[PATCH 04/28] xdr: add a new utility function to shift the head data of an xdr buffer

[Trond Myklebust]

On Mon, 2008-03-31 at 10:31 -0400, Kevin Coffman wrote:
> Add a new xdr helper function that shifts the data in an xdr
> buffer's head to make room for new data.
> 
> Also add an export for the helper function  write_bytes_to_xdr_buf().
> 
> Signed-off-by: Kevin Coffman <kwc at citi.umich.edu>
> ---
> 
>  include/linux/sunrpc/xdr.h |    2 ++
>  net/sunrpc/xdr.c           |   29 +++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
> index e4057d7..518d47c 100644
> --- a/include/linux/sunrpc/xdr.h
> +++ b/include/linux/sunrpc/xdr.h
> @@ -164,6 +164,8 @@ extern int xdr_buf_subsegment(struct xdr_buf *, struct xdr_buf *, unsigned int,
>  extern int xdr_buf_read_netobj(struct xdr_buf *, struct xdr_netobj *, unsigned int);
>  extern int read_bytes_from_xdr_buf(struct xdr_buf *, unsigned int, void *, unsigned int);
>  extern int write_bytes_to_xdr_buf(struct xdr_buf *, unsigned int, void *, unsigned int);
> +extern int xdr_shift_head_data(struct xdr_buf *, int, unsigned int);
> +
>  
>  /*
>   * Helper structure for copying from an sk_buff.
> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> index 995c3fd..de2c986 100644
> --- a/net/sunrpc/xdr.c
> +++ b/net/sunrpc/xdr.c
> @@ -428,6 +428,34 @@ xdr_shift_buf(struct xdr_buf *buf, size_t len)
>  EXPORT_SYMBOL(xdr_shift_buf);
>  
>  /**
> + * xdr_xdr_shift_head_data - Make room for new data within buf->head
> + * @buf: pointer to XDR buffer
> + * @offset: offset within the head to start moving data
      ^^^^^^^ @base: for consistency with similar XDR helpers

> + * @shiftlen: number of bytes to shift the data
> + */
> +int
> +xdr_shift_head_data(struct xdr_buf *buf, int offset, unsigned int shiftlen)
> +{
> +	u8 *p;
> +
> +	if (shiftlen == 0)
> +		return 0;
> +	/* make sure there is room (assuming head has a page) */
> +	if (buf->head[0].iov_len + shiftlen + offset > PAGE_CACHE_SIZE)
> +		return 1;
> +
> +	p = buf->head[0].iov_base + offset;
> +
> +	memmove(p + shiftlen, p, buf->head[0].iov_len - offset);
> +
> +	buf->head[0].iov_len += shiftlen;
> +	buf->len += shiftlen;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(xdr_shift_head_data);

You had better be certain that you don't have any tail data when doing
this. In almost all cases, the tail data will be allocated immediately
after the head data and so you end up scribbling over it.

> +/**
>   * 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
> @@ -758,6 +786,7 @@ int write_bytes_to_xdr_buf(struct xdr_buf *buf, unsigned int base, void *obj, un
>  	__write_bytes_to_xdr_buf(&subbuf, obj, len);
>  	return 0;
>  }
> +EXPORT_SYMBOL(write_bytes_to_xdr_buf);
>  
>  int
>  xdr_decode_word(struct xdr_buf *buf, unsigned int base, u32 *obj)
> 
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