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:

 - Currently a crash in the middle of copy-up, rename, unlink, rmdir or create
   over a whiteout may result in filesystem corruption on the overlay level.
   IOW these operations need to become atomic or at least the corruption needs
   to be detected.

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

Sedat Dilek <sedat.dilek@googlemail.com>
 - fix up permission to confirm to new API

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>

1 files changed, 566 insertions, 0 deletions

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
new file mode 100644
index 00000000000..0797efbc7be
--- /dev/null
+++ b/fs/overlayfs/readdir.c
@@ -0,0 +1,566 @@
+/*
+ *
+ * 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/slab.h>
+#include <linux/namei.h>
+#include <linux/file.h>
+#include <linux/xattr.h>
+#include <linux/rbtree.h>
+#include <linux/security.h>
+#include <linux/cred.h>
+#include "overlayfs.h"
+
+struct ovl_cache_entry {
+	const char *name;
+	unsigned int len;
+	unsigned int type;
+	u64 ino;
+	bool is_whiteout;
+	struct list_head l_node;
+	struct rb_node node;
+};
+
+struct ovl_readdir_data {
+	struct rb_root *root;
+	struct list_head *list;
+	struct list_head *middle;
+	struct dentry *dir;
+	int count;
+	int err;
+};
+
+struct ovl_dir_file {
+	bool is_real;
+	bool is_cached;
+	struct list_head cursor;
+	u64 cache_version;
+	struct list_head cache;
+	struct file *realfile;
+};
+
+static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
+{
+	return container_of(n, struct ovl_cache_entry, node);
+}
+
+static struct ovl_cache_entry *ovl_cache_entry_find(struct rb_root *root,
+						    const char *name, int len)
+{
+	struct rb_node *node = root->rb_node;
+	int cmp;
+
+	while (node) {
+		struct ovl_cache_entry *p = ovl_cache_entry_from_node(node);
+
+		cmp = strncmp(name, p->name, len);
+		if (cmp > 0)
+			node = p->node.rb_right;
+		else if (cmp < 0 || len < p->len)
+			node = p->node.rb_left;
+		else
+			return p;
+	}
+
+	return NULL;
+}
+
+static struct ovl_cache_entry *ovl_cache_entry_new(const char *name, int len,
+						   u64 ino, unsigned int d_type)
+{
+	struct ovl_cache_entry *p;
+
+	p = kmalloc(sizeof(*p) + len + 1, GFP_KERNEL);
+	if (p) {
+		char *name_copy = (char *) (p + 1);
+		memcpy(name_copy, name, len);
+		name_copy[len] = '\0';
+		p->name = name_copy;
+		p->len = len;
+		p->type = d_type;
+		p->ino = ino;
+		p->is_whiteout = false;
+	}
+
+	return p;
+}
+
+static int ovl_cache_entry_add_rb(struct ovl_readdir_data *rdd,
+				  const char *name, int len, u64 ino,
+				  unsigned int d_type)
+{
+	struct rb_node **newp = &rdd->root->rb_node;
+	struct rb_node *parent = NULL;
+	struct ovl_cache_entry *p;
+
+	while (*newp) {
+		int cmp;
+		struct ovl_cache_entry *tmp;
+
+		parent = *newp;
+		tmp = ovl_cache_entry_from_node(*newp);
+		cmp = strncmp(name, tmp->name, len);
+		if (cmp > 0)
+			newp = &tmp->node.rb_right;
+		else if (cmp < 0 || len < tmp->len)
+			newp = &tmp->node.rb_left;
+		else
+			return 0;
+	}
+
+	p = ovl_cache_entry_new(name, len, ino, d_type);
+	if (p == NULL)
+		return -ENOMEM;
+
+	list_add_tail(&p->l_node, rdd->list);
+	rb_link_node(&p->node, parent, newp);
+	rb_insert_color(&p->node, rdd->root);
+
+	return 0;
+}
+
+static int ovl_fill_lower(void *buf, const char *name, int namelen,
+			    loff_t offset, u64 ino, unsigned int d_type)
+{
+	struct ovl_readdir_data *rdd = buf;
+	struct ovl_cache_entry *p;
+
+	rdd->count++;
+	p = ovl_cache_entry_find(rdd->root, name, namelen);
+	if (p) {
+		list_move_tail(&p->l_node, rdd->middle);
+	} else {
+		p = ovl_cache_entry_new(name, namelen, ino, d_type);
+		if (p == NULL)
+			rdd->err = -ENOMEM;
+		else
+			list_add_tail(&p->l_node, rdd->middle);
+	}
+
+	return rdd->err;
+}
+
+static void ovl_cache_free(struct list_head *list)
+{
+	struct ovl_cache_entry *p;
+	struct ovl_cache_entry *n;
+
+	list_for_each_entry_safe(p, n, list, l_node)
+		kfree(p);
+
+	INIT_LIST_HEAD(list);
+}
+
+static int ovl_fill_upper(void *buf, const char *name, int namelen,
+			  loff_t offset, u64 ino, unsigned int d_type)
+{
+	struct ovl_readdir_data *rdd = buf;
+
+	rdd->count++;
+	return ovl_cache_entry_add_rb(rdd, name, namelen, ino, d_type);
+}
+
+static inline int ovl_dir_read(struct path *realpath,
+			       struct ovl_readdir_data *rdd, filldir_t filler)
+{
+	struct file *realfile;
+	int err;
+
+	realfile = ovl_path_open(realpath, O_RDONLY | O_DIRECTORY);
+	if (IS_ERR(realfile))
+		return PTR_ERR(realfile);
+
+	do {
+		rdd->count = 0;
+		rdd->err = 0;
+		err = vfs_readdir(realfile, filler, rdd);
+		if (err >= 0)
+			err = rdd->err;
+	} while (!err && rdd->count);
+	fput(realfile);
+
+	return 0;
+}
+
+static void ovl_dir_reset(struct file *file)
+{
+	struct ovl_dir_file *od = file->private_data;
+	enum ovl_path_type type = ovl_path_type(file->f_path.dentry);
+
+	if (ovl_dentry_version_get(file->f_path.dentry) != od->cache_version) {
+		list_del_init(&od->cursor);
+		ovl_cache_free(&od->cache);
+		od->is_cached = false;
+	}
+	WARN_ON(!od->is_real && type != OVL_PATH_MERGE);
+	if (od->is_real && type == OVL_PATH_MERGE) {
+		fput(od->realfile);
+		od->realfile = NULL;
+		od->is_real = false;
+	}
+}
+
+static int ovl_dir_mark_whiteouts(struct ovl_readdir_data *rdd)
+{
+	struct ovl_cache_entry *p;
+	struct dentry *dentry;
+	const struct cred *old_cred;
+	struct cred *override_cred;
+
+	override_cred = prepare_creds();
+	if (!override_cred) {
+		ovl_cache_free(rdd->list);
+		return -ENOMEM;
+	}
+
+	/*
+	 * CAP_SYS_ADMIN for getxattr
+	 * CAP_DAC_OVERRIDE for lookup
+	 */
+	cap_raise(override_cred->cap_effective, CAP_SYS_ADMIN);
+	cap_raise(override_cred->cap_effective, CAP_DAC_OVERRIDE);
+	old_cred = override_creds(override_cred);
+
+	mutex_lock(&rdd->dir->d_inode->i_mutex);
+	list_for_each_entry(p, rdd->list, l_node) {
+		if (p->type != DT_LNK)
+			continue;
+
+		dentry = lookup_one_len(p->name, rdd->dir, p->len);
+		if (IS_ERR(dentry))
+			continue;
+
+		p->is_whiteout = ovl_is_whiteout(dentry);
+		dput(dentry);
+	}
+	mutex_unlock(&rdd->dir->d_inode->i_mutex);
+
+	revert_creds(old_cred);
+	put_cred(override_cred);
+
+	return 0;
+}
+
+static inline int ovl_dir_read_merged(struct path *upperpath,
+				      struct path *lowerpath,
+				      struct ovl_readdir_data *rdd)
+{
+	int err;
+	struct rb_root root = RB_ROOT;
+	struct list_head middle;
+
+	rdd->root = &root;
+	if (upperpath->dentry) {
+		rdd->dir = upperpath->dentry;
+		err = ovl_dir_read(upperpath, rdd, ovl_fill_upper);
+		if (err)
+			goto out;
+
+		err = ovl_dir_mark_whiteouts(rdd);
+		if (err)
+			goto out;
+	}
+	/*
+	 * Insert lowerpath entries before upperpath ones, this allows
+	 * offsets to be reasonably constant
+	 */
+	list_add(&middle, rdd->list);
+	rdd->middle = &middle;
+	err = ovl_dir_read(lowerpath, rdd, ovl_fill_lower);
+	list_del(&middle);
+out:
+	rdd->root = NULL;
+
+	return err;
+}
+
+static void ovl_seek_cursor(struct ovl_dir_file *od, loff_t pos)
+{
+	struct list_head *l;
+	loff_t off;
+
+	l = od->cache.next;
+	for (off = 0; off < pos; off++) {
+		if (l == &od->cache)
+			break;
+		l = l->next;
+	}
+	list_move_tail(&od->cursor, l);
+}
+
+static int ovl_readdir(struct file *file, void *buf, filldir_t filler)
+{
+	struct ovl_dir_file *od = file->private_data;
+	int res;
+
+	if (!file->f_pos)
+		ovl_dir_reset(file);
+
+	if (od->is_real) {
+		res = vfs_readdir(od->realfile, filler, buf);
+		file->f_pos = od->realfile->f_pos;
+
+		return res;
+	}
+
+	if (!od->is_cached) {
+		struct path lowerpath;
+		struct path upperpath;
+		struct ovl_readdir_data rdd = { .list = &od->cache };
+
+		ovl_path_lower(file->f_path.dentry, &lowerpath);
+		ovl_path_upper(file->f_path.dentry, &upperpath);
+
+		res = ovl_dir_read_merged(&upperpath, &lowerpath, &rdd);
+		if (res) {
+			ovl_cache_free(rdd.list);
+			return res;
+		}
+
+		od->cache_version = ovl_dentry_version_get(file->f_path.dentry);
+		od->is_cached = true;
+
+		ovl_seek_cursor(od, file->f_pos);
+	}
+
+	while (od->cursor.next != &od->cache) {
+		int over;
+		loff_t off;
+		struct ovl_cache_entry *p;
+
+		p = list_entry(od->cursor.next, struct ovl_cache_entry, l_node);
+		off = file->f_pos;
+		if (!p->is_whiteout) {
+			over = filler(buf, p->name, p->len, off, p->ino,
+				      p->type);
+			if (over)
+				break;
+		}
+		file->f_pos++;
+		list_move(&od->cursor, &p->l_node);
+	}
+
+	return 0;
+}
+
+static loff_t ovl_dir_llseek(struct file *file, loff_t offset, int origin)
+{
+	loff_t res;
+	struct ovl_dir_file *od = file->private_data;
+
+	mutex_lock(&file->f_dentry->d_inode->i_mutex);
+	if (!file->f_pos)
+		ovl_dir_reset(file);
+
+	if (od->is_real) {
+		res = vfs_llseek(od->realfile, offset, origin);
+		file->f_pos = od->realfile->f_pos;
+	} else {
+		res = -EINVAL;
+
+		switch (origin) {
+		case SEEK_CUR:
+			offset += file->f_pos;
+			break;
+		case SEEK_SET:
+			break;
+		default:
+			goto out_unlock;
+		}
+		if (offset < 0)
+			goto out_unlock;
+
+		if (offset != file->f_pos) {
+			file->f_pos = offset;
+			if (od->is_cached)
+				ovl_seek_cursor(od, offset);
+		}
+		res = offset;
+	}
+out_unlock:
+	mutex_unlock(&file->f_dentry->d_inode->i_mutex);
+
+	return res;
+}
+
+static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
+			 int datasync)
+{
+	struct ovl_dir_file *od = file->private_data;
+
+	/* May need to reopen directory if it got copied up */
+	if (!od->realfile) {
+		struct path upperpath;
+
+		ovl_path_upper(file->f_path.dentry, &upperpath);
+		od->realfile = ovl_path_open(&upperpath, O_RDONLY);
+		if (IS_ERR(od->realfile))
+			return PTR_ERR(od->realfile);
+	}
+
+	return vfs_fsync_range(od->realfile, start, end, datasync);
+}
+
+static int ovl_dir_release(struct inode *inode, struct file *file)
+{
+	struct ovl_dir_file *od = file->private_data;
+
+	list_del(&od->cursor);
+	ovl_cache_free(&od->cache);
+	if (od->realfile)
+		fput(od->realfile);
+	kfree(od);
+
+	return 0;
+}
+
+static int ovl_dir_open(struct inode *inode, struct file *file)
+{
+	struct path realpath;
+	struct file *realfile;
+	struct ovl_dir_file *od;
+	enum ovl_path_type type;
+
+	od = kzalloc(sizeof(struct ovl_dir_file), GFP_KERNEL);
+	if (!od)
+		return -ENOMEM;
+
+	type = ovl_path_real(file->f_path.dentry, &realpath);
+	realfile = ovl_path_open(&realpath, file->f_flags);
+	if (IS_ERR(realfile)) {
+		kfree(od);
+		return PTR_ERR(realfile);
+	}
+	INIT_LIST_HEAD(&od->cache);
+	INIT_LIST_HEAD(&od->cursor);
+	od->is_cached = false;
+	od->realfile = realfile;
+	od->is_real = (type != OVL_PATH_MERGE);
+	file->private_data = od;
+
+	return 0;
+}
+
+const struct file_operations ovl_dir_operations = {
+	.read		= generic_read_dir,
+	.open		= ovl_dir_open,
+	.readdir	= ovl_readdir,
+	.llseek		= ovl_dir_llseek,
+	.fsync		= ovl_dir_fsync,
+	.release	= ovl_dir_release,
+};
+
+static int ovl_check_empty_dir(struct dentry *dentry, struct list_head *list)
+{
+	int err;
+	struct path lowerpath;
+	struct path upperpath;
+	struct ovl_cache_entry *p;
+	struct ovl_readdir_data rdd = { .list = list };
+
+	ovl_path_upper(dentry, &upperpath);
+	ovl_path_lower(dentry, &lowerpath);
+
+	err = ovl_dir_read_merged(&upperpath, &lowerpath, &rdd);
+	if (err)
+		return err;
+
+	err = 0;
+
+	list_for_each_entry(p, list, l_node) {
+		if (p->is_whiteout)
+			continue;
+
+		if (p->name[0] == '.') {
+			if (p->len == 1)
+				continue;
+			if (p->len == 2 && p->name[1] == '.')
+				continue;
+		}
+		err = -ENOTEMPTY;
+		break;
+	}
+
+	return err;
+}
+
+static int ovl_remove_whiteouts(struct dentry *dir, struct list_head *list)
+{
+	struct path upperpath;
+	struct dentry *upperdir;
+	struct ovl_cache_entry *p;
+	const struct cred *old_cred;
+	struct cred *override_cred;
+	int err;
+
+	ovl_path_upper(dir, &upperpath);
+	upperdir = upperpath.dentry;
+
+	override_cred = prepare_creds();
+	if (!override_cred)
+		return -ENOMEM;
+
+	/*
+	 * CAP_DAC_OVERRIDE for lookup and unlink
+	 * CAP_SYS_ADMIN for setxattr of "trusted" namespace
+	 * CAP_FOWNER for unlink in sticky directory
+	 */
+	cap_raise(override_cred->cap_effective, CAP_DAC_OVERRIDE);
+	cap_raise(override_cred->cap_effective, CAP_SYS_ADMIN);
+	cap_raise(override_cred->cap_effective, CAP_FOWNER);
+	old_cred = override_creds(override_cred);
+
+	err = vfs_setxattr(upperdir, ovl_opaque_xattr, "y", 1, 0);
+	if (err)
+		goto out_revert_creds;
+
+	mutex_lock_nested(&upperdir->d_inode->i_mutex, I_MUTEX_PARENT);
+	list_for_each_entry(p, list, l_node) {
+		struct dentry *dentry;
+		int ret;
+
+		if (!p->is_whiteout)
+			continue;
+
+		dentry = lookup_one_len(p->name, upperdir, p->len);
+		if (IS_ERR(dentry)) {
+			printk(KERN_WARNING
+			    "overlayfs: failed to lookup whiteout %.*s: %li\n",
+			    p->len, p->name, PTR_ERR(dentry));
+			continue;
+		}
+		ret = vfs_unlink(upperdir->d_inode, dentry);
+		dput(dentry);
+		if (ret)
+			printk(KERN_WARNING
+			    "overlayfs: failed to unlink whiteout %.*s: %i\n",
+			    p->len, p->name, ret);
+	}
+	mutex_unlock(&upperdir->d_inode->i_mutex);
+
+out_revert_creds:
+	revert_creds(old_cred);
+	put_cred(override_cred);
+
+	return err;
+}
+
+int ovl_check_empty_and_clear(struct dentry *dentry, enum ovl_path_type type)
+{
+	int err;
+	LIST_HEAD(list);
+
+	err = ovl_check_empty_dir(dentry, &list);
+	if (!err && type == OVL_PATH_MERGE)
+		err = ovl_remove_whiteouts(dentry, &list);
+
+	ovl_cache_free(&list);
+
+	return err;
+}