linux-kernel - [PATCH] ovl: ignore lower entries when checking purity of non-directory entries

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Date:	Sun, 31 Jan 2016 16:17:53 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	linux-fsdevel@...r.kernel.org, linux-unionfs@...r.kernel.org
Cc:	David Howells <dhowells@...hat.com>, linux-kernel@...r.kernel.org,
	Vivek Goyal <vgoyal@...hat.com>
Subject: [PATCH] ovl: ignore lower entries when checking purity of
 non-directory entries

After rename file dentry still holds reference to lower dentry from
previous location. This doesn't matter for data access because data
cames from upper dentry. But this stale lower dentry taints dentry
at new location and turns it into non-pure upper. Such file leaves
visible whiteout entry after remove in directory which shouldn't
have whiteouts at all.

Overlayfs already tracks pureness of file location in oe->opaque.
This patch just uses that for detecting actual path type.

Comment from Vivek Goyal's patch:

Here are the details of the problem. Do following.

$ mkdir upper lower work merged upper/dir/
$ touch lower/test
$ sudo mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work
merged
$ mv merged/test merged/dir/
$ rm merged/dir/test
$ ls -l merged/dir/
/usr/bin/ls: cannot access merged/dir/test: No such file or directory
total 0
c????????? ? ? ? ?            ? test

Basic problem seems to be that once a file has been unlinked, a
whiteout has been left behind which was not needed and hence it becomes
visible.

whiteout is visible because parent dir is of not type MERGE, hence
od->is_real is set during ovl_dir_open(). And that means ovl_iterate()
passes on iterate handling directly to underlying fs. Underlying fs does
not know/filter whiteouts so it becomes visible to user.

Why did we leave a whiteout to begin with when we should not have.
ovl_do_remove() checks for OVL_TYPE_PURE_UPPER() and does not leave
whiteout if file is pure upper. In this case file is not found to be
pure upper hence whiteout is left.

So why file was not PURE_UPPER in this case? I think because dentry is
still carrying some leftover state which was valid before rename. For example,
od->numlower was set to 1 as it was a lower file. After rename, this state
is not valid anymore as there is no such file in lower.

Signed-off-by: Konstantin Khlebnikov <koct9i@...il.com>
Reported-by: Viktor Stanchev <me@...torstanchev.com>
Diagnosed-by: Vivek Goyal <vgoyal@...hat.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=109611
---
 fs/overlayfs/dir.c   |    7 +++++++
 fs/overlayfs/super.c |   12 +++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index ed95272d57a6..edf83f325bca 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -903,6 +903,13 @@ static int ovl_rename2(struct inode *olddir, struct dentry *old,
 	if (!overwrite && new_is_dir && !old_opaque && new_opaque)
 		ovl_remove_opaque(newdentry);

+	/*
+	 * Old dentry now lives in different location. Dentries in
+	 * lowerstack are stale. We cannot drop them here because
+	 * access to them is lockless. This could be only pure upper
+	 * or opaque directory - numlower is zero. Or upper non-dir
+	 * entry - its pureness is tracked by flag opaque.
+	 */
 	if (old_opaque != new_opaque) {
 		ovl_dentry_set_opaque(old, new_opaque);
 		if (!overwrite)
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 8d826bd56b26..ba28b007005e 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -76,12 +76,14 @@ enum ovl_path_type ovl_path_type(struct dentry *dentry)
 	if (oe->__upperdentry) {
 		type = __OVL_PATH_UPPER;

-		if (oe->numlower) {
-			if (S_ISDIR(dentry->d_inode->i_mode))
-				type |= __OVL_PATH_MERGE;
-		} else if (!oe->opaque) {
+		/*
+		 * Non-dir dentry can hold lower dentry from previous
+		 * location. Its purity depends only on opaque flag.
+		 */
+		if (oe->numlower && S_ISDIR(dentry->d_inode->i_mode))
+			type |= __OVL_PATH_MERGE;
+		else if (!oe->opaque)
 			type |= __OVL_PATH_PURE;
-		}
 	} else {
 		if (oe->numlower > 1)
 			type |= __OVL_PATH_MERGE;