UBUNTU: ubuntu: overlayfs -- overlay filesystem

Overlayfs allows one, usually read-write, directory tree to be
overlaid onto another, read-only directory tree.  All modifications
go to the upper, writable layer.

This type of mechanism is most often used for live CDs but there's a
wide variety of other uses.

The implementation differs from other "union filesystem"
implementations in that after a file is opened all operations go
directly to the underlying, lower or upper, filesystems.  This
simplifies the implementation and allows native performance in these
cases.

The dentry tree is duplicated from the underlying filesystems, this
enables fast cached lookups without adding special support into the
VFS.  This uses slightly more memory than union mounts, but dentries
are relatively small.

Currently inodes are duplicated as well, but it is a possible
optimization to share inodes for non-directories.

Opening non directories results in the open forwarded to the
underlying filesystem.  This makes the behavior very similar to union
mounts (with the same limitations vs. fchmod/fchown on O_RDONLY file
descriptors).

Usage:

  mount -t overlay -olowerdir=/lower,upperdir=/upper overlay /mnt

Supported:

 - all operations

Missing:

 - ensure that filesystems part of the overlay are not modified outside
   the overlay

The following cotributions have been folded into this patch:

Neil Brown <neilb@suse.de>:
 - minimal remount support
 - use correct seek function for directories
 - initialise is_real before use
 - rename ovl_fill_cache to ovl_dir_read

Felix Fietkau <nbd@openwrt.org>:
 - fix a deadlock in ovl_dir_read_merged
 - fix a deadlock in ovl_remove_whiteouts

Erez Zadok <ezk@fsl.cs.sunysb.edu>
 - fix cleanup after WARN_ON

Also thanks to the following people for testing and reporting bugs:

  Jordi Pujol <jordipujolp@gmail.com>
  Andy Whitcroft <apw@canonical.com>
  Michal Suchanek <hramrach@centrum.cz>
  Felix Fietkau <nbd@openwrt.org>
  Erez Zadok <ezk@fsl.cs.sunysb.edu>
  Randy Dunlap <rdunlap@xenotime.net>

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andy Whitcroft <apw@canonical.com>

1 files changed, 582 insertions, 0 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
new file mode 100644
index 00000000000..a344b427473
--- /dev/null
+++ b/fs/overlayfs/super.c
@@ -0,0 +1,582 @@
+/*
+ *
+ * Copyright (C) 2011 Novell 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/fs.h>
+#include <linux/namei.h>
+#include <linux/xattr.h>
+#include <linux/security.h>
+#include <linux/mount.h>
+#include <linux/slab.h>
+#include <linux/parser.h>
+#include <linux/module.h>
+#include "overlayfs.h"
+
+MODULE_AUTHOR("Miklos Szeredi <miklos@szeredi.hu>");
+MODULE_DESCRIPTION("Overlay filesystem");
+MODULE_LICENSE("GPL");
+
+struct ovl_fs {
+	struct vfsmount *upper_mnt;
+	struct vfsmount *lower_mnt;
+};
+
+struct ovl_entry {
+	/*
+	 * Keep "double reference" on upper dentries, so that
+	 * d_delete() doesn't think it's OK to reset d_inode to NULL.
+	 */
+	struct dentry *__upperdentry;
+	struct dentry *lowerdentry;
+	union {
+		struct {
+			u64 version;
+			bool opaque;
+		};
+		struct rcu_head rcu;
+	};
+};
+
+const char *ovl_whiteout_xattr = "trusted.overlay.whiteout";
+const char *ovl_opaque_xattr = "trusted.overlay.opaque";
+
+
+enum ovl_path_type ovl_path_type(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	if (oe->__upperdentry) {
+		if (oe->lowerdentry && S_ISDIR(dentry->d_inode->i_mode))
+			return OVL_PATH_MERGE;
+		else
+			return OVL_PATH_UPPER;
+	} else {
+		return OVL_PATH_LOWER;
+	}
+}
+
+static struct dentry *ovl_upperdentry_dereference(struct ovl_entry *oe)
+{
+	struct dentry *upperdentry = ACCESS_ONCE(oe->__upperdentry);
+	smp_read_barrier_depends();
+	return upperdentry;
+}
+
+void ovl_path_upper(struct dentry *dentry, struct path *path)
+{
+	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	path->mnt = ofs->upper_mnt;
+	path->dentry = ovl_upperdentry_dereference(oe);
+}
+
+void ovl_path_lower(struct dentry *dentry, struct path *path)
+{
+	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	path->mnt = ofs->lower_mnt;
+	path->dentry = oe->lowerdentry;
+}
+
+enum ovl_path_type ovl_path_real(struct dentry *dentry, struct path *path)
+{
+
+	enum ovl_path_type type = ovl_path_type(dentry);
+
+	if (type == OVL_PATH_LOWER)
+		ovl_path_lower(dentry, path);
+	else
+		ovl_path_upper(dentry, path);
+
+	return type;
+}
+
+struct dentry *ovl_dentry_upper(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	return ovl_upperdentry_dereference(oe);
+}
+
+struct dentry *ovl_dentry_lower(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	return oe->lowerdentry;
+}
+
+struct dentry *ovl_dentry_real(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+	struct dentry *realdentry;
+
+	realdentry = ovl_upperdentry_dereference(oe);
+	if (!realdentry)
+		realdentry = oe->lowerdentry;
+
+	return realdentry;
+}
+
+struct dentry *ovl_entry_real(struct ovl_entry *oe, bool *is_upper)
+{
+	struct dentry *realdentry;
+
+	realdentry = ovl_upperdentry_dereference(oe);
+	if (realdentry) {
+		*is_upper = true;
+	} else {
+		realdentry = oe->lowerdentry;
+		*is_upper = false;
+	}
+	return realdentry;
+}
+
+bool ovl_dentry_is_opaque(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+	return oe->opaque;
+}
+
+void ovl_dentry_set_opaque(struct dentry *dentry, bool opaque)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+	oe->opaque = opaque;
+}
+
+void ovl_dentry_update(struct dentry *dentry, struct dentry *upperdentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	WARN_ON(!mutex_is_locked(&upperdentry->d_parent->d_inode->i_mutex));
+	WARN_ON(oe->__upperdentry);
+	BUG_ON(!upperdentry->d_inode);
+	smp_wmb();
+	oe->__upperdentry = dget(upperdentry);
+}
+
+void ovl_dentry_version_inc(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	WARN_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
+	oe->version++;
+}
+
+u64 ovl_dentry_version_get(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	WARN_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
+	return oe->version;
+}
+
+bool ovl_is_whiteout(struct dentry *dentry)
+{
+	int res;
+	char val;
+
+	if (!dentry)
+		return false;
+	if (!dentry->d_inode)
+		return false;
+	if (!S_ISLNK(dentry->d_inode->i_mode))
+		return false;
+
+	res = vfs_getxattr(dentry, ovl_whiteout_xattr, &val, 1);
+	if (res == 1 && val == 'y')
+		return true;
+
+	return false;
+}
+
+static bool ovl_is_opaquedir(struct dentry *dentry)
+{
+	int res;
+	char val;
+
+	if (!S_ISDIR(dentry->d_inode->i_mode))
+		return false;
+
+	res = vfs_getxattr(dentry, ovl_opaque_xattr, &val, 1);
+	if (res == 1 && val == 'y')
+		return true;
+
+	return false;
+}
+
+static void ovl_entry_free(struct rcu_head *head)
+{
+	struct ovl_entry *oe = container_of(head, struct ovl_entry, rcu);
+	kfree(oe);
+}
+
+static void ovl_dentry_release(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	if (oe) {
+		dput(oe->__upperdentry);
+		dput(oe->__upperdentry);
+		dput(oe->lowerdentry);
+		call_rcu(&oe->rcu, ovl_entry_free);
+	}
+}
+
+const struct dentry_operations ovl_dentry_operations = {
+	.d_release = ovl_dentry_release,
+};
+
+static struct ovl_entry *ovl_alloc_entry(void)
+{
+	return kzalloc(sizeof(struct ovl_entry), GFP_KERNEL);
+}
+
+static struct dentry *ovl_lookup_real(struct dentry *dir, struct qstr *name)
+{
+	struct dentry *dentry;
+
+	mutex_lock(&dir->d_inode->i_mutex);
+	dentry = lookup_one_len(name->name, dir, name->len);
+	mutex_unlock(&dir->d_inode->i_mutex);
+
+	if (IS_ERR(dentry)) {
+		if (PTR_ERR(dentry) == -ENOENT)
+			dentry = NULL;
+	} else if (!dentry->d_inode) {
+		dput(dentry);
+		dentry = NULL;
+	}
+	return dentry;
+}
+
+int ovl_do_lookup(struct dentry *dentry)
+{
+	struct ovl_entry *oe;
+	struct dentry *upperdir;
+	struct dentry *lowerdir;
+	struct dentry *upperdentry = NULL;
+	struct dentry *lowerdentry = NULL;
+	struct inode *inode = NULL;
+	int err;
+
+	err = -ENOMEM;
+	oe = ovl_alloc_entry();
+	if (!oe)
+		goto out;
+
+	upperdir = ovl_dentry_upper(dentry->d_parent);
+	lowerdir = ovl_dentry_lower(dentry->d_parent);
+
+	if (upperdir) {
+		upperdentry = ovl_lookup_real(upperdir, &dentry->d_name);
+		err = PTR_ERR(upperdentry);
+		if (IS_ERR(upperdentry))
+			goto out_put_dir;
+
+		if (lowerdir && upperdentry &&
+		    (S_ISLNK(upperdentry->d_inode->i_mode) ||
+		     S_ISDIR(upperdentry->d_inode->i_mode))) {
+			const struct cred *old_cred;
+			struct cred *override_cred;
+
+			err = -ENOMEM;
+			override_cred = prepare_creds();
+			if (!override_cred)
+				goto out_dput_upper;
+
+			/* CAP_SYS_ADMIN needed for getxattr */
+			cap_raise(override_cred->cap_effective, CAP_SYS_ADMIN);
+			old_cred = override_creds(override_cred);
+
+			if (ovl_is_opaquedir(upperdentry)) {
+				oe->opaque = true;
+			} else if (ovl_is_whiteout(upperdentry)) {
+				dput(upperdentry);
+				upperdentry = NULL;
+				oe->opaque = true;
+			}
+			revert_creds(old_cred);
+			put_cred(override_cred);
+		}
+	}
+	if (lowerdir && !oe->opaque) {
+		lowerdentry = ovl_lookup_real(lowerdir, &dentry->d_name);
+		err = PTR_ERR(lowerdentry);
+		if (IS_ERR(lowerdentry))
+			goto out_dput_upper;
+	}
+
+	if (lowerdentry && upperdentry &&
+	    (!S_ISDIR(upperdentry->d_inode->i_mode) ||
+	     !S_ISDIR(lowerdentry->d_inode->i_mode))) {
+		dput(lowerdentry);
+		lowerdentry = NULL;
+		oe->opaque = true;
+	}
+
+	if (lowerdentry || upperdentry) {
+		struct dentry *realdentry;
+
+		realdentry = upperdentry ? upperdentry : lowerdentry;
+		err = -ENOMEM;
+		inode = ovl_new_inode(dentry->d_sb, realdentry->d_inode->i_mode, oe);
+		if (!inode)
+			goto out_dput;
+	}
+
+	if (upperdentry)
+		oe->__upperdentry = dget(upperdentry);
+
+	if (lowerdentry)
+		oe->lowerdentry = lowerdentry;
+
+	dentry->d_fsdata = oe;
+	dentry->d_op = &ovl_dentry_operations;
+	d_add(dentry, inode);
+
+	return 0;
+
+out_dput:
+	dput(lowerdentry);
+out_dput_upper:
+	dput(upperdentry);
+out_put_dir:
+	kfree(oe);
+out:
+	return err;
+}
+
+static void ovl_put_super(struct super_block *sb)
+{
+	struct ovl_fs *ufs = sb->s_fs_info;
+
+	if (!(sb->s_flags & MS_RDONLY))
+		mnt_drop_write(ufs->upper_mnt);
+
+	mntput(ufs->upper_mnt);
+	mntput(ufs->lower_mnt);
+
+	kfree(ufs);
+}
+
+static int ovl_remount_fs(struct super_block *sb, int *flagsp, char *data)
+{
+	int flags = *flagsp;
+	struct ovl_fs *ufs = sb->s_fs_info;
+
+	/* When remounting rw or ro, we need to adjust the write access to the
+	 * upper fs.
+	 */
+	if (((flags ^ sb->s_flags) & MS_RDONLY) == 0)
+		/* No change to readonly status */
+		return 0;
+
+	if (flags & MS_RDONLY) {
+		mnt_drop_write(ufs->upper_mnt);
+		return 0;
+	} else
+		return mnt_want_write(ufs->upper_mnt);
+}
+
+static const struct super_operations ovl_super_operations = {
+	.put_super	= ovl_put_super,
+	.remount_fs	= ovl_remount_fs,
+};
+
+struct ovl_config {
+	char *lowerdir;
+	char *upperdir;
+};
+
+enum {
+	Opt_lowerdir,
+	Opt_upperdir,
+	Opt_err,
+};
+
+static const match_table_t ovl_tokens = {
+	{Opt_lowerdir,			"lowerdir=%s"},
+	{Opt_upperdir,			"upperdir=%s"},
+	{Opt_err,			NULL}
+};
+
+static int ovl_parse_opt(char *opt, struct ovl_config *config)
+{
+	char *p;
+
+	config->upperdir = NULL;
+	config->lowerdir = NULL;
+
+	while ((p = strsep(&opt, ",")) != NULL) {
+		int token;
+		substring_t args[MAX_OPT_ARGS];
+
+		if (!*p)
+			continue;
+
+		token = match_token(p, ovl_tokens, args);
+		switch (token) {
+		case Opt_upperdir:
+			kfree(config->upperdir);
+			config->upperdir = match_strdup(&args[0]);
+			if (!config->upperdir)
+				return -ENOMEM;
+			break;
+
+		case Opt_lowerdir:
+			kfree(config->lowerdir);
+			config->lowerdir = match_strdup(&args[0]);
+			if (!config->lowerdir)
+				return -ENOMEM;
+			break;
+
+		default:
+			return -EINVAL;
+		}
+	}
+	return 0;
+}
+
+static int ovl_fill_super(struct super_block *sb, void *data, int silent)
+{
+	struct path lowerpath;
+	struct path upperpath;
+	struct inode *root_inode;
+	struct dentry *root_dentry;
+	struct ovl_entry *oe;
+	struct ovl_fs *ufs;
+	struct ovl_config config;
+	int err;
+
+	err = ovl_parse_opt((char *) data, &config);
+	if (err)
+		goto out;
+
+	err = -EINVAL;
+	if (!config.upperdir || !config.lowerdir) {
+		printk(KERN_ERR "overlayfs: missing upperdir or lowerdir\n");
+		goto out_free_config;
+	}
+
+	err = -ENOMEM;
+	ufs = kmalloc(sizeof(struct ovl_fs), GFP_KERNEL);
+	if (!ufs)
+		goto out_free_config;
+
+	oe = ovl_alloc_entry();
+	if (oe == NULL)
+		goto out_free_ufs;
+
+	root_inode = ovl_new_inode(sb, S_IFDIR, oe);
+	if (!root_inode)
+		goto out_free_oe;
+
+	err = kern_path(config.upperdir, LOOKUP_FOLLOW, &upperpath);
+	if (err)
+		goto out_put_root;
+
+	err = kern_path(config.lowerdir, LOOKUP_FOLLOW, &lowerpath);
+	if (err)
+		goto out_put_upperpath;
+
+	err = -ENOTDIR;
+	if (!S_ISDIR(upperpath.dentry->d_inode->i_mode) ||
+	    !S_ISDIR(lowerpath.dentry->d_inode->i_mode))
+		goto out_put_lowerpath;
+
+	ufs->upper_mnt = clone_private_mount(&upperpath);
+	err = PTR_ERR(ufs->upper_mnt);
+	if (IS_ERR(ufs->upper_mnt)) {
+		printk(KERN_ERR "overlayfs: failed to clone upperpath\n");
+		goto out_put_lowerpath;
+	}
+
+	ufs->lower_mnt = clone_private_mount(&lowerpath);
+	err = PTR_ERR(ufs->lower_mnt);
+	if (IS_ERR(ufs->lower_mnt)) {
+		printk(KERN_ERR "overlayfs: failed to clone lowerpath\n");
+		goto out_put_upper_mnt;
+	}
+
+	if (!(sb->s_flags & MS_RDONLY)) {
+		err = mnt_want_write(ufs->upper_mnt);
+		if (err)
+			goto out_put_lower_mnt;
+	}
+
+	err = -ENOMEM;
+	root_dentry = d_alloc_root(root_inode);
+	if (!root_dentry)
+		goto out_drop_write;
+
+	mntput(upperpath.mnt);
+	mntput(lowerpath.mnt);
+
+	oe->__upperdentry = dget(upperpath.dentry);
+	oe->lowerdentry = lowerpath.dentry;
+
+	root_dentry->d_fsdata = oe;
+	root_dentry->d_op = &ovl_dentry_operations;
+
+	sb->s_op = &ovl_super_operations;
+	sb->s_root = root_dentry;
+	sb->s_fs_info = ufs;
+
+	return 0;
+
+out_drop_write:
+	if (!(sb->s_flags & MS_RDONLY))
+		mnt_drop_write(ufs->upper_mnt);
+out_put_lower_mnt:
+	mntput(ufs->lower_mnt);
+out_put_upper_mnt:
+	mntput(ufs->upper_mnt);
+out_put_lowerpath:
+	path_put(&lowerpath);
+out_put_upperpath:
+	path_put(&upperpath);
+out_put_root:
+	iput(root_inode);
+out_free_oe:
+	kfree(oe);
+out_free_ufs:
+	kfree(ufs);
+out_free_config:
+	kfree(config.lowerdir);
+	kfree(config.upperdir);
+out:
+	return err;
+}
+
+static struct dentry *ovl_mount(struct file_system_type *fs_type, int flags,
+				const char *dev_name, void *raw_data)
+{
+	return mount_nodev(fs_type, flags, raw_data, ovl_fill_super);
+}
+
+static struct file_system_type ovl_fs_type = {
+	.owner		= THIS_MODULE,
+	.name		= "overlayfs",
+	.mount		= ovl_mount,
+	.kill_sb	= kill_anon_super,
+};
+
+static int __init ovl_init(void)
+{
+	return register_filesystem(&ovl_fs_type);
+}
+
+static void __exit ovl_exit(void)
+{
+	unregister_filesystem(&ovl_fs_type);
+}
+
+module_init(ovl_init);
+module_exit(ovl_exit);