ovl: stack file ops

Implement file operations on a regular overlay file.  The underlying file
is opened separately and cached in ->private_data.

It might be worth making an exception for such files when accounting in
nr_file to confirm to userspace expectations.  We are only adding a small
overhead (248bytes for the struct file) since the real inode and dentry are
pinned by overlayfs anyway.

This patch doesn't have any effect, since the vfs will use d_real() to find
the real underlying file to open.  The patch at the end of the series will
actually enable this functionality.

AV: make it use open_with_fake_path(), don't mess with override_creds

SzM: still need to mess with override_creds() until no fs uses
current_cred() in their open method.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/overlayfs/Makefile

@@ -4,5 +4,5 @@
 
 obj-$(CONFIG_OVERLAY_FS) += overlay.o
 
-overlay-objs := super.o namei.o util.o inode.o dir.o readdir.o copy_up.o \
-		export.o
+overlay-objs := super.o namei.o util.o inode.o file.o dir.o readdir.o \
+		copy_up.o export.o

fs/overlayfs/file.c

+/*
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published by
+ * the Free Software Foundation.
+ */
+
+#include <linux/cred.h>
+#include <linux/file.h>
+#include <linux/xattr.h>
+#include "overlayfs.h"
+
+static struct file *ovl_open_realfile(const struct file *file)
+{
+	struct inode *inode = file_inode(file);
+	struct inode *upperinode = ovl_inode_upper(inode);
+	struct inode *realinode = upperinode ?: ovl_inode_lower(inode);
+	struct file *realfile;
+	const struct cred *old_cred;
+
+	old_cred = ovl_override_creds(inode->i_sb);
+	realfile = open_with_fake_path(&file->f_path, file->f_flags | O_NOATIME,
+				       realinode, current_cred());
+	revert_creds(old_cred);
+
+	pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
+		 file, file, upperinode ? 'u' : 'l', file->f_flags,
+		 realfile, IS_ERR(realfile) ? 0 : realfile->f_flags);
+
+	return realfile;
+}
+
+static int ovl_open(struct inode *inode, struct file *file)
+{
+	struct dentry *dentry = file_dentry(file);
+	struct file *realfile;
+	int err;
+
+	err = ovl_open_maybe_copy_up(dentry, file->f_flags);
+	if (err)
+		return err;
+
+	/* No longer need these flags, so don't pass them on to underlying fs */
+	file->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
+
+	realfile = ovl_open_realfile(file);
+	if (IS_ERR(realfile))
+		return PTR_ERR(realfile);
+
+	file->private_data = realfile;
+
+	return 0;
+}
+
+static int ovl_release(struct inode *inode, struct file *file)
+{
+	fput(file->private_data);
+
+	return 0;
+}
+
+static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct inode *realinode = ovl_inode_real(file_inode(file));
+
+	return generic_file_llseek_size(file, offset, whence,
+					realinode->i_sb->s_maxbytes,
+					i_size_read(realinode));
+}
+
+const struct file_operations ovl_file_operations = {
+	.open		= ovl_open,
+	.release	= ovl_release,
+	.llseek		= ovl_llseek,
+};

fs/overlayfs/inode.c

@@ -535,6 +535,7 @@ static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev,
 	switch (mode & S_IFMT) {
 	case S_IFREG:
 		inode->i_op = &ovl_file_inode_operations;
+		inode->i_fop = &ovl_file_operations;
 		break;
 
 	case S_IFDIR:

fs/overlayfs/overlayfs.h

@@ -377,6 +377,9 @@ struct dentry *ovl_create_real(struct inode *dir, struct dentry *newdentry,
 int ovl_cleanup(struct inode *dir, struct dentry *dentry);
 struct dentry *ovl_create_temp(struct dentry *workdir, struct ovl_cattr *attr);
 
+/* file.c */
+extern const struct file_operations ovl_file_operations;
+
 /* copy_up.c */
 int ovl_copy_up(struct dentry *dentry);
 int ovl_copy_up_flags(struct dentry *dentry, int flags);