[PATCH 1/6] ovl: fix corner case of non-unique st_dev;st_ino

[Amir Goldstein <amir73il@gmail.com>]

On non-samefs overlay without xino, non pure upper inodes should use a
pseudo_dev assigned to each unique lower fs and pure upper inodes use
the real upper st_dev.

It is fine for an overlay pure upper inode to use the same st_dev;st_ino
values as the real upper inode, because the content of those two
different filesystem objects is always the same.

In this case, however:
- two filesystems, A and B
- upper layer is on A
- lower layer 1 is also on A
- lower layer 2 is on B

Non pure upper overlay inode, whose origin is in layer 1 will have the
same st_dev;st_ino values as the real lower inode. This may result with
a false positive results of 'diff' between the real lower and copied up
overlay inode.

Fix this by using the upper st_dev;st_ino values in this case.
This breaks the property of constant st_dev;st_ino across copy up
of this case. This breakage will be fixed by a later patch.

Fixes: 5148626b806a ("ovl: allocate anon bdev per unique lower fs")
Cc: stable@vger.kernel.org # v4.17+
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/overlayfs/inode.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index bc14781886bf..b045cf1826fc 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -200,8 +200,14 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 			if (ovl_test_flag(OVL_INDEX, d_inode(dentry)) ||
 			    (!ovl_verify_lower(dentry->d_sb) &&
 			     (is_dir || lowerstat.nlink == 1))) {
-				stat->ino = lowerstat.ino;
 				lower_layer = ovl_layer_lower(dentry);
+				/*
+				 * Cannot use origin st_dev;st_ino because
+				 * origin inode content may differ from overlay
+				 * inode content.
+				 */
+				if (samefs || lower_layer->fsid)
+					stat->ino = lowerstat.ino;
 			}
 
 			/*
-- 
2.17.1