[PATCH v8 07/15] xfs: test for reflink after the dio write checks

[Chris Mason <mason@kernel.org>]

A direct write that is not block aligned can expose stale disk contents
on XFS if it races with the first clone of the file.

xfs_file_dio_write_unaligned() falls back to buffered I/O if the file is
reflinked, and tests for that right after it takes the iolock. But
xfs_file_write_checks() runs after the test and can drop the iolock, for
example if S_NOSEC is not set. A clone can run in that gap:

    direct write                        clone
    ============                        =====
    takes the iolock shared
    sees that the file is not reflinked
    drops the iolock
                                        shares the blocks
    takes the iolock exclusive
    writes a part of a new COW block
    maps the whole block into the file
      /* the rest was never written */

The fix used here is to move the test after xfs_file_write_checks().

Fixes: ed1128c2d0c8 ("xfs: reduce exclusive locking on unaligned dio")
Signed-off-by: Chris Mason <mason@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---

Notes:
    v7->v8:
    - No change.
    
    v6->v7:
    - No change.

 fs/xfs/xfs_file.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 8ee88910b487..d888470d092a 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -877,9 +877,15 @@ xfs_file_dio_write_unaligned(
 	if (ret)
 		return ret;
 
+	ret = xfs_file_write_checks(iocb, from, &iolock, NULL);
+	if (ret)
+		goto out_unlock;
+
 	/*
 	 * We can't properly handle unaligned direct I/O to reflink files yet,
-	 * as we can't unshare a partial block.
+	 * as we can't unshare a partial block.  xfs_file_write_checks() can
+	 * drop the iolock and take it again, and the file can be reflinked in
+	 * between, so test for that only now.
 	 */
 	if (xfs_is_cow_inode(ip)) {
 		trace_xfs_reflink_bounce_dio_write(iocb, from);
@@ -887,10 +893,6 @@ xfs_file_dio_write_unaligned(
 		goto out_unlock;
 	}
 
-	ret = xfs_file_write_checks(iocb, from, &iolock, NULL);
-	if (ret)
-		goto out_unlock;
-
 	/*
 	 * If we are doing exclusive unaligned I/O, this must be the only I/O
 	 * in-flight.  Otherwise we risk data corruption due to unwritten extent
-- 
2.55.0