[TG3]: Set minimal hw interrupt mitigation.

[linux-2.6/verdex.git] / net / sunrpc / xdr.c

/*
 * linux/net/sunrpc/xdr.c
 *
 * Generic XDR support.
 *
 * Copyright (C) 1995, 1996 Olaf Kirch <okir@monad.swb.de>
 */

#include <linux/types.h>
#include <linux/socket.h>
#include <linux/string.h>
#include <linux/kernel.h>
#include <linux/pagemap.h>
#include <linux/errno.h>
#include <linux/in.h>
#include <linux/net.h>
#include <net/sock.h>
#include <linux/sunrpc/xdr.h>
#include <linux/sunrpc/msg_prot.h>

/*
 * XDR functions for basic NFS types
 */
u32 *
xdr_encode_netobj(u32 *p, const struct xdr_netobj *obj)
{
        unsigned int    quadlen = XDR_QUADLEN(obj->len);

        p[quadlen] = 0;         /* zero trailing bytes */
        *p++ = htonl(obj->len);
        memcpy(p, obj->data, obj->len);
        return p + XDR_QUADLEN(obj->len);
}

u32 *
xdr_decode_netobj(u32 *p, struct xdr_netobj *obj)
{
        unsigned int    len;

        if ((len = ntohl(*p++)) > XDR_MAX_NETOBJ)
                return NULL;
        obj->len  = len;
        obj->data = (u8 *) p;
        return p + XDR_QUADLEN(len);
}

/**
 * xdr_encode_opaque_fixed - Encode fixed length opaque data
 * @p: pointer to current position in XDR buffer.
 * @ptr: pointer to data to encode (or NULL)
 * @nbytes: size of data.
 *
 * Copy the array of data of length nbytes at ptr to the XDR buffer
 * at position p, then align to the next 32-bit boundary by padding
 * with zero bytes (see RFC1832).
 * Note: if ptr is NULL, only the padding is performed.
 *
 * Returns the updated current XDR buffer position
 *
 */
u32 *xdr_encode_opaque_fixed(u32 *p, const void *ptr, unsigned int nbytes)
{
        if (likely(nbytes != 0)) {
                unsigned int quadlen = XDR_QUADLEN(nbytes);
                unsigned int padding = (quadlen << 2) - nbytes;

                if (ptr != NULL)
                        memcpy(p, ptr, nbytes);
                if (padding != 0)
                        memset((char *)p + nbytes, 0, padding);
                p += quadlen;
        }
        return p;
}
EXPORT_SYMBOL(xdr_encode_opaque_fixed);

/**
 * xdr_encode_opaque - Encode variable length opaque data
 * @p: pointer to current position in XDR buffer.
 * @ptr: pointer to data to encode (or NULL)
 * @nbytes: size of data.
 *
 * Returns the updated current XDR buffer position
 */
u32 *xdr_encode_opaque(u32 *p, const void *ptr, unsigned int nbytes)
{
        *p++ = htonl(nbytes);
        return xdr_encode_opaque_fixed(p, ptr, nbytes);
}
EXPORT_SYMBOL(xdr_encode_opaque);

u32 *
xdr_encode_string(u32 *p, const char *string)
{
        return xdr_encode_array(p, string, strlen(string));
}

u32 *
xdr_decode_string(u32 *p, char **sp, int *lenp, int maxlen)
{
        unsigned int    len;
        char            *string;

        if ((len = ntohl(*p++)) > maxlen)
                return NULL;
        if (lenp)
                *lenp = len;
        if ((len % 4) != 0) {
                string = (char *) p;
        } else {
                string = (char *) (p - 1);
                memmove(string, p, len);
        }
        string[len] = '\0';
        *sp = string;
        return p + XDR_QUADLEN(len);
}

u32 *
xdr_decode_string_inplace(u32 *p, char **sp, int *lenp, int maxlen)
{
        unsigned int    len;

        if ((len = ntohl(*p++)) > maxlen)
                return NULL;
        *lenp = len;
        *sp = (char *) p;
        return p + XDR_QUADLEN(len);
}

void
xdr_encode_pages(struct xdr_buf *xdr, struct page **pages, unsigned int base,
                 unsigned int len)
{
        struct kvec *tail = xdr->tail;
        u32 *p;

        xdr->pages = pages;
        xdr->page_base = base;
        xdr->page_len = len;

        p = (u32 *)xdr->head[0].iov_base + XDR_QUADLEN(xdr->head[0].iov_len);
        tail->iov_base = p;
        tail->iov_len = 0;

        if (len & 3) {
                unsigned int pad = 4 - (len & 3);

                *p = 0;
                tail->iov_base = (char *)p + (len & 3);
                tail->iov_len  = pad;
                len += pad;
        }
        xdr->buflen += len;
        xdr->len += len;
}

void
xdr_inline_pages(struct xdr_buf *xdr, unsigned int offset,
                 struct page **pages, unsigned int base, unsigned int len)
{
        struct kvec *head = xdr->head;
        struct kvec *tail = xdr->tail;
        char *buf = (char *)head->iov_base;
        unsigned int buflen = head->iov_len;

        head->iov_len  = offset;

        xdr->pages = pages;
        xdr->page_base = base;
        xdr->page_len = len;

        tail->iov_base = buf + offset;
        tail->iov_len = buflen - offset;

        xdr->buflen += len;
}

void
xdr_partial_copy_from_skb(struct xdr_buf *xdr, unsigned int base,
                          skb_reader_t *desc,
                          skb_read_actor_t copy_actor)
{
        struct page     **ppage = xdr->pages;
        unsigned int    len, pglen = xdr->page_len;
        int             ret;

        len = xdr->head[0].iov_len;
        if (base < len) {
                len -= base;
                ret = copy_actor(desc, (char *)xdr->head[0].iov_base + base, len);
                if (ret != len || !desc->count)
                        return;
                base = 0;
        } else
                base -= len;

        if (pglen == 0)
                goto copy_tail;
        if (base >= pglen) {
                base -= pglen;
                goto copy_tail;
        }
        if (base || xdr->page_base) {
                pglen -= base;
                base  += xdr->page_base;
                ppage += base >> PAGE_CACHE_SHIFT;
                base &= ~PAGE_CACHE_MASK;
        }
        do {
                char *kaddr;

                len = PAGE_CACHE_SIZE;
                kaddr = kmap_atomic(*ppage, KM_SKB_SUNRPC_DATA);
                if (base) {
                        len -= base;
                        if (pglen < len)
                                len = pglen;
                        ret = copy_actor(desc, kaddr + base, len);
                        base = 0;
                } else {
                        if (pglen < len)
                                len = pglen;
                        ret = copy_actor(desc, kaddr, len);
                }
                flush_dcache_page(*ppage);
                kunmap_atomic(kaddr, KM_SKB_SUNRPC_DATA);
                if (ret != len || !desc->count)
                        return;
                ppage++;
        } while ((pglen -= len) != 0);
copy_tail:
        len = xdr->tail[0].iov_len;
        if (base < len)
                copy_actor(desc, (char *)xdr->tail[0].iov_base + base, len - base);
}


int
xdr_sendpages(struct socket *sock, struct sockaddr *addr, int addrlen,
                struct xdr_buf *xdr, unsigned int base, int msgflags)
{
        struct page **ppage = xdr->pages;
        unsigned int len, pglen = xdr->page_len;
        int err, ret = 0;
        ssize_t (*sendpage)(struct socket *, struct page *, int, size_t, int);

        len = xdr->head[0].iov_len;
        if (base < len || (addr != NULL && base == 0)) {
                struct kvec iov = {
                        .iov_base = xdr->head[0].iov_base + base,
                        .iov_len  = len - base,
                };
                struct msghdr msg = {
                        .msg_name    = addr,
                        .msg_namelen = addrlen,
                        .msg_flags   = msgflags,
                };
                if (xdr->len > len)
                        msg.msg_flags |= MSG_MORE;

                if (iov.iov_len != 0)
                        err = kernel_sendmsg(sock, &msg, &iov, 1, iov.iov_len);
                else
                        err = kernel_sendmsg(sock, &msg, NULL, 0, 0);
                if (ret == 0)
                        ret = err;
                else if (err > 0)
                        ret += err;
                if (err != iov.iov_len)
                        goto out;
                base = 0;
        } else
                base -= len;

        if (pglen == 0)
                goto copy_tail;
        if (base >= pglen) {
                base -= pglen;
                goto copy_tail;
        }
        if (base || xdr->page_base) {
                pglen -= base;
                base  += xdr->page_base;
                ppage += base >> PAGE_CACHE_SHIFT;
                base &= ~PAGE_CACHE_MASK;
        }

        sendpage = sock->ops->sendpage ? : sock_no_sendpage;
        do {
                int flags = msgflags;

                len = PAGE_CACHE_SIZE;
                if (base)
                        len -= base;
                if (pglen < len)
                        len = pglen;

                if (pglen != len || xdr->tail[0].iov_len != 0)
                        flags |= MSG_MORE;

                /* Hmm... We might be dealing with highmem pages */
                if (PageHighMem(*ppage))
                        sendpage = sock_no_sendpage;
                err = sendpage(sock, *ppage, base, len, flags);
                if (ret == 0)
                        ret = err;
                else if (err > 0)
                        ret += err;
                if (err != len)
                        goto out;
                base = 0;
                ppage++;
        } while ((pglen -= len) != 0);
copy_tail:
        len = xdr->tail[0].iov_len;
        if (base < len) {
                struct kvec iov = {
                        .iov_base = xdr->tail[0].iov_base + base,
                        .iov_len  = len - base,
                };
                struct msghdr msg = {
                        .msg_flags   = msgflags,
                };
                err = kernel_sendmsg(sock, &msg, &iov, 1, iov.iov_len);
                if (ret == 0)
                        ret = err;
                else if (err > 0)
                        ret += err;
        }
out:
        return ret;
}


/*
 * Helper routines for doing 'memmove' like operations on a struct xdr_buf
 *
 * _shift_data_right_pages
 * @pages: vector of pages containing both the source and dest memory area.
 * @pgto_base: page vector address of destination
 * @pgfrom_base: page vector address of source
 * @len: number of bytes to copy
 *
 * Note: the addresses pgto_base and pgfrom_base are both calculated in
 *       the same way:
 *            if a memory area starts at byte 'base' in page 'pages[i]',
 *            then its address is given as (i << PAGE_CACHE_SHIFT) + base
 * Also note: pgfrom_base must be < pgto_base, but the memory areas
 *      they point to may overlap.
 */
static void
_shift_data_right_pages(struct page **pages, size_t pgto_base,
                size_t pgfrom_base, size_t len)
{
        struct page **pgfrom, **pgto;
        char *vfrom, *vto;
        size_t copy;

        BUG_ON(pgto_base <= pgfrom_base);

        pgto_base += len;
        pgfrom_base += len;

        pgto = pages + (pgto_base >> PAGE_CACHE_SHIFT);
        pgfrom = pages + (pgfrom_base >> PAGE_CACHE_SHIFT);

        pgto_base &= ~PAGE_CACHE_MASK;
        pgfrom_base &= ~PAGE_CACHE_MASK;

        do {
                /* Are any pointers crossing a page boundary? */
                if (pgto_base == 0) {
                        flush_dcache_page(*pgto);
                        pgto_base = PAGE_CACHE_SIZE;
                        pgto--;
                }
                if (pgfrom_base == 0) {
                        pgfrom_base = PAGE_CACHE_SIZE;
                        pgfrom--;
                }

                copy = len;
                if (copy > pgto_base)
                        copy = pgto_base;
                if (copy > pgfrom_base)
                        copy = pgfrom_base;
                pgto_base -= copy;
                pgfrom_base -= copy;

                vto = kmap_atomic(*pgto, KM_USER0);
                vfrom = kmap_atomic(*pgfrom, KM_USER1);
                memmove(vto + pgto_base, vfrom + pgfrom_base, copy);
                kunmap_atomic(vfrom, KM_USER1);
                kunmap_atomic(vto, KM_USER0);

        } while ((len -= copy) != 0);
        flush_dcache_page(*pgto);
}

/*
 * _copy_to_pages
 * @pages: array of pages
 * @pgbase: page vector address of destination
 * @p: pointer to source data
 * @len: length
 *
 * Copies data from an arbitrary memory location into an array of pages
 * The copy is assumed to be non-overlapping.
 */
static void
_copy_to_pages(struct page **pages, size_t pgbase, const char *p, size_t len)
{
        struct page **pgto;
        char *vto;
        size_t copy;

        pgto = pages + (pgbase >> PAGE_CACHE_SHIFT);
        pgbase &= ~PAGE_CACHE_MASK;

        do {
                copy = PAGE_CACHE_SIZE - pgbase;
                if (copy > len)
                        copy = len;

                vto = kmap_atomic(*pgto, KM_USER0);
                memcpy(vto + pgbase, p, copy);
                kunmap_atomic(vto, KM_USER0);

                pgbase += copy;
                if (pgbase == PAGE_CACHE_SIZE) {
                        flush_dcache_page(*pgto);
                        pgbase = 0;
                        pgto++;
                }
                p += copy;

        } while ((len -= copy) != 0);
        flush_dcache_page(*pgto);
}

/*
 * _copy_from_pages
 * @p: pointer to destination
 * @pages: array of pages
 * @pgbase: offset of source data
 * @len: length
 *
 * Copies data into an arbitrary memory location from an array of pages
 * The copy is assumed to be non-overlapping.
 */
static void
_copy_from_pages(char *p, struct page **pages, size_t pgbase, size_t len)
{
        struct page **pgfrom;
        char *vfrom;
        size_t copy;

        pgfrom = pages + (pgbase >> PAGE_CACHE_SHIFT);
        pgbase &= ~PAGE_CACHE_MASK;

        do {
                copy = PAGE_CACHE_SIZE - pgbase;
                if (copy > len)
                        copy = len;

                vfrom = kmap_atomic(*pgfrom, KM_USER0);
                memcpy(p, vfrom + pgbase, copy);
                kunmap_atomic(vfrom, KM_USER0);

                pgbase += copy;
                if (pgbase == PAGE_CACHE_SIZE) {
                        pgbase = 0;
                        pgfrom++;
                }
                p += copy;

        } while ((len -= copy) != 0);
}

/*
 * xdr_shrink_bufhead
 * @buf: xdr_buf
 * @len: bytes to remove from buf->head[0]
 *
 * Shrinks XDR buffer's header kvec buf->head[0] by 
 * 'len' bytes. The extra data is not lost, but is instead
 * moved into the inlined pages and/or the tail.
 */
static void
xdr_shrink_bufhead(struct xdr_buf *buf, size_t len)
{
        struct kvec *head, *tail;
        size_t copy, offs;
        unsigned int pglen = buf->page_len;

        tail = buf->tail;
        head = buf->head;
        BUG_ON (len > head->iov_len);

        /* Shift the tail first */
        if (tail->iov_len != 0) {
                if (tail->iov_len > len) {
                        copy = tail->iov_len - len;
                        memmove((char *)tail->iov_base + len,
                                        tail->iov_base, copy);
                }
                /* Copy from the inlined pages into the tail */
                copy = len;
                if (copy > pglen)
                        copy = pglen;
                offs = len - copy;
                if (offs >= tail->iov_len)
                        copy = 0;
                else if (copy > tail->iov_len - offs)
                        copy = tail->iov_len - offs;
                if (copy != 0)
                        _copy_from_pages((char *)tail->iov_base + offs,
                                        buf->pages,
                                        buf->page_base + pglen + offs - len,
                                        copy);
                /* Do we also need to copy data from the head into the tail ? */
                if (len > pglen) {
                        offs = copy = len - pglen;
                        if (copy > tail->iov_len)
                                copy = tail->iov_len;
                        memcpy(tail->iov_base,
                                        (char *)head->iov_base +
                                        head->iov_len - offs,
                                        copy);
                }
        }
        /* Now handle pages */
        if (pglen != 0) {
                if (pglen > len)
                        _shift_data_right_pages(buf->pages,
                                        buf->page_base + len,
                                        buf->page_base,
                                        pglen - len);
                copy = len;
                if (len > pglen)
                        copy = pglen;
                _copy_to_pages(buf->pages, buf->page_base,
                                (char *)head->iov_base + head->iov_len - len,
                                copy);
        }
        head->iov_len -= len;
        buf->buflen -= len;
        /* Have we truncated the message? */
        if (buf->len > buf->buflen)
                buf->len = buf->buflen;
}

/*
 * xdr_shrink_pagelen
 * @buf: xdr_buf
 * @len: bytes to remove from buf->pages
 *
 * Shrinks XDR buffer's page array buf->pages by 
 * 'len' bytes. The extra data is not lost, but is instead
 * moved into the tail.
 */
static void
xdr_shrink_pagelen(struct xdr_buf *buf, size_t len)
{
        struct kvec *tail;
        size_t copy;
        char *p;
        unsigned int pglen = buf->page_len;

        tail = buf->tail;
        BUG_ON (len > pglen);

        /* Shift the tail first */
        if (tail->iov_len != 0) {
                p = (char *)tail->iov_base + len;
                if (tail->iov_len > len) {
                        copy = tail->iov_len - len;
                        memmove(p, tail->iov_base, copy);
                } else
                        buf->buflen -= len;
                /* Copy from the inlined pages into the tail */
                copy = len;
                if (copy > tail->iov_len)
                        copy = tail->iov_len;
                _copy_from_pages((char *)tail->iov_base,
                                buf->pages, buf->page_base + pglen - len,
                                copy);
        }
        buf->page_len -= len;
        buf->buflen -= len;
        /* Have we truncated the message? */
        if (buf->len > buf->buflen)
                buf->len = buf->buflen;
}

void
xdr_shift_buf(struct xdr_buf *buf, size_t len)
{
        xdr_shrink_bufhead(buf, len);
}

/**
 * xdr_init_encode - Initialize a struct xdr_stream for sending data.
 * @xdr: pointer to xdr_stream struct
 * @buf: pointer to XDR buffer in which to encode data
 * @p: current pointer inside XDR buffer
 *
 * Note: at the moment the RPC client only passes the length of our
 *       scratch buffer in the xdr_buf's header kvec. Previously this
 *       meant we needed to call xdr_adjust_iovec() after encoding the
 *       data. With the new scheme, the xdr_stream manages the details
 *       of the buffer length, and takes care of adjusting the kvec
 *       length for us.
 */
void xdr_init_encode(struct xdr_stream *xdr, struct xdr_buf *buf, uint32_t *p)
{
        struct kvec *iov = buf->head;

        xdr->buf = buf;
        xdr->iov = iov;
        xdr->end = (uint32_t *)((char *)iov->iov_base + iov->iov_len);
        buf->len = iov->iov_len = (char *)p - (char *)iov->iov_base;
        xdr->p = p;
}
EXPORT_SYMBOL(xdr_init_encode);

/**
 * xdr_reserve_space - Reserve buffer space for sending
 * @xdr: pointer to xdr_stream
 * @nbytes: number of bytes to reserve
 *
 * Checks that we have enough buffer space to encode 'nbytes' more
 * bytes of data. If so, update the total xdr_buf length, and
 * adjust the length of the current kvec.
 */
uint32_t * xdr_reserve_space(struct xdr_stream *xdr, size_t nbytes)
{
        uint32_t *p = xdr->p;
        uint32_t *q;

        /* align nbytes on the next 32-bit boundary */
        nbytes += 3;
        nbytes &= ~3;
        q = p + (nbytes >> 2);
        if (unlikely(q > xdr->end || q < p))
                return NULL;
        xdr->p = q;
        xdr->iov->iov_len += nbytes;
        xdr->buf->len += nbytes;
        return p;
}
EXPORT_SYMBOL(xdr_reserve_space);

/**
 * xdr_write_pages - Insert a list of pages into an XDR buffer for sending
 * @xdr: pointer to xdr_stream
 * @pages: list of pages
 * @base: offset of first byte
 * @len: length of data in bytes
 *
 */
void xdr_write_pages(struct xdr_stream *xdr, struct page **pages, unsigned int base,
                 unsigned int len)
{
        struct xdr_buf *buf = xdr->buf;
        struct kvec *iov = buf->tail;
        buf->pages = pages;
        buf->page_base = base;
        buf->page_len = len;

        iov->iov_base = (char *)xdr->p;
        iov->iov_len  = 0;
        xdr->iov = iov;

        if (len & 3) {
                unsigned int pad = 4 - (len & 3);

                BUG_ON(xdr->p >= xdr->end);
                iov->iov_base = (char *)xdr->p + (len & 3);
                iov->iov_len  += pad;
                len += pad;
                *xdr->p++ = 0;
        }
        buf->buflen += len;
        buf->len += len;
}
EXPORT_SYMBOL(xdr_write_pages);

/**
 * xdr_init_decode - Initialize an xdr_stream for decoding data.
 * @xdr: pointer to xdr_stream struct
 * @buf: pointer to XDR buffer from which to decode data
 * @p: current pointer inside XDR buffer
 */
void xdr_init_decode(struct xdr_stream *xdr, struct xdr_buf *buf, uint32_t *p)
{
        struct kvec *iov = buf->head;
        unsigned int len = iov->iov_len;

        if (len > buf->len)
                len = buf->len;
        xdr->buf = buf;
        xdr->iov = iov;
        xdr->p = p;
        xdr->end = (uint32_t *)((char *)iov->iov_base + len);
}
EXPORT_SYMBOL(xdr_init_decode);

/**
 * xdr_inline_decode - Retrieve non-page XDR data to decode
 * @xdr: pointer to xdr_stream struct
 * @nbytes: number of bytes of data to decode
 *
 * Check if the input buffer is long enough to enable us to decode
 * 'nbytes' more bytes of data starting at the current position.
 * If so return the current pointer, then update the current
 * pointer position.
 */
uint32_t * xdr_inline_decode(struct xdr_stream *xdr, size_t nbytes)
{
        uint32_t *p = xdr->p;
        uint32_t *q = p + XDR_QUADLEN(nbytes);

        if (unlikely(q > xdr->end || q < p))
                return NULL;
        xdr->p = q;
        return p;
}
EXPORT_SYMBOL(xdr_inline_decode);

/**
 * xdr_read_pages - Ensure page-based XDR data to decode is aligned at current pointer position
 * @xdr: pointer to xdr_stream struct
 * @len: number of bytes of page data
 *
 * Moves data beyond the current pointer position from the XDR head[] buffer
 * into the page list. Any data that lies beyond current position + "len"
 * bytes is moved into the XDR tail[]. The current pointer is then
 * repositioned at the beginning of the XDR tail.
 */
void xdr_read_pages(struct xdr_stream *xdr, unsigned int len)
{
        struct xdr_buf *buf = xdr->buf;
        struct kvec *iov;
        ssize_t shift;
        unsigned int end;
        int padding;

        /* Realign pages to current pointer position */
        iov  = buf->head;
        shift = iov->iov_len + (char *)iov->iov_base - (char *)xdr->p;
        if (shift > 0)
                xdr_shrink_bufhead(buf, shift);

        /* Truncate page data and move it into the tail */
        if (buf->page_len > len)
                xdr_shrink_pagelen(buf, buf->page_len - len);
        padding = (XDR_QUADLEN(len) << 2) - len;
        xdr->iov = iov = buf->tail;
        /* Compute remaining message length.  */
        end = iov->iov_len;
        shift = buf->buflen - buf->len;
        if (shift < end)
                end -= shift;
        else if (shift > 0)
                end = 0;
        /*
         * Position current pointer at beginning of tail, and
         * set remaining message length.
         */
        xdr->p = (uint32_t *)((char *)iov->iov_base + padding);
        xdr->end = (uint32_t *)((char *)iov->iov_base + end);
}
EXPORT_SYMBOL(xdr_read_pages);

static struct kvec empty_iov = {.iov_base = NULL, .iov_len = 0};

void
xdr_buf_from_iov(struct kvec *iov, struct xdr_buf *buf)
{
        buf->head[0] = *iov;
        buf->tail[0] = empty_iov;
        buf->page_len = 0;
        buf->buflen = buf->len = iov->iov_len;
}

/* Sets subiov to the intersection of iov with the buffer of length len
 * starting base bytes after iov.  Indicates empty intersection by setting
 * length of subiov to zero.  Decrements len by length of subiov, sets base
 * to zero (or decrements it by length of iov if subiov is empty). */
static void
iov_subsegment(struct kvec *iov, struct kvec *subiov, int *base, int *len)
{
        if (*base > iov->iov_len) {
                subiov->iov_base = NULL;
                subiov->iov_len = 0;
                *base -= iov->iov_len;
        } else {
                subiov->iov_base = iov->iov_base + *base;
                subiov->iov_len = min(*len, (int)iov->iov_len - *base);
                *base = 0;
        }
        *len -= subiov->iov_len; 
}

/* Sets subbuf to the portion of buf of length len beginning base bytes
 * from the start of buf. Returns -1 if base of length are out of bounds. */
int
xdr_buf_subsegment(struct xdr_buf *buf, struct xdr_buf *subbuf,
                        int base, int len)
{
        int i;

        subbuf->buflen = subbuf->len = len;
        iov_subsegment(buf->head, subbuf->head, &base, &len);

        if (base < buf->page_len) {
                i = (base + buf->page_base) >> PAGE_CACHE_SHIFT;
                subbuf->pages = &buf->pages[i];
                subbuf->page_base = (base + buf->page_base) & ~PAGE_CACHE_MASK;
                subbuf->page_len = min((int)buf->page_len - base, len);
                len -= subbuf->page_len;
                base = 0;
        } else {
                base -= buf->page_len;
                subbuf->page_len = 0;
        }

        iov_subsegment(buf->tail, subbuf->tail, &base, &len);
        if (base || len)
                return -1;
        return 0;
}

/* obj is assumed to point to allocated memory of size at least len: */
int
read_bytes_from_xdr_buf(struct xdr_buf *buf, int base, void *obj, int len)
{
        struct xdr_buf subbuf;
        int this_len;
        int status;

        status = xdr_buf_subsegment(buf, &subbuf, base, len);
        if (status)
                goto out;
        this_len = min(len, (int)subbuf.head[0].iov_len);
        memcpy(obj, subbuf.head[0].iov_base, this_len);
        len -= this_len;
        obj += this_len;
        this_len = min(len, (int)subbuf.page_len);
        if (this_len)
                _copy_from_pages(obj, subbuf.pages, subbuf.page_base, this_len);
        len -= this_len;
        obj += this_len;
        this_len = min(len, (int)subbuf.tail[0].iov_len);
        memcpy(obj, subbuf.tail[0].iov_base, this_len);
out:
        return status;
}

static int
read_u32_from_xdr_buf(struct xdr_buf *buf, int base, u32 *obj)
{
        u32     raw;
        int     status;

        status = read_bytes_from_xdr_buf(buf, base, &raw, sizeof(*obj));
        if (status)
                return status;
        *obj = ntohl(raw);
        return 0;
}

/* If the netobj starting offset bytes from the start of xdr_buf is contained
 * entirely in the head or the tail, set object to point to it; otherwise
 * try to find space for it at the end of the tail, copy it there, and
 * set obj to point to it. */
int
xdr_buf_read_netobj(struct xdr_buf *buf, struct xdr_netobj *obj, int offset)
{
        u32     tail_offset = buf->head[0].iov_len + buf->page_len;
        u32     obj_end_offset;

        if (read_u32_from_xdr_buf(buf, offset, &obj->len))
                goto out;
        obj_end_offset = offset + 4 + obj->len;

        if (obj_end_offset <= buf->head[0].iov_len) {
                /* The obj is contained entirely in the head: */
                obj->data = buf->head[0].iov_base + offset + 4;
        } else if (offset + 4 >= tail_offset) {
                if (obj_end_offset - tail_offset
                                > buf->tail[0].iov_len)
                        goto out;
                /* The obj is contained entirely in the tail: */
                obj->data = buf->tail[0].iov_base
                        + offset - tail_offset + 4;
        } else {
                /* use end of tail as storage for obj:
                 * (We don't copy to the beginning because then we'd have
                 * to worry about doing a potentially overlapping copy.
                 * This assumes the object is at most half the length of the
                 * tail.) */
                if (obj->len > buf->tail[0].iov_len)
                        goto out;
                obj->data = buf->tail[0].iov_base + buf->tail[0].iov_len - 
                                obj->len;
                if (read_bytes_from_xdr_buf(buf, offset + 4,
                                        obj->data, obj->len))
                        goto out;

        }
        return 0;
out:
        return -1;
}