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

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

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);
 }
 
 /*