ovl: fix regression caused by exclusive upper/work dir protection

Enforcing exclusive ownership on upper/work dirs caused a docker
regression: https://github.com/moby/moby/issues/34672.

Euan spotted the regression and pointed to the offending commit.
Vivek has brought the regression to my attention and provided this
reproducer:

Terminal 1:

  mount -t overlay -o workdir=work,lowerdir=lower,upperdir=upper none
        merged/

Terminal 2:

  unshare -m

Terminal 1:

  umount merged
  mount -t overlay -o workdir=work,lowerdir=lower,upperdir=upper none
        merged/
  mount: /root/overlay-testing/merged: none already mounted or mount point
         busy

To fix the regression, I replaced the error with an alarming warning.
With index feature enabled, mount does fail, but logs a suggestion to
override exclusive dir protection by disabling index.
Note that index=off mount does take the inuse locks, so a concurrent
index=off will issue the warning and a concurrent index=on mount will fail.

Documentation was updated to reflect this change.

Fixes: 2cac0c00 ("ovl: get exclusive ownership on upper/work dirs")
Cc: <stable@vger.kernel.org> # v4.13
Reported-by: NEuan Kemp <euank@euank.com>
Reported-by: NVivek Goyal <vgoyal@redhat.com>
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

Documentation/filesystems/overlayfs.txt

@@ -210,8 +210,11 @@ path as another overlay mount and it may use a lower layer path that is
 beneath or above the path of another overlay lower layer path.
 
 Using an upper layer path and/or a workdir path that are already used by
-another overlay mount is not allowed and will fail with EBUSY.  Using
+another overlay mount is not allowed and may fail with EBUSY.  Using
 partially overlapping paths is not allowed but will not fail with EBUSY.
+If files are accessed from two overlayfs mounts which share or overlap the
+upper layer and/or workdir path the behavior of the overlay is undefined,
+though it will not result in a crash or deadlock.
 
 Mounting an overlay using an upper layer path, where the upper layer path
 was previously used by another mounted overlay in combination with a

fs/overlayfs/ovl_entry.h

@@ -37,6 +37,9 @@ struct ovl_fs {
 	bool noxattr;
 	/* sb common to all layers */
 	struct super_block *same_sb;
+	/* Did we take the inuse lock? */
+	bool upperdir_locked;
+	bool workdir_locked;
 };
 
 /* private information held for every overlayfs dentry */

fs/overlayfs/super.c

@@ -211,9 +211,10 @@ static void ovl_put_super(struct super_block *sb)
 
 	dput(ufs->indexdir);
 	dput(ufs->workdir);
-	ovl_inuse_unlock(ufs->workbasedir);
+	if (ufs->workdir_locked)
+		ovl_inuse_unlock(ufs->workbasedir);
 	dput(ufs->workbasedir);
-	if (ufs->upper_mnt)
+	if (ufs->upper_mnt && ufs->upperdir_locked)
 		ovl_inuse_unlock(ufs->upper_mnt->mnt_root);
 	mntput(ufs->upper_mnt);
 	for (i = 0; i < ufs->numlower; i++)
@@ -881,9 +882,13 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 			goto out_put_upperpath;
 
 		err = -EBUSY;
-		if (!ovl_inuse_trylock(upperpath.dentry)) {
-			pr_err("overlayfs: upperdir is in-use by another mount\n");
+		if (ovl_inuse_trylock(upperpath.dentry)) {
+			ufs->upperdir_locked = true;
+		} else if (ufs->config.index) {
+			pr_err("overlayfs: upperdir is in-use by another mount, mount with '-o index=off' to override exclusive upperdir protection.\n");
 			goto out_put_upperpath;
+		} else {
+			pr_warn("overlayfs: upperdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
 		}
 
 		err = ovl_mount_dir(ufs->config.workdir, &workpath);
@@ -901,9 +906,13 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		}
 
 		err = -EBUSY;
-		if (!ovl_inuse_trylock(workpath.dentry)) {
-			pr_err("overlayfs: workdir is in-use by another mount\n");
+		if (ovl_inuse_trylock(workpath.dentry)) {
+			ufs->workdir_locked = true;
+		} else if (ufs->config.index) {
+			pr_err("overlayfs: workdir is in-use by another mount, mount with '-o index=off' to override exclusive workdir protection.\n");
 			goto out_put_workpath;
+		} else {
+			pr_warn("overlayfs: workdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
 		}
 
 		ufs->workbasedir = workpath.dentry;
@@ -1156,11 +1165,13 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 out_free_lowertmp:
 	kfree(lowertmp);
 out_unlock_workdentry:
-	ovl_inuse_unlock(workpath.dentry);
+	if (ufs->workdir_locked)
+		ovl_inuse_unlock(workpath.dentry);
 out_put_workpath:
 	path_put(&workpath);
 out_unlock_upperdentry:
-	ovl_inuse_unlock(upperpath.dentry);
+	if (ufs->upperdir_locked)
+		ovl_inuse_unlock(upperpath.dentry);
 out_put_upperpath:
 	path_put(&upperpath);
 out_free_config: