[PATCH v3 1/1] shiftfs: uid/gid shifting bind mount

[James Bottomley]

This allows any subtree to be uid/gid shifted and bound elsewhere.  It
does this by operating simlarly to overlayfs.  Its primary use is for
shifting the underlying uids of filesystems used to support
unpriviliged (uid shifted) containers.  The usual use case here is
that the container is operating with an uid shifted unprivileged root
but sometimes needs to make use of or work with a filesystem image
that has root at real uid 0.

The mechanism is to allow any subordinate mount namespace to mount a
shiftfs filesystem (by marking it FS_USERNS_MOUNT) but only allowing
it to mount marked subtrees (using the -o mark option as root).  Once
mounted, the subtree is mapped via the super block user namespace so
that the interior ids of the mounting user namespace are the ids
written to the filesystem.

Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>

---

v3 - update to 4.14 (d_real changes)
v1 - based on original shiftfs with uid mappings now done via s_user_ns
v2 - fix revalidation of dentries
     add inode aliasing
---
 fs/Kconfig                 |   8 +
 fs/Makefile                |   1 +
 fs/shiftfs.c               | 783 +++++++++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/magic.h |   2 +
 4 files changed, 794 insertions(+)
 create mode 100644 fs/shiftfs.c

diff --git a/fs/Kconfig b/fs/Kconfig
index 7aee6d699fd6..8b9c2b8566a9 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -105,6 +105,14 @@ source "fs/autofs4/Kconfig"
 source "fs/fuse/Kconfig"
 source "fs/overlayfs/Kconfig"
 
+config SHIFT_FS
+	tristate "UID/GID shifting overlay filesystem for containers"
+	help
+	  This filesystem can overlay any mounted filesystem and shift
+	  the uid/gid the files appear at.  The idea is that
+	  unprivileged containers can use this to mount root volumes
+	  using this technique.
+
 menu "Caches"
 
 source "fs/fscache/Kconfig"
diff --git a/fs/Makefile b/fs/Makefile
index 7bbaca9c67b1..2aa3ad47a286 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -128,3 +128,4 @@ obj-y				+= exofs/ # Multiple modules
 obj-$(CONFIG_CEPH_FS)		+= ceph/
 obj-$(CONFIG_PSTORE)		+= pstore/
 obj-$(CONFIG_EFIVAR_FS)		+= efivarfs/
+obj-$(CONFIG_SHIFT_FS)		+= shiftfs.o
diff --git a/fs/shiftfs.c b/fs/shiftfs.c
new file mode 100644
index 000000000000..7984a93745d2
--- /dev/null
+++ b/fs/shiftfs.c
@@ -0,0 +1,783 @@
+#include <linux/cred.h>
+#include <linux/mount.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/namei.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/magic.h>
+#include <linux/parser.h>
+#include <linux/seq_file.h>
+#include <linux/statfs.h>
+#include <linux/slab.h>
+#include <linux/user_namespace.h>
+#include <linux/uidgid.h>
+#include <linux/xattr.h>
+
+struct shiftfs_super_info {
+	struct vfsmount *mnt;
+	struct user_namespace *userns;
+	bool mark;
+};
+
+static struct inode *shiftfs_new_inode(struct super_block *sb, umode_t mode,
+				       struct dentry *dentry);
+
+enum {
+	OPT_MARK,
+	OPT_LAST,
+};
+
+/* global filesystem options */
+static const match_table_t tokens = {
+	{ OPT_MARK, "mark" },
+	{ OPT_LAST, NULL }
+};
+
+static const struct cred *shiftfs_get_up_creds(struct super_block *sb)
+{
+	struct shiftfs_super_info *ssi = sb->s_fs_info;
+	struct cred *cred = prepare_creds();
+
+	if (!cred)
+		return NULL;
+
+	cred->fsuid = KUIDT_INIT(from_kuid(sb->s_user_ns, cred->fsuid));
+	cred->fsgid = KGIDT_INIT(from_kgid(sb->s_user_ns, cred->fsgid));
+	put_user_ns(cred->user_ns);
+	cred->user_ns = get_user_ns(ssi->userns);
+
+	return cred;
+}
+
+static const struct cred *shiftfs_new_creds(const struct cred **newcred,
+					    struct super_block *sb)
+{
+	const struct cred *cred = shiftfs_get_up_creds(sb);
+
+	*newcred = cred;
+
+	if (cred)
+		cred = override_creds(cred);
+	else
+		printk(KERN_ERR "shiftfs: Credential override failed: no memory\n");
+
+	return cred;
+}
+
+static void shiftfs_old_creds(const struct cred *oldcred,
+			      const struct cred **newcred)
+{
+	if (!*newcred)
+		return;
+
+	revert_creds(oldcred);
+	put_cred(*newcred);
+}
+
+static int shiftfs_parse_options(struct shiftfs_super_info *ssi, char *options)
+{
+	char *p;
+	substring_t args[MAX_OPT_ARGS];
+
+	ssi->mark = false;
+
+	while ((p = strsep(&options, ",")) != NULL) {
+		int token;
+
+		if (!*p)
+			continue;
+
+		token = match_token(p, tokens, args);
+		switch (token) {
+		case OPT_MARK:
+			ssi->mark = true;
+			break;
+		default:
+			return -EINVAL;
+		}
+	}
+	return 0;
+}
+
+static void shiftfs_d_release(struct dentry *dentry)
+{
+	struct dentry *real = dentry->d_fsdata;
+
+	dput(real);
+}
+
+static struct dentry *shiftfs_d_real(struct dentry *dentry,
+				     const struct inode *inode,
+				     unsigned int open_flags,
+				     unsigned int dreal_flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+
+	if (unlikely(real->d_flags & DCACHE_OP_REAL))
+		return real->d_op->d_real(real, real->d_inode,
+					  open_flags, dreal_flags);
+
+	return real;
+}
+
+static int shiftfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+
+	if (d_unhashed(real))
+		return 0;
+
+	if (!(real->d_flags & DCACHE_OP_WEAK_REVALIDATE))
+		return 1;
+
+	return real->d_op->d_weak_revalidate(real, flags);
+}
+
+static int shiftfs_d_revalidate(struct dentry *dentry, unsigned int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+	int ret;
+
+	if (d_unhashed(real))
+		return 0;
+
+	/*
+	 * inode state of underlying changed from positive to negative
+	 * or vice versa; force a lookup to update our view
+	 */
+	if (d_is_negative(real) != d_is_negative(dentry))
+		return 0;
+
+	if (!(real->d_flags & DCACHE_OP_REVALIDATE))
+		return 1;
+
+	ret = real->d_op->d_revalidate(real, flags);
+
+	if (ret == 0 && !(flags & LOOKUP_RCU))
+		d_invalidate(real);
+
+	return ret;
+}
+
+static const struct dentry_operations shiftfs_dentry_ops = {
+	.d_release	= shiftfs_d_release,
+	.d_real		= shiftfs_d_real,
+	.d_revalidate	= shiftfs_d_revalidate,
+	.d_weak_revalidate = shiftfs_d_weak_revalidate,
+};
+
+static int shiftfs_readlink(struct dentry *dentry, char __user *data,
+			    int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+	const struct inode_operations *iop = real->d_inode->i_op;
+
+	if (iop->readlink)
+		return iop->readlink(real, data, flags);
+
+	return -EINVAL;
+}
+
+static const char *shiftfs_get_link(struct dentry *dentry, struct inode *inode,
+				    struct delayed_call *done)
+{
+	if (dentry) {
+		struct dentry *real = dentry->d_fsdata;
+		struct inode *reali = real->d_inode;
+		const struct inode_operations *iop = reali->i_op;
+		const char *res = ERR_PTR(-EPERM);
+
+		if (iop->get_link)
+			res = iop->get_link(real, reali, done);
+
+		return res;
+	} else {
+		/* RCU lookup not supported */
+		return ERR_PTR(-ECHILD);
+	}
+}
+
+static int shiftfs_setxattr(struct dentry *dentry, struct inode *inode,
+			    const char *name, const void *value,
+			    size_t size, int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+	int err = -EOPNOTSUPP;
+	const struct cred *oldcred, *newcred;
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	err = vfs_setxattr(real, name, value, size, flags);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	return err;
+}
+
+static int shiftfs_xattr_get(const struct xattr_handler *handler,
+			     struct dentry *dentry, struct inode *inode,
+			     const char *name, void *value, size_t size)
+{
+	struct dentry *real = dentry->d_fsdata;
+	int err;
+	const struct cred *oldcred, *newcred;
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	err = vfs_getxattr(real, name, value, size);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	return err;
+}
+
+static ssize_t shiftfs_listxattr(struct dentry *dentry, char *list,
+				 size_t size)
+{
+	struct dentry *real = dentry->d_fsdata;
+	int err;
+	const struct cred *oldcred, *newcred;
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	err = vfs_listxattr(real, list, size);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	return err;
+}
+
+static int shiftfs_removexattr(struct dentry *dentry, const char *name)
+{
+	struct dentry *real = dentry->d_fsdata;
+	int err;
+	const struct cred *oldcred, *newcred;
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	err = vfs_removexattr(real, name);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	return err;
+}
+
+static int shiftfs_xattr_set(const struct xattr_handler *handler,
+			     struct dentry *dentry, struct inode *inode,
+			     const char *name, const void *value, size_t size,
+			     int flags)
+{
+	if (!value)
+		return shiftfs_removexattr(dentry, name);
+	return shiftfs_setxattr(dentry, inode, name, value, size, flags);
+}
+
+static void shiftfs_fill_inode(struct inode *inode, struct dentry *dentry)
+{
+	struct inode *reali;
+
+	if (!dentry)
+		return;
+
+	reali = dentry->d_inode;
+
+	if (!reali->i_op->get_link)
+		inode->i_opflags |= IOP_NOFOLLOW;
+
+	inode->i_mapping = reali->i_mapping;
+	inode->i_private = dentry;
+}
+
+static int shiftfs_make_object(struct inode *dir, struct dentry *dentry,
+			       umode_t mode, const char *symlink,
+			       struct dentry *hardlink, bool excl)
+{
+	struct dentry *real = dir->i_private, *new = dentry->d_fsdata;
+	struct inode *reali = real->d_inode, *newi;
+	const struct inode_operations *iop = reali->i_op;
+	int err;
+	const struct cred *oldcred, *newcred;
+	bool op_ok = false;
+
+	if (hardlink) {
+		op_ok = iop->link;
+	} else {
+		switch (mode & S_IFMT) {
+		case S_IFDIR:
+			op_ok = iop->mkdir;
+			break;
+		case S_IFREG:
+			op_ok = iop->create;
+			break;
+		case S_IFLNK:
+			op_ok = iop->symlink;
+		}
+	}
+	if (!op_ok)
+		return -EINVAL;
+
+
+	newi = shiftfs_new_inode(dentry->d_sb, mode, NULL);
+	if (!newi)
+		return -ENOMEM;
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+
+	inode_lock_nested(reali, I_MUTEX_PARENT);
+
+	err = -EINVAL;		/* shut gcc up about uninit var */
+	if (hardlink) {
+		struct dentry *realhardlink = hardlink->d_fsdata;
+
+		err = vfs_link(realhardlink, reali, new, NULL);
+	} else {
+		switch (mode & S_IFMT) {
+		case S_IFDIR:
+			err = vfs_mkdir(reali, new, mode);
+			break;
+		case S_IFREG:
+			err = vfs_create(reali, new, mode, excl);
+			break;
+		case S_IFLNK:
+			err = vfs_symlink(reali, new, symlink);
+		}
+	}
+
+	shiftfs_old_creds(oldcred, &newcred);
+
+	if (err)
+		goto out_dput;
+
+	shiftfs_fill_inode(newi, new);
+
+	d_instantiate(dentry, newi);
+
+	new = NULL;
+	newi = NULL;
+
+ out_dput:
+	dput(new);
+	iput(newi);
+	inode_unlock(reali);
+
+	return err;
+}
+
+static int shiftfs_create(struct inode *dir, struct dentry *dentry,
+			  umode_t mode,  bool excl)
+{
+	mode |= S_IFREG;
+
+	return shiftfs_make_object(dir, dentry, mode, NULL, NULL, excl);
+}
+
+static int shiftfs_mkdir(struct inode *dir, struct dentry *dentry,
+			 umode_t mode)
+{
+	mode |= S_IFDIR;
+
+	return shiftfs_make_object(dir, dentry, mode, NULL, NULL, false);
+}
+
+static int shiftfs_link(struct dentry *hardlink, struct inode *dir,
+			struct dentry *dentry)
+{
+	return shiftfs_make_object(dir, dentry, 0, NULL, hardlink, false);
+}
+
+static int shiftfs_symlink(struct inode *dir, struct dentry *dentry,
+			   const char *symlink)
+{
+	return shiftfs_make_object(dir, dentry, S_IFLNK, symlink, NULL, false);
+}
+
+static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
+{
+	struct dentry *real = dir->i_private, *new = dentry->d_fsdata;
+	struct inode *reali = real->d_inode;
+	int err;
+	const struct cred *oldcred, *newcred;
+
+	inode_lock_nested(reali, I_MUTEX_PARENT);
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+
+	if (rmdir)
+		err = vfs_rmdir(reali, new);
+	else
+		err = vfs_unlink(reali, new, NULL);
+
+	shiftfs_old_creds(oldcred, &newcred);
+	inode_unlock(reali);
+
+	return err;
+}
+
+static int shiftfs_unlink(struct inode *dir, struct dentry *dentry)
+{
+	return shiftfs_rm(dir, dentry, false);
+}
+
+static int shiftfs_rmdir(struct inode *dir, struct dentry *dentry)
+{
+	return shiftfs_rm(dir, dentry, true);
+}
+
+static int shiftfs_rename(struct inode *olddir, struct dentry *old,
+			  struct inode *newdir, struct dentry *new,
+			  unsigned int flags)
+{
+	struct dentry *rodd = olddir->i_private, *rndd = newdir->i_private,
+		*realold = old->d_fsdata,
+		*realnew = new->d_fsdata, *trap;
+	struct inode *realolddir = rodd->d_inode, *realnewdir = rndd->d_inode;
+	int err = -EINVAL;
+	const struct cred *oldcred, *newcred;
+
+	trap = lock_rename(rndd, rodd);
+
+	if (trap == realold || trap == realnew)
+		goto out_unlock;
+
+	oldcred = shiftfs_new_creds(&newcred, old->d_sb);
+
+	err = vfs_rename(realolddir, realold, realnewdir,
+			 realnew, NULL, flags);
+
+	shiftfs_old_creds(oldcred, &newcred);
+
+ out_unlock:
+	unlock_rename(rndd, rodd);
+
+	return err;
+}
+
+static struct dentry *shiftfs_lookup(struct inode *dir, struct dentry *dentry,
+				     unsigned int flags)
+{
+	struct dentry *real = dir->i_private, *new;
+	struct inode *reali = real->d_inode, *newi;
+	const struct cred *oldcred, *newcred;
+
+	inode_lock(reali);
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	new = lookup_one_len(dentry->d_name.name, real, dentry->d_name.len);
+	shiftfs_old_creds(oldcred, &newcred);
+	inode_unlock(reali);
+
+	if (IS_ERR(new))
+		return new;
+
+	dentry->d_fsdata = new;
+
+	newi = NULL;
+	if (!new->d_inode)
+		goto out;
+
+	newi = shiftfs_new_inode(dentry->d_sb, new->d_inode->i_mode, new);
+	if (!newi) {
+		dput(new);
+		return ERR_PTR(-ENOMEM);
+	}
+
+ out:
+	return d_splice_alias(newi, dentry);
+}
+
+static int shiftfs_permission(struct inode *inode, int mask)
+{
+	struct dentry *real = inode->i_private;
+	struct inode *reali = real->d_inode;
+	const struct inode_operations *iop = reali->i_op;
+	int err;
+	const struct cred *oldcred, *newcred;
+
+	if (mask & MAY_NOT_BLOCK)
+		return -ECHILD;
+
+	oldcred = shiftfs_new_creds(&newcred, inode->i_sb);
+	if (iop->permission)
+		err = iop->permission(reali, mask);
+	else
+		err = generic_permission(reali, mask);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	return err;
+}
+
+static int shiftfs_setattr(struct dentry *dentry, struct iattr *attr)
+{
+	struct dentry *real = dentry->d_fsdata;
+	struct inode *reali = real->d_inode;
+	const struct inode_operations *iop = reali->i_op;
+	struct iattr newattr = *attr;
+	const struct cred *oldcred, *newcred;
+	struct super_block *sb = dentry->d_sb;
+	int err;
+
+	newattr.ia_uid = KUIDT_INIT(from_kuid(sb->s_user_ns, attr->ia_uid));
+	newattr.ia_gid = KGIDT_INIT(from_kgid(sb->s_user_ns, attr->ia_gid));
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	inode_lock(reali);
+	if (iop->setattr)
+		err = iop->setattr(real, &newattr);
+	else
+		err = simple_setattr(real, &newattr);
+	inode_unlock(reali);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	if (err)
+		return err;
+
+	/* all OK, reflect the change on our inode */
+	setattr_copy(d_inode(dentry), attr);
+	return 0;
+}
+
+static int shiftfs_getattr(const struct path *path, struct kstat *stat,
+			   u32 request_mask, unsigned int query_flags)
+{
+	struct inode *inode = path->dentry->d_inode;
+	struct dentry *real = path->dentry->d_fsdata;
+	struct inode *reali = real->d_inode;
+	const struct inode_operations *iop = reali->i_op;
+	struct path newpath = { path->dentry->d_sb->s_fs_info, real };
+	int err = 0;
+
+	if (iop->getattr)
+		err = iop->getattr(&newpath, stat, request_mask, query_flags);
+	else
+		generic_fillattr(reali, stat);
+
+	if (err)
+		return err;
+
+	/* transform the underlying id */
+	stat->uid = make_kuid(inode->i_sb->s_user_ns, __kuid_val(stat->uid));
+	stat->gid = make_kgid(inode->i_sb->s_user_ns, __kgid_val(stat->gid));
+	return 0;
+}
+
+static const struct inode_operations shiftfs_inode_ops = {
+	.lookup		= shiftfs_lookup,
+	.getattr	= shiftfs_getattr,
+	.setattr	= shiftfs_setattr,
+	.permission	= shiftfs_permission,
+	.mkdir		= shiftfs_mkdir,
+	.symlink	= shiftfs_symlink,
+	.get_link	= shiftfs_get_link,
+	.readlink	= shiftfs_readlink,
+	.unlink		= shiftfs_unlink,
+	.rmdir		= shiftfs_rmdir,
+	.rename		= shiftfs_rename,
+	.link		= shiftfs_link,
+	.create		= shiftfs_create,
+	.mknod		= NULL,	/* no special files currently */
+	.listxattr	= shiftfs_listxattr,
+};
+
+static struct inode *shiftfs_new_inode(struct super_block *sb, umode_t mode,
+				       struct dentry *dentry)
+{
+	struct inode *inode;
+
+	inode = new_inode(sb);
+	if (!inode)
+		return NULL;
+
+	/*
+	 * our inode is completely vestigial.  All lookups, getattr
+	 * and permission checks are done on the underlying inode, so
+	 * what the user sees is entirely from the underlying inode.
+	 */
+	mode &= S_IFMT;
+
+	inode->i_ino = get_next_ino();
+	inode->i_mode = mode;
+	inode->i_flags |= S_NOATIME | S_NOCMTIME;
+
+	inode->i_op = &shiftfs_inode_ops;
+
+	shiftfs_fill_inode(inode, dentry);
+
+	return inode;
+}
+
+static int shiftfs_show_options(struct seq_file *m, struct dentry *dentry)
+{
+	struct super_block *sb = dentry->d_sb;
+	struct shiftfs_super_info *ssi = sb->s_fs_info;
+
+	if (ssi->mark)
+		seq_show_option(m, "mark", NULL);
+
+	return 0;
+}
+
+static int shiftfs_statfs(struct dentry *dentry, struct kstatfs *buf)
+{
+	struct super_block *sb = dentry->d_sb;
+	struct shiftfs_super_info *ssi = sb->s_fs_info;
+	struct dentry *root = sb->s_root;
+	struct dentry *realroot = root->d_fsdata;
+	struct path realpath = { .mnt = ssi->mnt, .dentry = realroot };
+	int err;
+
+	err = vfs_statfs(&realpath, buf);
+	if (err)
+		return err;
+
+	buf->f_type = sb->s_magic;
+
+	return 0;
+}
+
+static void shiftfs_put_super(struct super_block *sb)
+{
+	struct shiftfs_super_info *ssi = sb->s_fs_info;
+
+	mntput(ssi->mnt);
+	put_user_ns(ssi->userns);
+	kfree(ssi);
+}
+
+static const struct xattr_handler shiftfs_xattr_handler = {
+	.prefix = "",
+	.get    = shiftfs_xattr_get,
+	.set    = shiftfs_xattr_set,
+};
+
+const struct xattr_handler *shiftfs_xattr_handlers[] = {
+	&shiftfs_xattr_handler,
+	NULL
+};
+
+static const struct super_operations shiftfs_super_ops = {
+	.put_super	= shiftfs_put_super,
+	.show_options	= shiftfs_show_options,
+	.statfs		= shiftfs_statfs,
+};
+
+struct shiftfs_data {
+	void *data;
+	const char *path;
+};
+
+static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
+			      int silent)
+{
+	struct shiftfs_data *data = raw_data;
+	char *name = kstrdup(data->path, GFP_KERNEL);
+	int err = -ENOMEM;
+	struct shiftfs_super_info *ssi = NULL;
+	struct path path;
+	struct dentry *dentry;
+
+	if (!name)
+		goto out;
+
+	ssi = kzalloc(sizeof(*ssi), GFP_KERNEL);
+	if (!ssi)
+		goto out;
+
+	err = -EPERM;
+	err = shiftfs_parse_options(ssi, data->data);
+	if (err)
+		goto out;
+
+	/* to mark a mount point, must be real root */
+	if (ssi->mark && !capable(CAP_SYS_ADMIN))
+		goto out;
+
+	/* else to mount a mark, must be userns admin */
+	if (!ssi->mark && !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
+		goto out;
+
+	err = kern_path(name, LOOKUP_FOLLOW, &path);
+	if (err)
+		goto out;
+
+	err = -EPERM;
+
+	if (!S_ISDIR(path.dentry->d_inode->i_mode)) {
+		err = -ENOTDIR;
+		goto out_put;
+	}
+
+	sb->s_stack_depth = path.dentry->d_sb->s_stack_depth + 1;
+	if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+		printk(KERN_ERR "shiftfs: maximum stacking depth exceeded\n");
+		err = -EINVAL;
+		goto out_put;
+	}
+
+	if (ssi->mark) {
+		/*
+		 * this part is visible unshifted, so make sure no
+		 * executables that could be used to give suid
+		 * privileges
+		 */
+		sb->s_iflags = SB_I_NOEXEC;
+		ssi->mnt = path.mnt;
+		dentry = path.dentry;
+	} else {
+		struct shiftfs_super_info *mp_ssi;
+
+		/*
+		 * this leg executes if we're admin capable in
+		 * the namespace, so be very careful
+		 */
+		if (path.dentry->d_sb->s_magic != SHIFTFS_MAGIC)
+			goto out_put;
+		mp_ssi = path.dentry->d_sb->s_fs_info;
+		if (!mp_ssi->mark)
+			goto out_put;
+		ssi->mnt = mntget(mp_ssi->mnt);
+		dentry = dget(path.dentry->d_fsdata);
+		path_put(&path);
+	}
+	ssi->userns = get_user_ns(dentry->d_sb->s_user_ns);
+	sb->s_fs_info = ssi;
+	sb->s_magic = SHIFTFS_MAGIC;
+	sb->s_op = &shiftfs_super_ops;
+	sb->s_xattr = shiftfs_xattr_handlers;
+	sb->s_d_op = &shiftfs_dentry_ops;
+	sb->s_root = d_make_root(shiftfs_new_inode(sb, S_IFDIR, dentry));
+	sb->s_root->d_fsdata = dentry;
+
+	return 0;
+
+ out_put:
+	path_put(&path);
+ out:
+	kfree(name);
+	kfree(ssi);
+	return err;
+}
+
+static struct dentry *shiftfs_mount(struct file_system_type *fs_type,
+				    int flags, const char *dev_name, void *data)
+{
+	struct shiftfs_data d = { data, dev_name };
+
+	return mount_nodev(fs_type, flags, &d, shiftfs_fill_super);
+}
+
+static struct file_system_type shiftfs_type = {
+	.owner		= THIS_MODULE,
+	.name		= "shiftfs",
+	.mount		= shiftfs_mount,
+	.kill_sb	= kill_anon_super,
+	.fs_flags	= FS_USERNS_MOUNT,
+};
+
+static int __init shiftfs_init(void)
+{
+	return register_filesystem(&shiftfs_type);
+}
+
+static void __exit shiftfs_exit(void)
+{
+	unregister_filesystem(&shiftfs_type);
+}
+
+MODULE_ALIAS_FS("shiftfs");
+MODULE_AUTHOR("James Bottomley");
+MODULE_DESCRIPTION("uid/gid shifting bind filesystem");
+MODULE_LICENSE("GPL v2");
+module_init(shiftfs_init)
+module_exit(shiftfs_exit)
diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index e439565df838..1b3db66384a2 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -88,4 +88,6 @@
 #define BALLOON_KVM_MAGIC	0x13661366
 #define ZSMALLOC_MAGIC		0x58295829
 
+#define SHIFTFS_MAGIC		0x6a656a62
+
 #endif /* __LINUX_MAGIC_H__ */
-- 
2.13.7