[4.2.y-ckt stable] Patch "ovl: ignore lower entries when checking purity of non-directory entries" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ovl: ignore lower entries when checking purity of non-directory entries

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt6.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 62b4fa28996611bcb846154956e6faa38cab5fe8 Mon Sep 17 00:00:00 2001
From: Konstantin Khlebnikov <koct9i at gmail.com>
Date: Sun, 31 Jan 2016 16:17:53 +0300
Subject: ovl: ignore lower entries when checking purity of non-directory
 entries

commit 45d11738969633ec07ca35d75d486bf2d8918df6 upstream.

After rename file dentry still holds reference to lower dentry from
previous location. This doesn't matter for data access because data comes
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 at gmail.com>
Reported-by: Viktor Stanchev <me at viktorstanchev.com>
Suggested-by: Vivek Goyal <vgoyal at redhat.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=109611
Acked-by: Vivek Goyal <vgoyal at redhat.com>
Signed-off-by: Miklos Szeredi <miklos at szeredi.hu>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 36d6a5b..a2b1d7c 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -904,6 +904,13 @@ 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 96a122f..000b2ed 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;
--
2.7.0