Re: [PATCH 03/11] xfs: remote attribute read too short

Dave Chinner
2013-05-21T22:53:39Z

On Tue, May 21, 2013 at 03:59:24PM -0500, Ben Myers wrote:
> On Tue, May 21, 2013 at 06:02:02PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > Reading a maximally size remote attribute fails when CRCs are
> > enabled with this verification error:
> >
> > XFS (vdb): remote attribute header does not match required off/len/owner)
> >
> > There are two reasons for this, the first being that the
> > length of the buffer being read is determined from the
> > args->rmtblkcnt which doesn't take into account CRC headers. Hence
> > the mapped length ends up being too short and so we need to
> > calculate it directly from the value length.
> >
> > The second is that the byte count of valid data within a buffer is
> > capped by the length of the data and so doesn't take into account
> > that the buffer might be longer due to headers. Hence we need to
> > calculate the data space in the buffer first before calculating the
> > actual byte count of data.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> > fs/xfs/xfs_attr_remote.c | 15 +++++++++------
> > 1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_attr_remote.c b/fs/xfs/xfs_attr_remote.c
> > index aad95b0..bcdc07c 100644
> > --- a/fs/xfs/xfs_attr_remote.c
> > +++ b/fs/xfs/xfs_attr_remote.c
> > @@ -52,9 +52,11 @@ xfs_attr3_rmt_blocks(
> > struct xfs_mount *mp,
> > int attrlen)
> > {
> > - int buflen = XFS_ATTR3_RMT_BUF_SPACE(mp,
> > - mp->m_sb.sb_blocksize);
> > - return (attrlen + buflen - 1) / buflen;
> > + if (xfs_sb_version_hascrc(&mp->m_sb)) {
> > + int buflen = XFS_ATTR3_RMT_BUF_SPACE(mp, mp->m_sb.sb_blocksize);
> > + return (attrlen + buflen - 1) / buflen;
> > + }
> > + return XFS_B_TO_FSB(mp, attrlen);
>
> xfs_attr_rmtval_set, xfs_attr3_rmt_blocks, and XFS_ATTR3_RMT_BUF_SPACE all have
> checks for crcs on the superblock. It's like I'm seeing stars.

Sure - I'm not concerned about how efficient the code is at this
point. I just want it to work correctly. If that means we check for
CRC support more frequently than we need to, then that's no big deal
right now.

>
> > }
> >
> > static bool
> > @@ -206,8 +208,9 @@ xfs_attr_rmtval_get(
> >
> > while (valuelen > 0) {
> > nmap = ATTR_RMTVALUE_MAPSIZE;
> > + blkcnt = xfs_attr3_rmt_blocks(mp, valuelen);
> > error = xfs_bmapi_read(args->dp, (xfs_fileoff_t)lblkno,
> > - args->rmtblkcnt, map, &nmap,
> > + blkcnt, map, &nmap,
>
> Isn't blkcnt wrong on the 2nd or greater iteration of this loop? Looks like an old bug.

Yes, args->rmtblkcnt is wrong on the 2nd+ loop iteration. And yes,
it's an old, old bug.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com