Re: [PATCH 8/8] xfs: ensure post-EOF zeroing happens after zeroing part of a file

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Jun 21, 2018 at 11:29:07PM -0700, Christoph Hellwig wrote:
> On Thu, Jun 21, 2018 at 11:31:53AM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > If a user asks us to zero_range part of a file, the end of the range is
> > EOF, and not aligned to a page boundary, invoke writeback of the EOF
> > page to ensure that the post-EOF part of the page is zeroed.  This
> > ensures that we don't expose stale memory contents via mmap, if in a
> > clumsy manner.
> > 
> > Found by running generic/127 when it runs zero_range and mapread at EOF
> > one after the other.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/xfs_bmap_util.c |   15 ++++++++++++++-
> >  1 file changed, 14 insertions(+), 1 deletion(-)
> > 
> > 
> > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > index abc37b0899c0..c94d376e4152 100644
> > --- a/fs/xfs/xfs_bmap_util.c
> > +++ b/fs/xfs/xfs_bmap_util.c
> > @@ -1208,7 +1208,20 @@ xfs_free_file_space(
> >  		return 0;
> >  	if (offset + len > XFS_ISIZE(ip))
> >  		len = XFS_ISIZE(ip) - offset;
> > -	return iomap_zero_range(VFS_I(ip), offset, len, NULL, &xfs_iomap_ops);
> > +	error = iomap_zero_range(VFS_I(ip), offset, len, NULL, &xfs_iomap_ops);
> > +	if (error)
> > +		return error;
> > +
> > +	/*
> > +	 * If we zeroed right up to EOF and EOF straddles a page boundary we
> > +	 * must make sure that the post-EOF area is also zeroed because the
> > +	 * page could be mmap'd and iomap_zero_range doesn't do that for us.
> > +	 * Writeback of the eof page will do this, albeit clumsily.
> > +	 */
> > +	if (offset + len < XFS_ISIZE(ip) || ((offset + len) & PAGE_MASK) == 0)
> > +		return 0;
> > +	return filemap_write_and_wait_range(VFS_I(ip)->i_mapping,
> > +			(offset + len) & ~PAGE_MASK, LLONG_MAX);
> 
> Total nitpick, but between the kernel logic flow and comment I'd invert
> the check:
> 
> 	if (offset + len >= XFS_ISIZE(ip) && (offset + len) & PAGE_MASK) {
> 		error = filemap_write_and_wait_range(VFS_I(ip)->i_mapping,
> 			(offset + len) & ~PAGE_MASK, LLONG_MAX);
> 	}
> 
> 	return error;
> 
> Otherwise looks good:
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Ok, will update & test overnight.

--D

> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html