[PATCH 1/2] vfs: add file_dentry()

[Miklos Szeredi <miklos@szeredi.hu>]

From: Miklos Szeredi <mszeredi@redhat.com>

Regular files opened on overlayfs will result in the file being opened on
the underlying filesystem, while f_path points to the overlayfs
mount/dentry.

This confuses filesystems which get the dentry from struct file and assume
it's theirs.

Add a new helper, file_dentry() [*], to get the filesystem's own dentry
from the file.  This simply compares file_inode(file->f_path.dentry) to
file_inode(file) and if they are equal returns file->f_path.dentry (this is
the common, non-overlayfs case).

In the uncommon case (regular file on overlayfs) it will call into
overlayfs's ->d_native_dentry() to get the underlying dentry matching
file_inode(file).

[*] If possible, it's better simply to use file_inode() instead.

Fixes: 4bacc9c9234c ("overlayfs: Make f_path always point to the overlay and f_inode to the underlay")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Tested-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/open.c              |   11 +++++++++++
 fs/overlayfs/super.c   |   16 ++++++++++++++++
 include/linux/dcache.h |    1 +
 include/linux/fs.h     |    2 ++
 4 files changed, 30 insertions(+)

--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1234,6 +1234,8 @@ static inline struct inode *file_inode(c
 	return f->f_inode;
 }
 
+extern struct dentry *file_dentry(const struct file *file);
+
 static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl)
 {
 	return locks_lock_inode_wait(file_inode(filp), fl);
--- a/fs/open.c
+++ b/fs/open.c
@@ -831,6 +831,17 @@ char *file_path(struct file *filp, char
 }
 EXPORT_SYMBOL(file_path);
 
+struct dentry *file_dentry(const struct file *file)
+{
+	struct dentry *dentry = file->f_path.dentry;
+
+	if (likely(d_inode(dentry) == file_inode(file)))
+		return dentry;
+	else
+		return dentry->d_op->d_native_dentry(dentry, file_inode(file));
+}
+EXPORT_SYMBOL(file_dentry);
+
 /**
  * vfs_open - open the file at the given path
  * @path: path to open
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -336,14 +336,30 @@ static int ovl_dentry_weak_revalidate(st
 	return ret;
 }
 
+static struct dentry *ovl_d_native_dentry(struct dentry *dentry,
+					 struct inode *inode)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+	struct dentry *realentry = ovl_upperdentry_dereference(oe);
+
+	if (realentry && inode == d_inode(realentry))
+		return realentry;
+	realentry = __ovl_dentry_lower(oe);
+	if (realentry && inode == d_inode(realentry))
+		return realentry;
+	BUG();
+}
+
 static const struct dentry_operations ovl_dentry_operations = {
 	.d_release = ovl_dentry_release,
 	.d_select_inode = ovl_d_select_inode,
+	.d_native_dentry = ovl_d_native_dentry,
 };
 
 static const struct dentry_operations ovl_reval_dentry_operations = {
 	.d_release = ovl_dentry_release,
 	.d_select_inode = ovl_d_select_inode,
+	.d_native_dentry = ovl_d_native_dentry,
 	.d_revalidate = ovl_dentry_revalidate,
 	.d_weak_revalidate = ovl_dentry_weak_revalidate,
 };
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -161,6 +161,7 @@ struct dentry_operations {
 	struct vfsmount *(*d_automount)(struct path *);
 	int (*d_manage)(struct dentry *, bool);
 	struct inode *(*d_select_inode)(struct dentry *, unsigned);
+	struct dentry *(*d_native_dentry)(struct dentry *, struct inode *);
 } ____cacheline_aligned;
 
 /*