Re: [PATCH v2 19/23] xfs: disable direct read path for fs-verity sealed files

Darrick J. Wong
2023-04-05T15:12:26Z

On Wed, Apr 05, 2023 at 05:01:42PM +0200, Andrey Albershteyn wrote:
> On Tue, Apr 04, 2023 at 09:10:47AM -0700, Darrick J. Wong wrote:
> > On Tue, Apr 04, 2023 at 04:53:15PM +0200, Andrey Albershteyn wrote:
> > > The direct path is not supported on verity files. Attempts to use direct
> > > I/O path on such files should fall back to buffered I/O path.
> > >
> > > Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> > > ---
> > > fs/xfs/xfs_file.c | 14 +++++++++++---
> > > 1 file changed, 11 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > > index 947b5c436172..9e072e82f6c1 100644
> > > --- a/fs/xfs/xfs_file.c
> > > +++ b/fs/xfs/xfs_file.c
> > > @@ -244,7 +244,8 @@ xfs_file_dax_read(
> > > struct kiocb *iocb,
> > > struct iov_iter *to)
> > > {
> > > - struct xfs_inode *ip = XFS_I(iocb->ki_filp->f_mapping->host);
> > > + struct inode *inode = iocb->ki_filp->f_mapping->host;
> > > + struct xfs_inode *ip = XFS_I(inode);
> > > ssize_t ret = 0;
> > >
> > > trace_xfs_file_dax_read(iocb, to);
> > > @@ -297,10 +298,17 @@ xfs_file_read_iter(
> > >
> > > if (IS_DAX(inode))
> > > ret = xfs_file_dax_read(iocb, to);
> > > - else if (iocb->ki_flags & IOCB_DIRECT)
> > > + else if (iocb->ki_flags & IOCB_DIRECT && !fsverity_active(inode))
> > > ret = xfs_file_dio_read(iocb, to);
> > > - else
> > > + else {
> > > + /*
> > > + * In case fs-verity is enabled, we also fallback to the
> > > + * buffered read from the direct read path. Therefore,
> > > + * IOCB_DIRECT is set and need to be cleared
> > > + */
> > > + iocb->ki_flags &= ~IOCB_DIRECT;
> > > ret = xfs_file_buffered_read(iocb, to);
> >
> > XFS doesn't usually allow directio fallback to the pagecache. Why
> > would fsverity be any different?
>
> Didn't know that, this is what happens on ext4 so I did the same.
> Then it probably make sense to just error on DIRECT on verity
> sealed file.

Thinking about this a little more -- I suppose we shouldn't just go
breaking directio reads from a verity file if we can help it. Is there
a way to ask fsverity to perform its validation against some arbitrary
memory buffer that happens to be fs-block aligned? In which case we
could support fsblock-aligned directio reads without falling back to the
page cache?

--D

> >
> > --D
> >
> > > + }
> > >
> > > if (ret > 0)
> > > XFS_STATS_ADD(mp, xs_read_bytes, ret);
> > > --
> > > 2.38.4
> > >
> >
>
> --
> - Andrey
>