[RFC 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, except that since
there's only a single underlying layer, all dentry lookups go through
this.  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.

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

---

Changes so far: fixed up locking and addressed viro's comments

use negative dentries on the underlying cached in d_fsdata to remove
the extra lookup_one_len() calls

Add show_options/statfs callbacks

Add proper Kconfig plumbing

diff --git a/fs/Kconfig b/fs/Kconfig
index 6725f59..a9b0834 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -94,6 +94,14 @@ 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 85b6e13..ff9890e 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 0000000..d352377
--- /dev/null
+++ b/fs/shiftfs.c
@@ -0,0 +1,833 @@
+#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>
+
+struct shiftfs_super_info {
+	struct vfsmount *mnt;
+	struct uid_gid_map uid_map, gid_map;
+};
+
+static struct inode *shiftfs_new_inode(struct super_block *sb, umode_t mode,
+				       struct dentry *dentry);
+
+enum {
+	OPT_UIDMAP,
+	OPT_GIDMAP,
+	OPT_LAST,
+};
+
+/* global filesystem options */
+static const match_table_t tokens = {
+	{ OPT_UIDMAP, "uidmap=%u:%u:%u" },
+	{ OPT_GIDMAP, "gidmap=%u:%u:%u" },
+	{ OPT_LAST, NULL }
+};
+
+/*
+ * code stolen from user_namespace.c ... except that these functions
+ * return the same id back if unmapped ... should probably have a
+ * library?
+ */
+static u32 map_id_down(struct uid_gid_map *map, u32 id)
+{
+	unsigned idx, extents;
+	u32 first, last;
+
+	/* Find the matching extent */
+	extents = map->nr_extents;
+	smp_rmb();
+	for (idx = 0; idx < extents; idx++) {
+		first = map->extent[idx].first;
+		last = first + map->extent[idx].count - 1;
+		if (id >= first && id <= last)
+			break;
+	}
+	/* Map the id or note failure */
+	if (idx < extents)
+		id = (id - first) + map->extent[idx].lower_first;
+
+	return id;
+}
+
+static u32 map_id_up(struct uid_gid_map *map, u32 id)
+{
+	unsigned idx, extents;
+	u32 first, last;
+
+	/* Find the matching extent */
+	extents = map->nr_extents;
+	smp_rmb();
+	for (idx = 0; idx < extents; idx++) {
+		first = map->extent[idx].lower_first;
+		last = first + map->extent[idx].count - 1;
+		if (id >= first && id <= last)
+			break;
+	}
+	/* Map the id or note failure */
+	if (idx < extents)
+		id = (id - first) + map->extent[idx].first;
+
+	return id;
+}
+
+static bool mappings_overlap(struct uid_gid_map *new_map,
+			     struct uid_gid_extent *extent)
+{
+	u32 upper_first, lower_first, upper_last, lower_last;
+	unsigned idx;
+
+	upper_first = extent->first;
+	lower_first = extent->lower_first;
+	upper_last = upper_first + extent->count - 1;
+	lower_last = lower_first + extent->count - 1;
+
+	for (idx = 0; idx < new_map->nr_extents; idx++) {
+		u32 prev_upper_first, prev_lower_first;
+		u32 prev_upper_last, prev_lower_last;
+		struct uid_gid_extent *prev;
+
+		prev = &new_map->extent[idx];
+
+		prev_upper_first = prev->first;
+		prev_lower_first = prev->lower_first;
+		prev_upper_last = prev_upper_first + prev->count - 1;
+		prev_lower_last = prev_lower_first + prev->count - 1;
+
+		/* Does the upper range intersect a previous extent? */
+		if ((prev_upper_first <= upper_last) &&
+		    (prev_upper_last >= upper_first))
+			return true;
+
+		/* Does the lower range intersect a previous extent? */
+		if ((prev_lower_first <= lower_last) &&
+		    (prev_lower_last >= lower_first))
+			return true;
+	}
+	return false;
+}
+/* end code stolen from user_namespace.c */
+
+static const struct cred *shiftfs_get_up_creds(struct super_block *sb)
+{
+	struct cred *cred = prepare_creds();
+	struct shiftfs_super_info *ssi = sb->s_fs_info;
+
+	if (!cred)
+		return NULL;
+
+	cred->fsuid = KUIDT_INIT(map_id_up(&ssi->uid_map, __kuid_val(cred->fsuid)));
+	cred->fsgid = KGIDT_INIT(map_id_up(&ssi->gid_map, __kgid_val(cred->fsgid)));
+
+	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 "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];
+	int from, to, count;
+	struct uid_gid_map *map, *maps[2] = {
+		[OPT_UIDMAP] = &ssi->uid_map,
+		[OPT_GIDMAP] = &ssi->gid_map,
+	};
+
+	while ((p = strsep(&options, ",")) != NULL) {
+		int token;
+		struct uid_gid_extent ext;
+
+		if (!*p)
+			continue;
+
+		token = match_token(p, tokens, args);
+		if (token != OPT_UIDMAP && token != OPT_GIDMAP)
+			return -EINVAL;
+		if (match_int(&args[0], &from) ||
+		    match_int(&args[1], &to) ||
+		    match_int(&args[2], &count))
+			return -EINVAL;
+		map = maps[token];
+		if (map->nr_extents >= UID_GID_MAP_MAX_EXTENTS)
+			return -EINVAL;
+		ext.first = from;
+		ext.lower_first = to;
+		ext.count = count;
+		if (mappings_overlap(map, &ext))
+			return -EINVAL;
+		map->extent[map->nr_extents++] = ext;
+	}
+	return 0;
+}
+
+static void shiftfs_d_release(struct dentry *dentry)
+{
+	struct dentry *real = dentry->d_fsdata;
+
+	dput(real);
+}
+
+static const struct dentry_operations shiftfs_dentry_ops = {
+	.d_release	= shiftfs_d_release,
+};
+
+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, const char *name,
+			    const void *value, size_t size, int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+	const struct inode_operations *iop = real->d_inode->i_op;
+	int err = -EOPNOTSUPP;
+
+	if (iop->setxattr) {
+		const struct cred *oldcred, *newcred;
+
+		oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+		err = iop->setxattr(real, name, value, size, flags);
+		shiftfs_old_creds(oldcred, &newcred);
+	}
+
+	return err;
+}
+
+static ssize_t shiftfs_getxattr(struct dentry *dentry, const char *name,
+				void *value, size_t size)
+{
+	struct dentry *real = dentry->d_fsdata;
+	const struct inode_operations *iop = real->d_inode->i_op;
+	int err = -EOPNOTSUPP;
+
+	if (iop->getxattr) {
+		const struct cred *oldcred, *newcred;
+
+		oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+		err = iop->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;
+	const struct inode_operations *iop = real->d_inode->i_op;
+
+	if (iop->listxattr)
+		return iop->listxattr(real, list, size);
+
+	return -EINVAL;
+}
+
+static int shiftfs_removexattr(struct dentry *dentry, const char *name)
+{
+	struct dentry *real = dentry->d_fsdata;
+	const struct inode_operations *iop = real->d_inode->i_op;
+
+	if (iop->removexattr)
+		return iop->removexattr(real, name);
+
+	return -EINVAL;
+}
+
+static void shiftfs_fill_inode(struct inode *inode, struct dentry *dentry)
+{
+	struct inode *reali;
+	struct shiftfs_super_info *ssi = inode->i_sb->s_fs_info;
+
+	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;
+
+	inode->i_uid = KUIDT_INIT(map_id_down(&ssi->uid_map, __kuid_val(reali->i_uid)));
+	inode->i_gid = KGIDT_INIT(map_id_down(&ssi->gid_map, __kgid_val(reali->i_gid)));
+}
+
+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_rename2(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;
+
+	/* note: violation of usual fs rules here: dentries are never
+	 * added with d_add.  This is because we want no dentry cache
+	 * for shiftfs.  All lookups proceed through the dentry cache
+	 * of the underlying filesystem, meaning we always see any
+	 * changes in the underlying */
+
+	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;
+
+	if (!new->d_inode)
+		return NULL;
+
+	newi = shiftfs_new_inode(dentry->d_sb, new->d_inode->i_mode, new);
+	if (!newi) {
+		dput(new);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	d_instantiate(dentry, newi);
+
+	return NULL;
+}
+
+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;
+
+	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 shiftfs_super_info *ssi = dentry->d_sb->s_fs_info;
+	int err;
+
+	newattr.ia_uid = KUIDT_INIT(map_id_up(&ssi->uid_map, __kuid_val(attr->ia_uid)));
+	newattr.ia_gid = KGIDT_INIT(map_id_up(&ssi->gid_map, __kgid_val(attr->ia_gid)));
+
+	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
+	if (iop->setattr)
+		err = iop->setattr(real, &newattr);
+	else
+		err = simple_setattr(real, &newattr);
+	shiftfs_old_creds(oldcred, &newcred);
+
+	return err;
+}
+
+static int shiftfs_getattr(struct vfsmount *mnt, struct dentry *dentry,
+			   struct kstat *stat)
+{
+	struct inode *inode = dentry->d_inode;
+	struct dentry *real = inode->i_private;
+	struct inode *reali = real->d_inode;
+	const struct inode_operations *iop = reali->i_op;
+	int err = 0;
+
+	mnt = dentry->d_sb->s_fs_info;
+
+	if (iop->getattr)
+		err = iop->getattr(mnt, real, stat);
+	else
+		generic_fillattr(reali, stat);
+
+	if (err)
+		return err;
+
+	stat->uid = inode->i_uid;
+	stat->gid = inode->i_gid;
+	return 0;
+}
+
+struct shiftfs_fop_carrier {
+	struct inode *inode;
+	int (*release)(struct inode *, struct file *);
+	struct file_operations fop;
+};
+
+static int shiftfs_release(struct inode *inode, struct file *file)
+{
+	struct shiftfs_fop_carrier *sfc;
+	int err = 0;
+
+	sfc = container_of(file->f_op, struct shiftfs_fop_carrier, fop);
+
+	if (sfc->release)
+		err = sfc->release(inode, file);
+
+	file->f_inode = sfc->inode;
+	file->f_op = sfc->inode->i_fop;
+	fops_put(inode->i_fop);
+
+	kfree(sfc);
+
+	return err;
+}
+
+static int shiftfs_open(struct inode *inode, struct file *file)
+{
+	struct dentry *real = inode->i_private;
+	struct inode *reali = real->d_inode;
+	const struct file_operations *fop;
+	struct shiftfs_fop_carrier *sfc;
+	int err = 0;
+
+	sfc = kmalloc(sizeof(*sfc), GFP_KERNEL);
+	if (!sfc)
+		return -ENOMEM;
+
+	if (real->d_flags & DCACHE_OP_SELECT_INODE)
+		reali = real->d_op->d_select_inode(real, file->f_flags);
+
+	fop = fops_get(reali->i_fop);
+	sfc->inode = inode;
+	memcpy(&sfc->fop, fop, sizeof(*fop));
+	sfc->release = sfc->fop.release;
+	sfc->fop.release = shiftfs_release;
+
+	file->f_op = &sfc->fop;
+	file->f_inode = reali;
+
+	if (fop->open)
+		err = fop->open(reali, file);
+
+	return err;
+}
+
+static const struct inode_operations shiftfs_inode_ops = {
+	/* intercepted */
+	.lookup		= shiftfs_lookup,
+	.getattr	= shiftfs_getattr,
+	.setattr	= shiftfs_setattr,
+	.permission	= shiftfs_permission,
+
+	/*pass though */
+	.mkdir		= shiftfs_mkdir,
+	.symlink	= shiftfs_symlink,
+	.get_link	= shiftfs_get_link,
+	.readlink	= shiftfs_readlink,
+	.unlink		= shiftfs_unlink,
+	.rmdir		= shiftfs_rmdir,
+	.rename2	= shiftfs_rename2,
+	.link		= shiftfs_link,
+	.create		= shiftfs_create,
+	.mknod		= NULL,	/* no special files currently */
+	.setxattr	= shiftfs_setxattr,
+	.getxattr	= shiftfs_getxattr,
+	.listxattr	= shiftfs_listxattr,
+	.removexattr	= shiftfs_removexattr,
+};
+
+static const struct file_operations shiftfs_file_ops = {
+	.open		= shiftfs_open,
+};
+
+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;
+
+	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;
+	inode->i_fop = &shiftfs_file_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;
+
+	static const char *options[] = { "uidmap", "gidmap" };
+	const struct uid_gid_map *map[ARRAY_SIZE(options)] =
+		{ &ssi->uid_map, &ssi->gid_map };
+	int i, j;
+
+	for (i = 0; i < ARRAY_SIZE(options); i++) {
+		for (j = 0; j < map[i]->nr_extents; j++) {
+			const struct uid_gid_extent *ext = &map[i]->extent[j];
+
+			seq_show_option(m, options[i], NULL);
+			seq_printf(m, "=%u:%u:%u", ext->first,
+				   ext->lower_first, ext->count);
+		}
+	}
+
+	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);
+	kfree(ssi);
+}
+
+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;
+
+	if (!name)
+		goto out;
+
+	ssi = kzalloc(sizeof(*ssi), GFP_KERNEL);
+	if (!ssi)
+		goto out;
+
+	err = -EPERM;
+	if (!capable(CAP_SYS_ADMIN))
+		goto out;
+
+	err = shiftfs_parse_options(ssi, data->data);
+	if (err)
+		goto out;
+
+	err = kern_path(name, LOOKUP_FOLLOW, &path);
+	if (err)
+		goto out;
+
+	if (!S_ISDIR(path.dentry->d_inode->i_mode)) {
+		err = -ENOTDIR;
+		goto out_put;
+	}
+	ssi->mnt = path.mnt;
+
+	sb->s_fs_info = ssi;
+	sb->s_magic = SHIFTFS_MAGIC;
+	sb->s_op = &shiftfs_super_ops;
+	sb->s_d_op = &shiftfs_dentry_ops;
+	sb->s_root = d_make_root(shiftfs_new_inode(sb, S_IFDIR, path.dentry));
+	sb->s_root->d_fsdata = path.dentry;
+
+	return 0;
+
+ out_put:
+	path_put(&path);
+ out:
+	kfree(name);
+	if (err)
+		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,
+};
+
+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 0de181a..d7992f5 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -79,4 +79,6 @@
 #define NSFS_MAGIC		0x6e736673
 #define BPF_FS_MAGIC		0xcafe4a11
 
+#define SHIFTFS_MAGIC		0x6a656a62
+
 #endif /* __LINUX_MAGIC_H__ */