[PATCH v9 09/14] xfs: test for always COW after the dio write checks

[Chris Mason <mason@kernel.org>]

A direct write to a zoned file from vectors that are not block aligned
can silently corrupt the file's data, if the realtime flag is cleared
and set again while the write starts.

Every bio of an always COW write has to cover whole blocks, so
xfs_file_dio_write_aligned() passes IOMAP_DIO_FSBLOCK_ALIGNED for such
an inode. But it tests for always COW before it takes the IOLOCK, and
the realtime flag of a file with no blocks can change until then:

    xfs_file_dio_write_zoned()            FS_IOC_FSSETXATTR
    ==========================            =================
    xfs_zoned_write_space_reserve()
                                          clears the realtime flag
    xfs_file_dio_write_aligned()
      xfs_is_always_cow_inode() is false
                                          sets the realtime flag
      xfs_ilock_iocb_for_write()
      xfs_file_write_checks()
      iomap_dio_rw()
        splits a bio inside a block

The fix used here is to test for always COW once the IOLOCK is held.

Before commit 8caec6c9fef7 ("xfs: support sub-block aligned vectors in
always COW mode"), xfs_file_dio_write() made the same test with no lock
held, to send such vectors down the unaligned path. So v6.15 to v6.18
have the same race, and need a different version of this fix.

Fixes: 2e2383405824 ("xfs: implement direct writes to zoned RT devices")
Signed-off-by: Chris Mason <mason@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---

Notes:
    v8->v9: no change
    
    v7->v8:
    - Added the Reviewed-by of Christoph Hellwig.
    
    v6->v7:
    - New patch.

 fs/xfs/xfs_file.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 515ddafc6673..5fa8b07a779d 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -708,14 +708,6 @@ xfs_file_dio_write_aligned(
 	unsigned int		dio_flags = 0;
 	ssize_t			ret;
 
-	/*
-	 * For always COW inodes, each bio must be aligned to the file system
-	 * block size and not just the device sector size because we need to
-	 * allocate a block-aligned amount of space for each write.
-	 */
-	if (xfs_is_always_cow_inode(ip))
-		dio_flags |= IOMAP_DIO_FSBLOCK_ALIGNED;
-
 	ret = xfs_ilock_iocb_for_write(iocb, &iolock);
 	if (ret)
 		return ret;
@@ -743,6 +735,15 @@ xfs_file_dio_write_aligned(
 		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
 		iolock = XFS_IOLOCK_SHARED;
 	}
+
+	/*
+	 * For always COW inodes, each bio must be aligned to the file system
+	 * block size and not just the device sector size because we need to
+	 * allocate a block-aligned amount of space for each write.  The
+	 * realtime flag can change until we hold the IOLOCK, so test only now.
+	 */
+	if (xfs_is_always_cow_inode(ip))
+		dio_flags |= IOMAP_DIO_FSBLOCK_ALIGNED;
 	if (mapping_stable_writes(iocb->ki_filp->f_mapping))
 		dio_flags |= IOMAP_DIO_BOUNCE;
 	trace_xfs_file_direct_write(iocb, from);
-- 
2.55.0