Re: [Qemu-devel] [PATCH] 9pfs: fix offset error in v9fs_xattr_read()

[Stefano Stabellini]

On Sat, 21 Jan 2017, Greg Kurz wrote:
> The current code tries to copy `read_count' bytes starting at offset
> `offset' from a `read_count`-sized iovec. This causes v9fs_pack() to
> fail with ENOBUFS.
> 
> Since the PDU iovec is already partially filled with `offset' bytes,
> let's skip them when creating `qiov_full' and have v9fs_pack() to
> copy the whole of it. Moreover, this is consistent with the other
> places where v9fs_init_qiov_from_pdu() is called.
> 
> This fixes commit "bcb8998fac16 9pfs: call v9fs_init_qiov_from_pdu
> before v9fs_pack".

Sorry about that!


> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  hw/9pfs/9p.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index fa58877570f6..f0eef1a3ef53 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1685,8 +1685,8 @@ static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, 
> V9fsFidState *fidp,
>      }
>      offset += err;
>  
> -    v9fs_init_qiov_from_pdu(&qiov_full, pdu, 0, read_count, false);
> -    err = v9fs_pack(qiov_full.iov, qiov_full.niov, offset,
> +    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, read_count, false);

v9fs_init_qiov_from_pdu calls init_in_iov_from_pdu passing read_count as
size argument. offset is not passed to init_in_iov_from_pdu, it is only
used to initialized qiov_full.

In other words, don't we need to:

    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, read_count + offset, 
false);

To make sure that qiov_full has "read_count + offset" bytes in it, and
qiov_full is initialized skipping the first "offset" bytes?


> +    err = v9fs_pack(qiov_full.iov, qiov_full.niov, 0,
>                      ((char *)fidp->fs.xattr.value) + off,
>                      read_count);
>      qemu_iovec_destroy(&qiov_full);
>