[PATCH][SRU][Disco] UBUNTU: SAUCE: shiftfs: enable overlayfs on shiftfs

[Christian Brauner]

From: Christian Brauner <christian.brauner at canonical.com>

BugLink: https://bugs.launchpad.net/bugs/1838677

This patch enables overlayfs to use shiftfs as an underlay.

Currently it is not possible to use overlayfs on top of shiftfs. This
means Docker inside of LXD cannot make user of the overlay2 graph driver
which is blocking users such as Travis from making use of it
efficiently.

Co-Developed-by: Seth Forshee <seth.forshee at canonical.com>
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Signed-off-by: Christian Brauner <christian.brauner at canonical.com>
---
 fs/open.c            | 4 +++-
 fs/overlayfs/file.c  | 1 +
 fs/overlayfs/super.c | 5 +++--
 fs/shiftfs.c         | 4 +++-
 include/linux/fs.h   | 3 ++-
 5 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 14502dde6403..b8268a334dc9 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -914,13 +914,15 @@ struct file *dentry_open(const struct path *path, int flags,
 EXPORT_SYMBOL(dentry_open);
 
 struct file *open_with_fake_path(const struct path *path, int flags,
-				struct inode *inode, const struct cred *cred)
+				 struct inode *inode, struct dentry *dentry,
+				 const struct cred *cred)
 {
 	struct file *f = alloc_empty_file_noaccount(flags, cred);
 	if (!IS_ERR(f)) {
 		int error;
 
 		f->f_path = *path;
+		f->f_path.dentry = dentry;
 		error = do_dentry_open(f, inode, NULL);
 		if (error) {
 			fput(f);
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 6f6eb638a320..fe61b86c843b 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -33,6 +33,7 @@ static struct file *ovl_open_realfile(const struct file *file,
 
 	old_cred = ovl_override_creds(inode->i_sb);
 	realfile = open_with_fake_path(&file->f_path, flags, realinode,
+				       ovl_dentry_real(file->f_path.dentry),
 				       current_cred());
 	revert_creds(old_cred);
 
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index bb2a6f80ffd0..46def89a3664 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -748,13 +748,14 @@ static int ovl_mount_dir(const char *name, struct path *path)
 		ovl_unescape(tmp);
 		err = ovl_mount_dir_noesc(tmp, path);
 
-		if (!err)
-			if (ovl_dentry_remote(path->dentry)) {
+		if (!err) {
+			if ((path->dentry->d_sb->s_magic != SHIFTFS_MAGIC) && ovl_dentry_remote(path->dentry)) {
 				pr_err("overlayfs: filesystem on '%s' not supported as upperdir\n",
 				       tmp);
 				path_put_init(path);
 				err = -EINVAL;
 			}
+		}
 		kfree(tmp);
 	}
 	return err;
diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index 49f6714e9f95..400c3062365c 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -1048,7 +1048,9 @@ static struct file *shiftfs_open_realfile(const struct file *file,
 	oldcred = shiftfs_override_creds(inode->i_sb);
 	/* XXX: open_with_fake_path() not gauranteed to stay around, if
 	 * removed use dentry_open() */
-	lowerf = open_with_fake_path(realpath, file->f_flags, loweri, info->creator_cred);
+	lowerf = open_with_fake_path(realpath, file->f_flags, loweri,
+				     realpath->dentry,
+				     info->creator_cred);
 	revert_creds(oldcred);
 
 	return lowerf;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 5dd548c73f8d..4cb7a1124c5f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2524,7 +2524,8 @@ extern struct file *file_open_root(struct dentry *, struct vfsmount *,
 				   const char *, int, umode_t);
 extern struct file * dentry_open(const struct path *, int, const struct cred *);
 extern struct file * open_with_fake_path(const struct path *, int,
-					 struct inode*, const struct cred *);
+					 struct inode*, struct dentry *dentry,
+					 const struct cred *);
 static inline struct file *file_clone_open(struct file *file)
 {
 	return dentry_open(&file->f_path, file->f_flags, file->f_cred);
-- 
2.22.0