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

[Greg Kurz <groug@kaod.org>]

On Mon, 23 Jan 2017 12:20:57 -0800 (PST)
Stefano Stabellini <sstabellini@kernel.org> wrote:

> 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 <groug@kaod.org>
> > ---
> >  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?
> 

If we do that then qemu_iovec_concat() will skip offset bytes and then copy
read_count + offset bytes or am I missing something ?

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