linux-ext4 - Re: [PATCH v2 19/23] xfs: disable direct read path for fs-verity sealed files

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Message-ID: <20230405221039.GP3223426@dread.disaster.area>
Date:   Thu, 6 Apr 2023 08:10:39 +1000
From:   Dave Chinner <david@...morbit.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Andrey Albershteyn <aalbersh@...hat.com>, dchinner@...hat.com,
        ebiggers@...nel.org, hch@...radead.org, linux-xfs@...r.kernel.org,
        fsverity@...ts.linux.dev, rpeterso@...hat.com, agruenba@...hat.com,
        xiang@...nel.org, chao@...nel.org,
        damien.lemoal@...nsource.wdc.com, jth@...nel.org,
        linux-erofs@...ts.ozlabs.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com
Subject: Re: [PATCH v2 19/23] xfs: disable direct read path for fs-verity
 sealed files

On Wed, Apr 05, 2023 at 08:09:27AM -0700, Darrick J. Wong wrote:
> 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@...hat.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?

The memory buffer doesn't even need to be fs-block aligned - it just
needs to be a pointer to memory the kernel can read...

We also need fsverity to be able to handle being passed mapped
kernel memory rather than pages/folios for the merkle tree
interfaces. That way we can just pass it the mapped buffer memory
straight from the xfs-buf and we don't have to do the whacky "copy
from xattr xfs_bufs into pages so fsverity can take temporary
reference counts on what it thinks are page cache pages" as it walks
the merkle tree.

-Dave.
-- 
Dave Chinner
david@...morbit.com