#include <trace/events/skb.h>
#include <net/busy_poll.h>

/*
 *      Is a socket 'connection oriented' ?
 */
static inline int connection_based(struct sock *sk)
{
        return sk->sk_type == SOCK_SEQPACKET || sk->sk_type == SOCK_STREAM;
}

static int receiver_wake_function(wait_queue_entry_t *wait, unsigned int mode, int sync,
                                  void *key)
{
        unsigned long bits = (unsigned long)key;

        /*
         * Avoid a wakeup if event not interesting for us
         */
        if (bits && !(bits & (POLLIN | POLLERR)))
                return 0;
        return autoremove_wake_function(wait, mode, sync, key);
}
/*
 * Wait for the last received packet to be different from skb
 */
int __skb_wait_for_more_packets(struct sock *sk, int *err, long *timeo_p,
                                const struct sk_buff *skb)
{
        int error;
        DEFINE_WAIT_FUNC(wait, receiver_wake_function);

        prepare_to_wait_exclusive(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);

        /* Socket errors? */
        error = sock_error(sk);
        if (error)
                goto out_err;

        if (sk->sk_receive_queue.prev != skb)
                goto out;

        /* Socket shut down? */
        if (sk->sk_shutdown & RCV_SHUTDOWN)
                goto out_noerr;

        /* Sequenced packets can come disconnected.
         * If so we report the problem
         */
        error = -ENOTCONN;
        if (connection_based(sk) &&
            !(sk->sk_state == TCP_ESTABLISHED || sk->sk_state == TCP_LISTEN))
                goto out_err;

        /* handle signals */
        if (signal_pending(current))
                goto interrupted;

        error = 0;
        *timeo_p = schedule_timeout(*timeo_p);
out:
        finish_wait(sk_sleep(sk), &wait);
        return error;
interrupted:
        error = sock_intr_errno(*timeo_p);
out_err:
        *err = error;
        goto out;
out_noerr:
        *err = 0;
        error = 1;
        goto out;
}
EXPORT_SYMBOL(__skb_wait_for_more_packets);

static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
{
        struct sk_buff *nskb;

        if (skb->peeked)
                return skb;

        /* We have to unshare an skb before modifying it. */
        if (!skb_shared(skb))
                goto done;

        nskb = skb_clone(skb, GFP_ATOMIC);
        if (!nskb)
                return ERR_PTR(-ENOMEM);

        skb->prev->next = nskb;
        skb->next->prev = nskb;
        nskb->prev = skb->prev;
        nskb->next = skb->next;

        consume_skb(skb);
        skb = nskb;

done:
        skb->peeked = 1;

        return skb;
}

struct sk_buff *__skb_try_recv_from_queue(struct sock *sk,
                                          struct sk_buff_head *queue,
                                          unsigned int flags,
                                          void (*destructor)(struct sock *sk,
                                                           struct sk_buff *skb),
                                          int *peeked, int *off, int *err,
                                          struct sk_buff **last)
{
        bool peek_at_off = false;
        struct sk_buff *skb;
        int _off = 0;

        if (unlikely(flags & MSG_PEEK && *off >= 0)) {
                peek_at_off = true;
                _off = *off;
        }

        *last = queue->prev;
        skb_queue_walk(queue, skb) {
                if (flags & MSG_PEEK) {
                        if (peek_at_off && _off >= skb->len &&
                            (_off || skb->peeked)) {
                                _off -= skb->len;
                                continue;
                        }
                        if (!skb->len) {
                                skb = skb_set_peeked(skb);
                                if (unlikely(IS_ERR(skb))) {
                                        *err = PTR_ERR(skb);
                                        return NULL;
                                }
                        }
                        *peeked = 1;
                        refcount_inc(&skb->users);
                } else {
                        __skb_unlink(skb, queue);
                        if (destructor)
                                destructor(sk, skb);
                }
                *off = _off;
                return skb;
        }
        return NULL;
}

/**
 *      __skb_try_recv_datagram - Receive a datagram skbuff
 *      @sk: socket
 *      @flags: MSG\_ flags
 *      @destructor: invoked under the receive lock on successful dequeue
 *      @peeked: returns non-zero if this packet has been seen before
 *      @off: an offset in bytes to peek skb from. Returns an offset
 *            within an skb where data actually starts
 *      @err: error code returned
 *      @last: set to last peeked message to inform the wait function
 *             what to look for when peeking
 *
 *      Get a datagram skbuff, understands the peeking, nonblocking wakeups
 *      and possible races. This replaces identical code in packet, raw and
 *      udp, as well as the IPX AX.25 and Appletalk. It also finally fixes
 *      the long standing peek and read race for datagram sockets. If you
 *      alter this routine remember it must be re-entrant.
 *
 *      This function will lock the socket if a skb is returned, so
 *      the caller needs to unlock the socket in that case (usually by
 *      calling skb_free_datagram). Returns NULL with @err set to
 *      -EAGAIN if no data was available or to some other value if an
 *      error was detected.
 *
 *      * It does not lock socket since today. This function is
 *      * free of race conditions. This measure should/can improve
 *      * significantly datagram socket latencies at high loads,
 *      * when data copying to user space takes lots of time.
 *      * (BTW I've just killed the last cli() in IP/IPv6/core/netlink/packet
 *      *  8) Great win.)
 *      *                                           --ANK (980729)
 *
 *      The order of the tests when we find no data waiting are specified
 *      quite explicitly by POSIX 1003.1g, don't change them without having
 *      the standard around please.
 */
struct sk_buff *__skb_try_recv_datagram(struct sock *sk, unsigned int flags,
                                        void (*destructor)(struct sock *sk,
                                                           struct sk_buff *skb),
                                        int *peeked, int *off, int *err,
                                        struct sk_buff **last)
{
        struct sk_buff_head *queue = &sk->sk_receive_queue;
        struct sk_buff *skb;
        unsigned long cpu_flags;
        /*
         * Caller is allowed not to check sk->sk_err before skb_recv_datagram()
         */
        int error = sock_error(sk);

        if (error)
                goto no_packet;

        *peeked = 0;
        do {
                /* Again only user level code calls this function, so nothing
                 * interrupt level will suddenly eat the receive_queue.
                 *
                 * Look at current nfs client by the way...
                 * However, this function was correct in any case. 8)
                 */
                spin_lock_irqsave(&queue->lock, cpu_flags);
                skb = __skb_try_recv_from_queue(sk, queue, flags, destructor,
                                                peeked, off, &error, last);
                spin_unlock_irqrestore(&queue->lock, cpu_flags);
                if (error)
                        goto no_packet;
                if (skb)
                        return skb;

                if (!sk_can_busy_loop(sk))
                        break;

                sk_busy_loop(sk, flags & MSG_DONTWAIT);
        } while (!skb_queue_empty(&sk->sk_receive_queue));

        error = -EAGAIN;

no_packet:
        *err = error;
        return NULL;
}
EXPORT_SYMBOL(__skb_try_recv_datagram);

struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
                                    void (*destructor)(struct sock *sk,
                                                       struct sk_buff *skb),
                                    int *peeked, int *off, int *err)
{
        struct sk_buff *skb, *last;
        long timeo;

        timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);

        do {
                skb = __skb_try_recv_datagram(sk, flags, destructor, peeked,
                                              off, err, &last);
                if (skb)
                        return skb;

                if (*err != -EAGAIN)
                        break;
        } while (timeo &&
                !__skb_wait_for_more_packets(sk, err, &timeo, last));

        return NULL;
}
EXPORT_SYMBOL(__skb_recv_datagram);

struct sk_buff *skb_recv_datagram(struct sock *sk, unsigned int flags,
                                  int noblock, int *err)
{
        int peeked, off = 0;

        return __skb_recv_datagram(sk, flags | (noblock ? MSG_DONTWAIT : 0),
                                   NULL, &peeked, &off, err);
}
EXPORT_SYMBOL(skb_recv_datagram);

void skb_free_datagram(struct sock *sk, struct sk_buff *skb)
{
        consume_skb(skb);
        sk_mem_reclaim_partial(sk);
}
EXPORT_SYMBOL(skb_free_datagram);

void __skb_free_datagram_locked(struct sock *sk, struct sk_buff *skb, int len)
{
        bool slow;

        if (!skb_unref(skb)) {
                sk_peek_offset_bwd(sk, len);
                return;
        }

        slow = lock_sock_fast(sk);
        sk_peek_offset_bwd(sk, len);
        skb_orphan(skb);
        sk_mem_reclaim_partial(sk);
        unlock_sock_fast(sk, slow);

        /* skb is now orphaned, can be freed outside of locked section */
        __kfree_skb(skb);
}
EXPORT_SYMBOL(__skb_free_datagram_locked);

int __sk_queue_drop_skb(struct sock *sk, struct sk_buff_head *sk_queue,
                        struct sk_buff *skb, unsigned int flags,
                        void (*destructor)(struct sock *sk,
                                           struct sk_buff *skb))
{
        int err = 0;

        if (flags & MSG_PEEK) {
                err = -ENOENT;
                spin_lock_bh(&sk_queue->lock);
                if (skb->next) {
                        __skb_unlink(skb, sk_queue);
                        refcount_dec(&skb->users);
                        if (destructor)
                                destructor(sk, skb);
                        err = 0;
                }
                spin_unlock_bh(&sk_queue->lock);
        }

        atomic_inc(&sk->sk_drops);
        return err;
}
EXPORT_SYMBOL(__sk_queue_drop_skb);

/**
 *      skb_kill_datagram - Free a datagram skbuff forcibly
 *      @sk: socket
 *      @skb: datagram skbuff
 *      @flags: MSG\_ flags
 *
 *      This function frees a datagram skbuff that was received by
 *      skb_recv_datagram.  The flags argument must match the one
 *      used for skb_recv_datagram.
 *
 *      If the MSG_PEEK flag is set, and the packet is still on the
 *      receive queue of the socket, it will be taken off the queue
 *      before it is freed.
 *
 *      This function currently only disables BH when acquiring the
 *      sk_receive_queue lock.  Therefore it must not be used in a
 *      context where that lock is acquired in an IRQ context.
 *
 *      It returns 0 if the packet was removed by us.
 */

int skb_kill_datagram(struct sock *sk, struct sk_buff *skb, unsigned int flags)
{
        int err = __sk_queue_drop_skb(sk, &sk->sk_receive_queue, skb, flags,
                                      NULL);

        kfree_skb(skb);
        sk_mem_reclaim_partial(sk);
        return err;
}
EXPORT_SYMBOL(skb_kill_datagram);

/**
 *      skb_copy_datagram_iter - Copy a datagram to an iovec iterator.
 *      @skb: buffer to copy
 *      @offset: offset in the buffer to start copying from
 *      @to: iovec iterator to copy to
 *      @len: amount of data to copy from buffer to iovec
 */
int skb_copy_datagram_iter(const struct sk_buff *skb, int offset,
                           struct iov_iter *to, int len)
{
        int start = skb_headlen(skb);
        int i, copy = start - offset, start_off = offset, n;
        struct sk_buff *frag_iter;

        trace_skb_copy_datagram_iovec(skb, len);

        /* Copy header. */
        if (copy > 0) {
                if (copy > len)
                        copy = len;
                n = copy_to_iter(skb->data + offset, copy, to);
                offset += n;
                if (n != copy)
                        goto short_copy;
                if ((len -= copy) == 0)
                        return 0;
        }

        /* Copy paged appendix. Hmm... why does this look so complicated? */
        for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
                int end;
                const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];

                WARN_ON(start > offset + len);

                end = start + skb_frag_size(frag);
                if ((copy = end - offset) > 0) {
                        if (copy > len)
                                copy = len;
                        n = copy_page_to_iter(skb_frag_page(frag),
                                              frag->page_offset + offset -
                                              start, copy, to);
                        offset += n;
                        if (n != copy)
                                goto short_copy;
                        if (!(len -= copy))
                                return 0;
                }
                start = end;
        }

        skb_walk_frags(skb, frag_iter) {
                int end;

                WARN_ON(start > offset + len);

                end = start + frag_iter->len;
                if ((copy = end - offset) > 0) {
                        if (copy > len)
                                copy = len;
                        if (skb_copy_datagram_iter(frag_iter, offset - start,
                                                   to, copy))
                                goto fault;
                        if ((len -= copy) == 0)
                                return 0;
                        offset += copy;
                }
                start = end;
        }
        if (!len)
                return 0;

        /* This is not really a user copy fault, but rather someone
         * gave us a bogus length on the skb.  We should probably
         * print a warning here as it may indicate a kernel bug.
         */

fault:
        iov_iter_revert(to, offset - start_off);
        return -EFAULT;

short_copy:
        if (iov_iter_count(to))
                goto fault;

        return 0;
}
EXPORT_SYMBOL(skb_copy_datagram_iter);

/**
 *      skb_copy_datagram_from_iter - Copy a datagram from an iov_iter.
 *      @skb: buffer to copy
 *      @offset: offset in the buffer to start copying to
 *      @from: the copy source
 *      @len: amount of data to copy to buffer from iovec
 *
 *      Returns 0 or -EFAULT.
 */
int skb_copy_datagram_from_iter(struct sk_buff *skb, int offset,
                                 struct iov_iter *from,
                                 int len)
{
        int start = skb_headlen(skb);
        int i, copy = start - offset;
        struct sk_buff *frag_iter;

        /* Copy header. */
        if (copy > 0) {
                if (copy > len)
                        copy = len;
                if (copy_from_iter(skb->data + offset, copy, from) != copy)
                        goto fault;
                if ((len -= copy) == 0)
                        return 0;
                offset += copy;
        }

        /* Copy paged appendix. Hmm... why does this look so complicated? */
        for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
                int end;
                const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];

                WARN_ON(start > offset + len);

                end = start + skb_frag_size(frag);
                if ((copy = end - offset) > 0) {
                        size_t copied;

                        if (copy > len)
                                copy = len;
                        copied = copy_page_from_iter(skb_frag_page(frag),
                                          frag->page_offset + offset - start,
                                          copy, from);
                        if (copied != copy)
                                goto fault;

                        if (!(len -= copy))
                                return 0;
                        offset += copy;
                }
                start = end;
        }

        skb_walk_frags(skb, frag_iter) {
                int end;

                WARN_ON(start > offset + len);

                end = start + frag_iter->len;
                if ((copy = end - offset) > 0) {
                        if (copy > len)
                                copy = len;
                        if (skb_copy_datagram_from_iter(frag_iter,
                                                        offset - start,
                                                        from, copy))
                                goto fault;
                        if ((len -= copy) == 0)
                                return 0;
                        offset += copy;
                }
                start = end;
        }
        if (!len)
                return 0;

fault:
        return -EFAULT;
}
EXPORT_SYMBOL(skb_copy_datagram_from_iter);

int __zerocopy_sg_from_iter(struct sock *sk, struct sk_buff *skb,
                            struct iov_iter *from, size_t length)
{
        int frag = skb_shinfo(skb)->nr_frags;

        while (length && iov_iter_count(from)) {
                struct page *pages[MAX_SKB_FRAGS];
                size_t start;
                ssize_t copied;
                unsigned long truesize;
                int n = 0;

                if (frag == MAX_SKB_FRAGS)
                        return -EMSGSIZE;

                copied = iov_iter_get_pages(from, pages, length,
                                            MAX_SKB_FRAGS - frag, &start);
                if (copied < 0)
                        return -EFAULT;

                iov_iter_advance(from, copied);
                length -= copied;

                truesize = PAGE_ALIGN(copied + start);
                skb->data_len += copied;
                skb->len += copied;
                skb->truesize += truesize;
                if (sk && sk->sk_type == SOCK_STREAM) {
                        sk->sk_wmem_queued += truesize;
                        sk_mem_charge(sk, truesize);
                } else {
                        refcount_add(truesize, &skb->sk->sk_wmem_alloc);
                }
                while (copied) {
                        int size = min_t(int, copied, PAGE_SIZE - start);
                        skb_fill_page_desc(skb, frag++, pages[n], start, size);
                        start = 0;
                        copied -= size;
                        n++;
                }
        }
        return 0;
}
EXPORT_SYMBOL(__zerocopy_sg_from_iter);

/**
 *      zerocopy_sg_from_iter - Build a zerocopy datagram from an iov_iter
 *      @skb: buffer to copy
 *      @from: the source to copy from
 *
 *      The function will first copy up to headlen, and then pin the userspace
 *      pages and build frags through them.
 *
 *      Returns 0, -EFAULT or -EMSGSIZE.
 */
int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *from)
{
        int copy = min_t(int, skb_headlen(skb), iov_iter_count(from));

        /* copy up to skb headlen */
        if (skb_copy_datagram_from_iter(skb, 0, from, copy))
                return -EFAULT;

        return __zerocopy_sg_from_iter(NULL, skb, from, ~0U);
}
EXPORT_SYMBOL(zerocopy_sg_from_iter);

static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
                                      struct iov_iter *to, int len,
                                      __wsum *csump)
{
        int start = skb_headlen(skb);
        int i, copy = start - offset, start_off = offset;
        struct sk_buff *frag_iter;
        int pos = 0;
        int n;

        /* Copy header. */
        if (copy > 0) {
                if (copy > len)
                        copy = len;
                n = csum_and_copy_to_iter(skb->data + offset, copy, csump, to);
                offset += n;
                if (n != copy)
                        goto fault;
                if ((len -= copy) == 0)
                        return 0;
                pos = copy;
        }

        for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
                int end;
                const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];

                WARN_ON(start > offset + len);

                end = start + skb_frag_size(frag);
                if ((copy = end - offset) > 0) {
                        __wsum csum2 = 0;
                        struct page *page = skb_frag_page(frag);
                        u8  *vaddr = kmap(page);

                        if (copy > len)
                                copy = len;
                        n = csum_and_copy_to_iter(vaddr + frag->page_offset +
                                                  offset - start, copy,
                                                  &csum2, to);
                        kunmap(page);
                        offset += n;
                        if (n != copy)
                                goto fault;
                        *csump = csum_block_add(*csump, csum2, pos);
                        if (!(len -= copy))
                                return 0;
                        pos += copy;
                }
                start = end;
        }

        skb_walk_frags(skb, frag_iter) {
                int end;

                WARN_ON(start > offset + len);

                end = start + frag_iter->len;
                if ((copy = end - offset) > 0) {
                        __wsum csum2 = 0;
                        if (copy > len)
                                copy = len;
                        if (skb_copy_and_csum_datagram(frag_iter,
                                                       offset - start,
                                                       to, copy,
                                                       &csum2))
                                goto fault;
                        *csump = csum_block_add(*csump, csum2, pos);
                        if ((len -= copy) == 0)
                                return 0;
                        offset += copy;
                        pos += copy;
                }
                start = end;
        }
        if (!len)
                return 0;

fault:
        iov_iter_revert(to, offset - start_off);
        return -EFAULT;
}

__sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
{
        __sum16 sum;

        sum = csum_fold(skb_checksum(skb, 0, len, skb->csum));
        if (likely(!sum)) {
                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
                    !skb->csum_complete_sw)
                        netdev_rx_csum_fault(skb->dev);
        }
        if (!skb_shared(skb))
                skb->csum_valid = !sum;
        return sum;
}
EXPORT_SYMBOL(__skb_checksum_complete_head);

__sum16 __skb_checksum_complete(struct sk_buff *skb)
{
        __wsum csum;
        __sum16 sum;

        csum = skb_checksum(skb, 0, skb->len, 0);

        /* skb->csum holds pseudo checksum */
        sum = csum_fold(csum_add(skb->csum, csum));
        if (likely(!sum)) {
                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
                    !skb->csum_complete_sw)
                        netdev_rx_csum_fault(skb->dev);
        }

        if (!skb_shared(skb)) {
                /* Save full packet checksum */
                skb->csum = csum;
                skb->ip_summed = CHECKSUM_COMPLETE;
                skb->csum_complete_sw = 1;
                skb->csum_valid = !sum;
        }

        return sum;
}
EXPORT_SYMBOL(__skb_checksum_complete);

/**
 *      skb_copy_and_csum_datagram_msg - Copy and checksum skb to user iovec.
 *      @skb: skbuff
 *      @hlen: hardware length
 *      @msg: destination
 *
 *      Caller _must_ check that skb will fit to this iovec.
 *
 *      Returns: 0       - success.
 *               -EINVAL - checksum failure.
 *               -EFAULT - fault during copy.
 */
int skb_copy_and_csum_datagram_msg(struct sk_buff *skb,
                                   int hlen, struct msghdr *msg)
{
        __wsum csum;
        int chunk = skb->len - hlen;

        if (!chunk)
                return 0;

        if (msg_data_left(msg) < chunk) {
                if (__skb_checksum_complete(skb))
                        return -EINVAL;
                if (skb_copy_datagram_msg(skb, hlen, msg, chunk))
                        goto fault;
        } else {
                csum = csum_partial(skb->data, hlen, skb->csum);
                if (skb_copy_and_csum_datagram(skb, hlen, &msg->msg_iter,
                                               chunk, &csum))
                        goto fault;

                if (csum_fold(csum)) {
                        iov_iter_revert(&msg->msg_iter, chunk);
                        return -EINVAL;
                }

                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE))
                        netdev_rx_csum_fault(skb->dev);
        }
        return 0;
fault:
        return -EFAULT;
}
EXPORT_SYMBOL(skb_copy_and_csum_datagram_msg);

/**
 *      datagram_poll - generic datagram poll
 *      @file: file struct
 *      @sock: socket
 *      @wait: poll table
 *
 *      Datagram poll: Again totally generic. This also handles
 *      sequenced packet sockets providing the socket receive queue
 *      is only ever holding data ready to receive.
 *
 *      Note: when you *don't* use this routine for this protocol,
 *      and you use a different write policy from sock_writeable()
 *      then please supply your own write_space callback.
 */
unsigned int datagram_poll(struct file *file, struct socket *sock,
                           poll_table *wait)
{
        struct sock *sk = sock->sk;
        unsigned int mask;

        sock_poll_wait(file, sk_sleep(sk), wait);
        mask = 0;

        /* exceptional events? */
        if (sk->sk_err || !skb_queue_empty(&sk->sk_error_queue))
                mask |= POLLERR |
                        (sock_flag(sk, SOCK_SELECT_ERR_QUEUE) ? POLLPRI : 0);

        if (sk->sk_shutdown & RCV_SHUTDOWN)
                mask |= POLLRDHUP | POLLIN | POLLRDNORM;
        if (sk->sk_shutdown == SHUTDOWN_MASK)
                mask |= POLLHUP;

        /* readable? */
        if (!skb_queue_empty(&sk->sk_receive_queue))
                mask |= POLLIN | POLLRDNORM;

        /* Connection-based need to check for termination and startup */
        if (connection_based(sk)) {
                if (sk->sk_state == TCP_CLOSE)
                        mask |= POLLHUP;
                /* connection hasn't started yet? */
                if (sk->sk_state == TCP_SYN_SENT)
                        return mask;
        }

        /* writable? */
        if (sock_writeable(sk))
                mask |= POLLOUT | POLLWRNORM | POLLWRBAND;
        else
                sk_set_bit(SOCKWQ_ASYNC_NOSPACE, sk);

        return mask;
}
EXPORT_SYMBOL(datagram_poll);