Add support for shiftfs

UID/GID shifting overlay filesystem for containers

Signed-off-by: Mike Pagano <mpagano@gentoo.org>

2 files changed, 2206 insertions, 0 deletions

diff --git a/0000_README b/0000_README
index 25a88270..b2d54828 100644
--- a/0000_README
+++ b/0000_README
@@ -419,6 +419,10 @@ Patch:  4567_distro-Gentoo-Kconfig.patch
 From:   Tom Wijsman <TomWij@gentoo.org>
 Desc:   Add Gentoo Linux support config settings and defaults.
 
+Patch:  5000_shifts-ubuntu-20.04.patch
+From:   https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal
+Desc:   UID/GID shifting overlay filesystem for containers 
+
 Patch:  5011_enable-cpu-optimizations-for-gcc8.patch
 From:   https://github.com/graysky2/kernel_gcc_patch/
 Desc:   Kernel patch for >= gccv8 enables kernel >= v4.13 optimizations for additional CPUs.
diff --git a/5000_shifts-ubuntu-20.04.patch b/5000_shifts-ubuntu-20.04.patch
new file mode 100644
index 00000000..f213d939
--- /dev/null
+++ b/5000_shifts-ubuntu-20.04.patch
@@ -0,0 +1,2202 @@
+--- /dev/null	2021-01-06 15:31:07.232620794 -0500
++++ b/fs/shiftfs.c	2021-01-06 19:04:01.754354287 -0500
+@@ -0,0 +1,2156 @@
++#include <linux/btrfs.h>
++#include <linux/capability.h>
++#include <linux/cred.h>
++#include <linux/mount.h>
++#include <linux/fdtable.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/security.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>
++#include <linux/posix_acl.h>
++#include <linux/posix_acl_xattr.h>
++#include <linux/uio.h>
++
++struct shiftfs_super_info {
++	struct vfsmount *mnt;
++	struct user_namespace *userns;
++	/* creds of process who created the super block */
++	const struct cred *creator_cred;
++	bool mark;
++	unsigned int passthrough;
++	unsigned int passthrough_mark;
++};
++
++static void shiftfs_fill_inode(struct inode *inode, unsigned long ino,
++			       umode_t mode, dev_t dev, struct dentry *dentry);
++
++#define SHIFTFS_PASSTHROUGH_NONE 0
++#define SHIFTFS_PASSTHROUGH_STAT 1
++#define SHIFTFS_PASSTHROUGH_IOCTL 2
++#define SHIFTFS_PASSTHROUGH_ALL                                                \
++	(SHIFTFS_PASSTHROUGH_STAT | SHIFTFS_PASSTHROUGH_IOCTL)
++
++static inline bool shiftfs_passthrough_ioctls(struct shiftfs_super_info *info)
++{
++	if (!(info->passthrough & SHIFTFS_PASSTHROUGH_IOCTL))
++		return false;
++
++	return true;
++}
++
++static inline bool shiftfs_passthrough_statfs(struct shiftfs_super_info *info)
++{
++	if (!(info->passthrough & SHIFTFS_PASSTHROUGH_STAT))
++		return false;
++
++	return true;
++}
++
++enum {
++	OPT_MARK,
++	OPT_PASSTHROUGH,
++	OPT_LAST,
++};
++
++/* global filesystem options */
++static const match_table_t tokens = {
++	{ OPT_MARK, "mark" },
++	{ OPT_PASSTHROUGH, "passthrough=%u" },
++	{ OPT_LAST, NULL }
++};
++
++static const struct cred *shiftfs_override_creds(const struct super_block *sb)
++{
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++
++	return override_creds(sbinfo->creator_cred);
++}
++
++static inline void shiftfs_revert_object_creds(const struct cred *oldcred,
++					       struct cred *newcred)
++{
++	revert_creds(oldcred);
++	put_cred(newcred);
++}
++
++static kuid_t shift_kuid(struct user_namespace *from, struct user_namespace *to,
++			 kuid_t kuid)
++{
++	uid_t uid = from_kuid(from, kuid);
++	return make_kuid(to, uid);
++}
++
++static kgid_t shift_kgid(struct user_namespace *from, struct user_namespace *to,
++			 kgid_t kgid)
++{
++	gid_t gid = from_kgid(from, kgid);
++	return make_kgid(to, gid);
++}
++
++static int shiftfs_override_object_creds(const struct super_block *sb,
++					 const struct cred **oldcred,
++					 struct cred **newcred,
++					 struct dentry *dentry, umode_t mode,
++					 bool hardlink)
++{
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++	kuid_t fsuid = current_fsuid();
++	kgid_t fsgid = current_fsgid();
++
++	*oldcred = shiftfs_override_creds(sb);
++
++	*newcred = prepare_creds();
++	if (!*newcred) {
++		revert_creds(*oldcred);
++		return -ENOMEM;
++	}
++
++	(*newcred)->fsuid = shift_kuid(sb->s_user_ns, sbinfo->userns, fsuid);
++	(*newcred)->fsgid = shift_kgid(sb->s_user_ns, sbinfo->userns, fsgid);
++
++	if (!hardlink) {
++		int err = security_dentry_create_files_as(dentry, mode,
++							  &dentry->d_name,
++							  *oldcred, *newcred);
++		if (err) {
++			shiftfs_revert_object_creds(*oldcred, *newcred);
++			return err;
++		}
++	}
++
++	put_cred(override_creds(*newcred));
++	return 0;
++}
++
++static void shiftfs_copyattr(struct inode *from, struct inode *to)
++{
++	struct user_namespace *from_ns = from->i_sb->s_user_ns;
++	struct user_namespace *to_ns = to->i_sb->s_user_ns;
++
++	to->i_uid = shift_kuid(from_ns, to_ns, from->i_uid);
++	to->i_gid = shift_kgid(from_ns, to_ns, from->i_gid);
++	to->i_mode = from->i_mode;
++	to->i_atime = from->i_atime;
++	to->i_mtime = from->i_mtime;
++	to->i_ctime = from->i_ctime;
++	i_size_write(to, i_size_read(from));
++}
++
++static void shiftfs_copyflags(struct inode *from, struct inode *to)
++{
++	unsigned int mask = S_SYNC | S_IMMUTABLE | S_APPEND | S_NOATIME;
++
++	inode_set_flags(to, from->i_flags & mask, mask);
++}
++
++static void shiftfs_file_accessed(struct file *file)
++{
++	struct inode *upperi, *loweri;
++
++	if (file->f_flags & O_NOATIME)
++		return;
++
++	upperi = file_inode(file);
++	loweri = upperi->i_private;
++
++	if (!loweri)
++		return;
++
++	upperi->i_mtime = loweri->i_mtime;
++	upperi->i_ctime = loweri->i_ctime;
++
++	touch_atime(&file->f_path);
++}
++
++static int shiftfs_parse_mount_options(struct shiftfs_super_info *sbinfo,
++				       char *options)
++{
++	char *p;
++	substring_t args[MAX_OPT_ARGS];
++
++	sbinfo->mark = false;
++	sbinfo->passthrough = 0;
++
++	while ((p = strsep(&options, ",")) != NULL) {
++		int err, intarg, token;
++
++		if (!*p)
++			continue;
++
++		token = match_token(p, tokens, args);
++		switch (token) {
++		case OPT_MARK:
++			sbinfo->mark = true;
++			break;
++		case OPT_PASSTHROUGH:
++			err = match_int(&args[0], &intarg);
++			if (err)
++				return err;
++
++			if (intarg & ~SHIFTFS_PASSTHROUGH_ALL)
++				return -EINVAL;
++
++			sbinfo->passthrough = intarg;
++			break;
++		default:
++			return -EINVAL;
++		}
++	}
++
++	return 0;
++}
++
++static void shiftfs_d_release(struct dentry *dentry)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++
++	if (lowerd)
++		dput(lowerd);
++}
++
++static struct dentry *shiftfs_d_real(struct dentry *dentry,
++				     const struct inode *inode)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++
++	if (inode && d_inode(dentry) == inode)
++		return dentry;
++
++	lowerd = d_real(lowerd, inode);
++	if (lowerd && (!inode || inode == d_inode(lowerd)))
++		return lowerd;
++
++	WARN(1, "shiftfs_d_real(%pd4, %s:%lu): real dentry not found\n", dentry,
++	     inode ? inode->i_sb->s_id : "NULL", inode ? inode->i_ino : 0);
++	return dentry;
++}
++
++static int shiftfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags)
++{
++	int err = 1;
++	struct dentry *lowerd = dentry->d_fsdata;
++
++	if (d_is_negative(lowerd) != d_is_negative(dentry))
++		return 0;
++
++	if ((lowerd->d_flags & DCACHE_OP_WEAK_REVALIDATE))
++		err = lowerd->d_op->d_weak_revalidate(lowerd, flags);
++
++	if (d_really_is_positive(dentry)) {
++		struct inode *inode = d_inode(dentry);
++		struct inode *loweri = d_inode(lowerd);
++
++		shiftfs_copyattr(loweri, inode);
++	}
++
++	return err;
++}
++
++static int shiftfs_d_revalidate(struct dentry *dentry, unsigned int flags)
++{
++	int err = 1;
++	struct dentry *lowerd = dentry->d_fsdata;
++
++	if (d_unhashed(lowerd) ||
++	    ((d_is_negative(lowerd) != d_is_negative(dentry))))
++		return 0;
++
++	if (flags & LOOKUP_RCU)
++		return -ECHILD;
++
++	if ((lowerd->d_flags & DCACHE_OP_REVALIDATE))
++		err = lowerd->d_op->d_revalidate(lowerd, flags);
++
++	if (d_really_is_positive(dentry)) {
++		struct inode *inode = d_inode(dentry);
++		struct inode *loweri = d_inode(lowerd);
++
++		shiftfs_copyattr(loweri, inode);
++	}
++
++	return err;
++}
++
++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 const char *shiftfs_get_link(struct dentry *dentry, struct inode *inode,
++				    struct delayed_call *done)
++{
++	const char *p;
++	const struct cred *oldcred;
++	struct dentry *lowerd;
++
++	/* RCU lookup not supported */
++	if (!dentry)
++		return ERR_PTR(-ECHILD);
++
++	lowerd = dentry->d_fsdata;
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	p = vfs_get_link(lowerd, done);
++	revert_creds(oldcred);
++
++	return p;
++}
++
++static int shiftfs_setxattr(struct dentry *dentry, struct inode *inode,
++			    const char *name, const void *value,
++			    size_t size, int flags)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++	int err;
++	const struct cred *oldcred;
++
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	err = vfs_setxattr(lowerd, name, value, size, flags);
++	revert_creds(oldcred);
++
++	shiftfs_copyattr(lowerd->d_inode, inode);
++
++	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 *lowerd = dentry->d_fsdata;
++	int err;
++	const struct cred *oldcred;
++
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	err = vfs_getxattr(lowerd, name, value, size);
++	revert_creds(oldcred);
++
++	return err;
++}
++
++static ssize_t shiftfs_listxattr(struct dentry *dentry, char *list,
++				 size_t size)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++	int err;
++	const struct cred *oldcred;
++
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	err = vfs_listxattr(lowerd, list, size);
++	revert_creds(oldcred);
++
++	return err;
++}
++
++static int shiftfs_removexattr(struct dentry *dentry, const char *name)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++	int err;
++	const struct cred *oldcred;
++
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	err = vfs_removexattr(lowerd, name);
++	revert_creds(oldcred);
++
++	/* update c/mtime */
++	shiftfs_copyattr(lowerd->d_inode, d_inode(dentry));
++
++	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 int shiftfs_inode_test(struct inode *inode, void *data)
++{
++	return inode->i_private == data;
++}
++
++static int shiftfs_inode_set(struct inode *inode, void *data)
++{
++	inode->i_private = data;
++	return 0;
++}
++
++static int shiftfs_create_object(struct inode *diri, struct dentry *dentry,
++				 umode_t mode, const char *symlink,
++				 struct dentry *hardlink, bool excl)
++{
++	int err;
++	const struct cred *oldcred;
++	struct cred *newcred;
++	void *loweri_iop_ptr = NULL;
++	umode_t modei = mode;
++	struct super_block *dir_sb = diri->i_sb;
++	struct dentry *lowerd_new = dentry->d_fsdata;
++	struct inode *inode = NULL, *loweri_dir = diri->i_private;
++	const struct inode_operations *loweri_dir_iop = loweri_dir->i_op;
++	struct dentry *lowerd_link = NULL;
++
++	if (hardlink) {
++		loweri_iop_ptr = loweri_dir_iop->link;
++	} else {
++		switch (mode & S_IFMT) {
++		case S_IFDIR:
++			loweri_iop_ptr = loweri_dir_iop->mkdir;
++			break;
++		case S_IFREG:
++			loweri_iop_ptr = loweri_dir_iop->create;
++			break;
++		case S_IFLNK:
++			loweri_iop_ptr = loweri_dir_iop->symlink;
++			break;
++		case S_IFSOCK:
++			/* fall through */
++		case S_IFIFO:
++			loweri_iop_ptr = loweri_dir_iop->mknod;
++			break;
++		}
++	}
++	if (!loweri_iop_ptr) {
++		err = -EINVAL;
++		goto out_iput;
++	}
++
++	inode_lock_nested(loweri_dir, I_MUTEX_PARENT);
++
++	if (!hardlink) {
++		inode = new_inode(dir_sb);
++		if (!inode) {
++			err = -ENOMEM;
++			goto out_iput;
++		}
++
++		/*
++		 * new_inode() will have added the new inode to the super
++		 * block's list of inodes. Further below we will call
++		 * inode_insert5() Which would perform the same operation again
++		 * thereby corrupting the list. To avoid this raise I_CREATING
++		 * in i_state which will cause inode_insert5() to skip this
++		 * step. I_CREATING will be cleared by d_instantiate_new()
++		 * below.
++		 */
++		spin_lock(&inode->i_lock);
++		inode->i_state |= I_CREATING;
++		spin_unlock(&inode->i_lock);
++
++		inode_init_owner(inode, diri, mode);
++		modei = inode->i_mode;
++	}
++
++	err = shiftfs_override_object_creds(dentry->d_sb, &oldcred, &newcred,
++					    dentry, modei, hardlink != NULL);
++	if (err)
++		goto out_iput;
++
++	if (hardlink) {
++		lowerd_link = hardlink->d_fsdata;
++		err = vfs_link(lowerd_link, loweri_dir, lowerd_new, NULL);
++	} else {
++		switch (modei & S_IFMT) {
++		case S_IFDIR:
++			err = vfs_mkdir(loweri_dir, lowerd_new, modei);
++			break;
++		case S_IFREG:
++			err = vfs_create(loweri_dir, lowerd_new, modei, excl);
++			break;
++		case S_IFLNK:
++			err = vfs_symlink(loweri_dir, lowerd_new, symlink);
++			break;
++		case S_IFSOCK:
++			/* fall through */
++		case S_IFIFO:
++			err = vfs_mknod(loweri_dir, lowerd_new, modei, 0);
++			break;
++		default:
++			err = -EINVAL;
++			break;
++		}
++	}
++
++	shiftfs_revert_object_creds(oldcred, newcred);
++
++	if (!err && WARN_ON(!lowerd_new->d_inode))
++		err = -EIO;
++	if (err)
++		goto out_iput;
++
++	if (hardlink) {
++		inode = d_inode(hardlink);
++		ihold(inode);
++
++		/* copy up times from lower inode */
++		shiftfs_copyattr(d_inode(lowerd_link), inode);
++		set_nlink(d_inode(hardlink), d_inode(lowerd_link)->i_nlink);
++		d_instantiate(dentry, inode);
++	} else {
++		struct inode *inode_tmp;
++		struct inode *loweri_new = d_inode(lowerd_new);
++
++		inode_tmp = inode_insert5(inode, (unsigned long)loweri_new,
++					  shiftfs_inode_test, shiftfs_inode_set,
++					  loweri_new);
++		if (unlikely(inode_tmp != inode)) {
++			pr_err_ratelimited("shiftfs: newly created inode found in cache\n");
++			iput(inode_tmp);
++			err = -EINVAL;
++			goto out_iput;
++		}
++
++		ihold(loweri_new);
++		shiftfs_fill_inode(inode, loweri_new->i_ino, loweri_new->i_mode,
++				   0, lowerd_new);
++		d_instantiate_new(dentry, inode);
++	}
++
++	shiftfs_copyattr(loweri_dir, diri);
++	if (loweri_iop_ptr == loweri_dir_iop->mkdir)
++		set_nlink(diri, loweri_dir->i_nlink);
++
++	inode = NULL;
++
++out_iput:
++	iput(inode);
++	inode_unlock(loweri_dir);
++
++	return err;
++}
++
++static int shiftfs_create(struct inode *dir, struct dentry *dentry,
++			  umode_t mode,  bool excl)
++{
++	mode |= S_IFREG;
++
++	return shiftfs_create_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_create_object(dir, dentry, mode, NULL, NULL, false);
++}
++
++static int shiftfs_link(struct dentry *hardlink, struct inode *dir,
++			struct dentry *dentry)
++{
++	return shiftfs_create_object(dir, dentry, 0, NULL, hardlink, false);
++}
++
++static int shiftfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode,
++			 dev_t rdev)
++{
++	if (!S_ISFIFO(mode) && !S_ISSOCK(mode))
++		return -EPERM;
++
++	return shiftfs_create_object(dir, dentry, mode, NULL, NULL, false);
++}
++
++static int shiftfs_symlink(struct inode *dir, struct dentry *dentry,
++			   const char *symlink)
++{
++	return shiftfs_create_object(dir, dentry, S_IFLNK, symlink, NULL, false);
++}
++
++static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++	struct inode *loweri = dir->i_private;
++	struct inode *inode = d_inode(dentry);
++	int err;
++	const struct cred *oldcred;
++
++	dget(lowerd);
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	inode_lock_nested(loweri, I_MUTEX_PARENT);
++	if (rmdir)
++		err = vfs_rmdir(loweri, lowerd);
++	else
++		err = vfs_unlink(loweri, lowerd, NULL);
++	revert_creds(oldcred);
++
++	if (!err) {
++		d_drop(dentry);
++
++		if (rmdir)
++			clear_nlink(inode);
++		else
++			drop_nlink(inode);
++	}
++	inode_unlock(loweri);
++
++	shiftfs_copyattr(loweri, dir);
++	dput(lowerd);
++
++	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 *lowerd_dir_old = old->d_parent->d_fsdata,
++		      *lowerd_dir_new = new->d_parent->d_fsdata,
++		      *lowerd_old = old->d_fsdata, *lowerd_new = new->d_fsdata,
++		      *trapd;
++	struct inode *loweri_dir_old = lowerd_dir_old->d_inode,
++		     *loweri_dir_new = lowerd_dir_new->d_inode;
++	int err = -EINVAL;
++	const struct cred *oldcred;
++
++	trapd = lock_rename(lowerd_dir_new, lowerd_dir_old);
++
++	if (trapd == lowerd_old || trapd == lowerd_new)
++		goto out_unlock;
++
++	oldcred = shiftfs_override_creds(old->d_sb);
++	err = vfs_rename(loweri_dir_old, lowerd_old, loweri_dir_new, lowerd_new,
++			 NULL, flags);
++	revert_creds(oldcred);
++
++	shiftfs_copyattr(loweri_dir_old, olddir);
++	shiftfs_copyattr(loweri_dir_new, newdir);
++
++out_unlock:
++	unlock_rename(lowerd_dir_new, lowerd_dir_old);
++
++	return err;
++}
++
++static struct dentry *shiftfs_lookup(struct inode *dir, struct dentry *dentry,
++				     unsigned int flags)
++{
++	struct dentry *new;
++	struct inode *newi;
++	const struct cred *oldcred;
++	struct dentry *lowerd = dentry->d_parent->d_fsdata;
++	struct inode *inode = NULL, *loweri = lowerd->d_inode;
++
++	inode_lock(loweri);
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	new = lookup_one_len(dentry->d_name.name, lowerd, dentry->d_name.len);
++	revert_creds(oldcred);
++	inode_unlock(loweri);
++
++	if (IS_ERR(new))
++		return new;
++
++	dentry->d_fsdata = new;
++
++	newi = new->d_inode;
++	if (!newi)
++		goto out;
++
++	inode = iget5_locked(dentry->d_sb, (unsigned long)newi,
++			     shiftfs_inode_test, shiftfs_inode_set, newi);
++	if (!inode) {
++		dput(new);
++		return ERR_PTR(-ENOMEM);
++	}
++	if (inode->i_state & I_NEW) {
++		/*
++		 * inode->i_private set by shiftfs_inode_set(), but we still
++		 * need to take a reference
++		*/
++		ihold(newi);
++		shiftfs_fill_inode(inode, newi->i_ino, newi->i_mode, 0, new);
++		unlock_new_inode(inode);
++	}
++
++out:
++	return d_splice_alias(inode, dentry);
++}
++
++static int shiftfs_permission(struct inode *inode, int mask)
++{
++	int err;
++	const struct cred *oldcred;
++	struct inode *loweri = inode->i_private;
++
++	if (!loweri) {
++		WARN_ON(!(mask & MAY_NOT_BLOCK));
++		return -ECHILD;
++	}
++
++	err = generic_permission(inode, mask);
++	if (err)
++		return err;
++
++	oldcred = shiftfs_override_creds(inode->i_sb);
++	err = inode_permission(loweri, mask);
++	revert_creds(oldcred);
++
++	return err;
++}
++
++static int shiftfs_fiemap(struct inode *inode,
++			  struct fiemap_extent_info *fieinfo, u64 start,
++			  u64 len)
++{
++	int err;
++	const struct cred *oldcred;
++	struct inode *loweri = inode->i_private;
++
++	if (!loweri->i_op->fiemap)
++		return -EOPNOTSUPP;
++
++	oldcred = shiftfs_override_creds(inode->i_sb);
++	if (fieinfo->fi_flags & FIEMAP_FLAG_SYNC)
++		filemap_write_and_wait(loweri->i_mapping);
++	err = loweri->i_op->fiemap(loweri, fieinfo, start, len);
++	revert_creds(oldcred);
++
++	return err;
++}
++
++static int shiftfs_tmpfile(struct inode *dir, struct dentry *dentry,
++			   umode_t mode)
++{
++	int err;
++	const struct cred *oldcred;
++	struct dentry *lowerd = dentry->d_fsdata;
++	struct inode *loweri = dir->i_private;
++
++	if (!loweri->i_op->tmpfile)
++		return -EOPNOTSUPP;
++
++	oldcred = shiftfs_override_creds(dir->i_sb);
++	err = loweri->i_op->tmpfile(loweri, lowerd, mode);
++	revert_creds(oldcred);
++
++	return err;
++}
++
++static int shiftfs_setattr(struct dentry *dentry, struct iattr *attr)
++{
++	struct dentry *lowerd = dentry->d_fsdata;
++	struct inode *loweri = lowerd->d_inode;
++	struct iattr newattr;
++	const struct cred *oldcred;
++	struct super_block *sb = dentry->d_sb;
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++	int err;
++
++	err = setattr_prepare(dentry, attr);
++	if (err)
++		return err;
++
++	newattr = *attr;
++	newattr.ia_uid = shift_kuid(sb->s_user_ns, sbinfo->userns, attr->ia_uid);
++	newattr.ia_gid = shift_kgid(sb->s_user_ns, sbinfo->userns, attr->ia_gid);
++
++	/*
++	 * mode change is for clearing setuid/setgid bits. Allow lower fs
++	 * to interpret this in its own way.
++	 */
++	if (newattr.ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID))
++		newattr.ia_valid &= ~ATTR_MODE;
++
++	inode_lock(loweri);
++	oldcred = shiftfs_override_creds(dentry->d_sb);
++	err = notify_change(lowerd, &newattr, NULL);
++	revert_creds(oldcred);
++	inode_unlock(loweri);
++
++	shiftfs_copyattr(loweri, d_inode(dentry));
++
++	return err;
++}
++
++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 *lowerd = path->dentry->d_fsdata;
++	struct inode *loweri = lowerd->d_inode;
++	struct shiftfs_super_info *info = path->dentry->d_sb->s_fs_info;
++	struct path newpath = { .mnt = info->mnt, .dentry = lowerd };
++	struct user_namespace *from_ns = loweri->i_sb->s_user_ns;
++	struct user_namespace *to_ns = inode->i_sb->s_user_ns;
++	const struct cred *oldcred;
++	int err;
++
++	oldcred = shiftfs_override_creds(inode->i_sb);
++	err = vfs_getattr(&newpath, stat, request_mask, query_flags);
++	revert_creds(oldcred);
++
++	if (err)
++		return err;
++
++	/* transform the underlying id */
++	stat->uid = shift_kuid(from_ns, to_ns, stat->uid);
++	stat->gid = shift_kgid(from_ns, to_ns, stat->gid);
++	return 0;
++}
++
++#ifdef CONFIG_SHIFT_FS_POSIX_ACL
++
++static int
++shift_acl_ids(struct user_namespace *from, struct user_namespace *to,
++	      struct posix_acl *acl)
++{
++	int i;
++
++	for (i = 0; i < acl->a_count; i++) {
++		struct posix_acl_entry *e = &acl->a_entries[i];
++		switch(e->e_tag) {
++		case ACL_USER:
++			e->e_uid = shift_kuid(from, to, e->e_uid);
++			if (!uid_valid(e->e_uid))
++				return -EOVERFLOW;
++			break;
++		case ACL_GROUP:
++			e->e_gid = shift_kgid(from, to, e->e_gid);
++			if (!gid_valid(e->e_gid))
++				return -EOVERFLOW;
++			break;
++		}
++	}
++	return 0;
++}
++
++static void
++shift_acl_xattr_ids(struct user_namespace *from, struct user_namespace *to,
++		    void *value, size_t size)
++{
++	struct posix_acl_xattr_header *header = value;
++	struct posix_acl_xattr_entry *entry = (void *)(header + 1), *end;
++	int count;
++	kuid_t kuid;
++	kgid_t kgid;
++
++	if (!value)
++		return;
++	if (size < sizeof(struct posix_acl_xattr_header))
++		return;
++	if (header->a_version != cpu_to_le32(POSIX_ACL_XATTR_VERSION))
++		return;
++
++	count = posix_acl_xattr_count(size);
++	if (count < 0)
++		return;
++	if (count == 0)
++		return;
++
++	for (end = entry + count; entry != end; entry++) {
++		switch(le16_to_cpu(entry->e_tag)) {
++		case ACL_USER:
++			kuid = make_kuid(&init_user_ns, le32_to_cpu(entry->e_id));
++			kuid = shift_kuid(from, to, kuid);
++			entry->e_id = cpu_to_le32(from_kuid(&init_user_ns, kuid));
++			break;
++		case ACL_GROUP:
++			kgid = make_kgid(&init_user_ns, le32_to_cpu(entry->e_id));
++			kgid = shift_kgid(from, to, kgid);
++			entry->e_id = cpu_to_le32(from_kgid(&init_user_ns, kgid));
++			break;
++		default:
++			break;
++		}
++	}
++}
++
++static struct posix_acl *shiftfs_get_acl(struct inode *inode, int type)
++{
++	struct inode *loweri = inode->i_private;
++	const struct cred *oldcred;
++	struct posix_acl *lower_acl, *acl = NULL;
++	struct user_namespace *from_ns = loweri->i_sb->s_user_ns;
++	struct user_namespace *to_ns = inode->i_sb->s_user_ns;
++	int size;
++	int err;
++
++	if (!IS_POSIXACL(loweri))
++		return NULL;
++
++	oldcred = shiftfs_override_creds(inode->i_sb);
++	lower_acl = get_acl(loweri, type);
++	revert_creds(oldcred);
++
++	if (lower_acl && !IS_ERR(lower_acl)) {
++		/* XXX: export posix_acl_clone? */
++		size = sizeof(struct posix_acl) +
++		       lower_acl->a_count * sizeof(struct posix_acl_entry);
++		acl = kmemdup(lower_acl, size, GFP_KERNEL);
++		posix_acl_release(lower_acl);
++
++		if (!acl)
++			return ERR_PTR(-ENOMEM);
++
++		refcount_set(&acl->a_refcount, 1);
++
++		err = shift_acl_ids(from_ns, to_ns, acl);
++		if (err) {
++			kfree(acl);
++			return ERR_PTR(err);
++		}
++	}
++
++	return acl;
++}
++
++static int
++shiftfs_posix_acl_xattr_get(const struct xattr_handler *handler,
++			   struct dentry *dentry, struct inode *inode,
++			   const char *name, void *buffer, size_t size)
++{
++	struct inode *loweri = inode->i_private;
++	int ret;
++
++	ret = shiftfs_xattr_get(NULL, dentry, inode, handler->name,
++				buffer, size);
++	if (ret < 0)
++		return ret;
++
++	inode_lock(loweri);
++	shift_acl_xattr_ids(loweri->i_sb->s_user_ns, inode->i_sb->s_user_ns,
++			    buffer, size);
++	inode_unlock(loweri);
++	return ret;
++}
++
++static int
++shiftfs_posix_acl_xattr_set(const struct xattr_handler *handler,
++			    struct dentry *dentry, struct inode *inode,
++			    const char *name, const void *value,
++			    size_t size, int flags)
++{
++	struct inode *loweri = inode->i_private;
++	int err;
++
++	if (!IS_POSIXACL(loweri) || !loweri->i_op->set_acl)
++		return -EOPNOTSUPP;
++	if (handler->flags == ACL_TYPE_DEFAULT && !S_ISDIR(inode->i_mode))
++		return value ? -EACCES : 0;
++	if (!inode_owner_or_capable(inode))
++		return -EPERM;
++
++	if (value) {
++		shift_acl_xattr_ids(inode->i_sb->s_user_ns,
++				    loweri->i_sb->s_user_ns,
++				    (void *)value, size);
++		err = shiftfs_setxattr(dentry, inode, handler->name, value,
++				       size, flags);
++	} else {
++		err = shiftfs_removexattr(dentry, handler->name);
++	}
++
++	if (!err)
++		shiftfs_copyattr(loweri, inode);
++
++	return err;
++}
++
++static const struct xattr_handler
++shiftfs_posix_acl_access_xattr_handler = {
++	.name = XATTR_NAME_POSIX_ACL_ACCESS,
++	.flags = ACL_TYPE_ACCESS,
++	.get = shiftfs_posix_acl_xattr_get,
++	.set = shiftfs_posix_acl_xattr_set,
++};
++
++static const struct xattr_handler
++shiftfs_posix_acl_default_xattr_handler = {
++	.name = XATTR_NAME_POSIX_ACL_DEFAULT,
++	.flags = ACL_TYPE_DEFAULT,
++	.get = shiftfs_posix_acl_xattr_get,
++	.set = shiftfs_posix_acl_xattr_set,
++};
++
++#else /* !CONFIG_SHIFT_FS_POSIX_ACL */
++
++#define shiftfs_get_acl NULL
++
++#endif /* CONFIG_SHIFT_FS_POSIX_ACL */
++
++static const struct inode_operations shiftfs_dir_inode_operations = {
++	.lookup		= shiftfs_lookup,
++	.mkdir		= shiftfs_mkdir,
++	.symlink	= shiftfs_symlink,
++	.unlink		= shiftfs_unlink,
++	.rmdir		= shiftfs_rmdir,
++	.rename		= shiftfs_rename,
++	.link		= shiftfs_link,
++	.setattr	= shiftfs_setattr,
++	.create		= shiftfs_create,
++	.mknod		= shiftfs_mknod,
++	.permission	= shiftfs_permission,
++	.getattr	= shiftfs_getattr,
++	.listxattr	= shiftfs_listxattr,
++	.get_acl	= shiftfs_get_acl,
++};
++
++static const struct inode_operations shiftfs_file_inode_operations = {
++	.fiemap		= shiftfs_fiemap,
++	.getattr	= shiftfs_getattr,
++	.get_acl	= shiftfs_get_acl,
++	.listxattr	= shiftfs_listxattr,
++	.permission	= shiftfs_permission,
++	.setattr	= shiftfs_setattr,
++	.tmpfile	= shiftfs_tmpfile,
++};
++
++static const struct inode_operations shiftfs_special_inode_operations = {
++	.getattr	= shiftfs_getattr,
++	.get_acl	= shiftfs_get_acl,
++	.listxattr	= shiftfs_listxattr,
++	.permission	= shiftfs_permission,
++	.setattr	= shiftfs_setattr,
++};
++
++static const struct inode_operations shiftfs_symlink_inode_operations = {
++	.getattr	= shiftfs_getattr,
++	.get_link	= shiftfs_get_link,
++	.listxattr	= shiftfs_listxattr,
++	.setattr	= shiftfs_setattr,
++};
++
++static struct file *shiftfs_open_realfile(const struct file *file,
++					  struct inode *realinode)
++{
++	struct file *realfile;
++	const struct cred *old_cred;
++	struct inode *inode = file_inode(file);
++	struct dentry *lowerd = file->f_path.dentry->d_fsdata;
++	struct shiftfs_super_info *info = inode->i_sb->s_fs_info;
++	struct path realpath = { .mnt = info->mnt, .dentry = lowerd };
++
++	old_cred = shiftfs_override_creds(inode->i_sb);
++	realfile = open_with_fake_path(&realpath, file->f_flags, realinode,
++				       info->creator_cred);
++	revert_creds(old_cred);
++
++	return realfile;
++}
++
++#define SHIFTFS_SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT)
++
++static int shiftfs_change_flags(struct file *file, unsigned int flags)
++{
++	struct inode *inode = file_inode(file);
++	int err;
++
++	/* if some flag changed that cannot be changed then something's amiss */
++	if (WARN_ON((file->f_flags ^ flags) & ~SHIFTFS_SETFL_MASK))
++		return -EIO;
++
++	flags &= SHIFTFS_SETFL_MASK;
++
++	if (((flags ^ file->f_flags) & O_APPEND) && IS_APPEND(inode))
++		return -EPERM;
++
++	if (flags & O_DIRECT) {
++		if (!file->f_mapping->a_ops ||
++		    !file->f_mapping->a_ops->direct_IO)
++			return -EINVAL;
++	}
++
++	if (file->f_op->check_flags) {
++		err = file->f_op->check_flags(flags);
++		if (err)
++			return err;
++	}
++
++	spin_lock(&file->f_lock);
++	file->f_flags = (file->f_flags & ~SHIFTFS_SETFL_MASK) | flags;
++	spin_unlock(&file->f_lock);
++
++	return 0;
++}
++
++static int shiftfs_open(struct inode *inode, struct file *file)
++{
++	struct file *realfile;
++
++	realfile = shiftfs_open_realfile(file, inode->i_private);
++	if (IS_ERR(realfile))
++		return PTR_ERR(realfile);
++
++	file->private_data = realfile;
++	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO. */
++	file->f_mapping = realfile->f_mapping;
++
++	return 0;
++}
++
++static int shiftfs_dir_open(struct inode *inode, struct file *file)
++{
++	struct file *realfile;
++	const struct cred *oldcred;
++	struct dentry *lowerd = file->f_path.dentry->d_fsdata;
++	struct shiftfs_super_info *info = inode->i_sb->s_fs_info;
++	struct path realpath = { .mnt = info->mnt, .dentry = lowerd };
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	realfile = dentry_open(&realpath, file->f_flags | O_NOATIME,
++			       info->creator_cred);
++	revert_creds(oldcred);
++	if (IS_ERR(realfile))
++		return PTR_ERR(realfile);
++
++	file->private_data = realfile;
++
++	return 0;
++}
++
++static int shiftfs_release(struct inode *inode, struct file *file)
++{
++	struct file *realfile = file->private_data;
++
++	if (realfile)
++		fput(realfile);
++
++	return 0;
++}
++
++static int shiftfs_dir_release(struct inode *inode, struct file *file)
++{
++	return shiftfs_release(inode, file);
++}
++
++static loff_t shiftfs_dir_llseek(struct file *file, loff_t offset, int whence)
++{
++	struct file *realfile = file->private_data;
++
++	return vfs_llseek(realfile, offset, whence);
++}
++
++static loff_t shiftfs_file_llseek(struct file *file, loff_t offset, int whence)
++{
++	struct inode *realinode = file_inode(file)->i_private;
++
++	return generic_file_llseek_size(file, offset, whence,
++					realinode->i_sb->s_maxbytes,
++					i_size_read(realinode));
++}
++
++/* XXX: Need to figure out what to to about atime updates, maybe other
++ * timestamps too ... ref. ovl_file_accessed() */
++
++static rwf_t shiftfs_iocb_to_rwf(struct kiocb *iocb)
++{
++	int ifl = iocb->ki_flags;
++	rwf_t flags = 0;
++
++	if (ifl & IOCB_NOWAIT)
++		flags |= RWF_NOWAIT;
++	if (ifl & IOCB_HIPRI)
++		flags |= RWF_HIPRI;
++	if (ifl & IOCB_DSYNC)
++		flags |= RWF_DSYNC;
++	if (ifl & IOCB_SYNC)
++		flags |= RWF_SYNC;
++
++	return flags;
++}
++
++static int shiftfs_real_fdget(const struct file *file, struct fd *lowerfd)
++{
++	struct file *realfile;
++
++	if (file->f_op->open != shiftfs_open &&
++	    file->f_op->open != shiftfs_dir_open)
++		return -EINVAL;
++
++	realfile = file->private_data;
++	lowerfd->flags = 0;
++	lowerfd->file = realfile;
++
++	/* Did the flags change since open? */
++	if (unlikely(file->f_flags & ~lowerfd->file->f_flags))
++		return shiftfs_change_flags(lowerfd->file, file->f_flags);
++
++	return 0;
++}
++
++static ssize_t shiftfs_read_iter(struct kiocb *iocb, struct iov_iter *iter)
++{
++	struct file *file = iocb->ki_filp;
++	struct fd lowerfd;
++	const struct cred *oldcred;
++	ssize_t ret;
++
++	if (!iov_iter_count(iter))
++		return 0;
++
++	ret = shiftfs_real_fdget(file, &lowerfd);
++	if (ret)
++		return ret;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	ret = vfs_iter_read(lowerfd.file, iter, &iocb->ki_pos,
++			    shiftfs_iocb_to_rwf(iocb));
++	revert_creds(oldcred);
++
++	shiftfs_file_accessed(file);
++
++	fdput(lowerfd);
++	return ret;
++}
++
++static ssize_t shiftfs_write_iter(struct kiocb *iocb, struct iov_iter *iter)
++{
++	struct file *file = iocb->ki_filp;
++	struct inode *inode = file_inode(file);
++	struct fd lowerfd;
++	const struct cred *oldcred;
++	ssize_t ret;
++
++	if (!iov_iter_count(iter))
++		return 0;
++
++	inode_lock(inode);
++	/* Update mode */
++	shiftfs_copyattr(inode->i_private, inode);
++	ret = file_remove_privs(file);
++	if (ret)
++		goto out_unlock;
++
++	ret = shiftfs_real_fdget(file, &lowerfd);
++	if (ret)
++		goto out_unlock;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	file_start_write(lowerfd.file);
++	ret = vfs_iter_write(lowerfd.file, iter, &iocb->ki_pos,
++			     shiftfs_iocb_to_rwf(iocb));
++	file_end_write(lowerfd.file);
++	revert_creds(oldcred);
++
++	/* Update size */
++	shiftfs_copyattr(inode->i_private, inode);
++
++	fdput(lowerfd);
++
++out_unlock:
++	inode_unlock(inode);
++	return ret;
++}
++
++static int shiftfs_fsync(struct file *file, loff_t start, loff_t end,
++			 int datasync)
++{
++	struct fd lowerfd;
++	const struct cred *oldcred;
++	int ret;
++
++	ret = shiftfs_real_fdget(file, &lowerfd);
++	if (ret)
++		return ret;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	ret = vfs_fsync_range(lowerfd.file, start, end, datasync);
++	revert_creds(oldcred);
++
++	fdput(lowerfd);
++	return ret;
++}
++
++static int shiftfs_mmap(struct file *file, struct vm_area_struct *vma)
++{
++	struct file *realfile = file->private_data;
++	const struct cred *oldcred;
++	int ret;
++
++	if (!realfile->f_op->mmap)
++		return -ENODEV;
++
++	if (WARN_ON(file != vma->vm_file))
++		return -EIO;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	vma->vm_file = get_file(realfile);
++	ret = call_mmap(vma->vm_file, vma);
++	revert_creds(oldcred);
++
++	shiftfs_file_accessed(file);
++
++	if (ret) {
++		/*
++		 * Drop refcount from new vm_file value and restore original
++		 * vm_file value
++		 */
++		vma->vm_file = file;
++		fput(realfile);
++	} else {
++		/* Drop refcount from previous vm_file value */
++		fput(file);
++	}
++
++	return ret;
++}
++
++static long shiftfs_fallocate(struct file *file, int mode, loff_t offset,
++			      loff_t len)
++{
++	struct inode *inode = file_inode(file);
++	struct inode *loweri = inode->i_private;
++	struct fd lowerfd;
++	const struct cred *oldcred;
++	int ret;
++
++	ret = shiftfs_real_fdget(file, &lowerfd);
++	if (ret)
++		return ret;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	ret = vfs_fallocate(lowerfd.file, mode, offset, len);
++	revert_creds(oldcred);
++
++	/* Update size */
++	shiftfs_copyattr(loweri, inode);
++
++	fdput(lowerfd);
++	return ret;
++}
++
++static int shiftfs_fadvise(struct file *file, loff_t offset, loff_t len,
++			   int advice)
++{
++	struct fd lowerfd;
++	const struct cred *oldcred;
++	int ret;
++
++	ret = shiftfs_real_fdget(file, &lowerfd);
++	if (ret)
++		return ret;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	ret = vfs_fadvise(lowerfd.file, offset, len, advice);
++	revert_creds(oldcred);
++
++	fdput(lowerfd);
++	return ret;
++}
++
++static int shiftfs_override_ioctl_creds(int cmd, const struct super_block *sb,
++					const struct cred **oldcred,
++					struct cred **newcred)
++{
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++	kuid_t fsuid = current_fsuid();
++	kgid_t fsgid = current_fsgid();
++
++	*oldcred = shiftfs_override_creds(sb);
++
++	*newcred = prepare_creds();
++	if (!*newcred) {
++		revert_creds(*oldcred);
++		return -ENOMEM;
++	}
++
++	(*newcred)->fsuid = shift_kuid(sb->s_user_ns, sbinfo->userns, fsuid);
++	(*newcred)->fsgid = shift_kgid(sb->s_user_ns, sbinfo->userns, fsgid);
++
++	/* clear all caps to prevent bypassing capable() checks */
++	cap_clear((*newcred)->cap_bset);
++	cap_clear((*newcred)->cap_effective);
++	cap_clear((*newcred)->cap_inheritable);
++	cap_clear((*newcred)->cap_permitted);
++
++	if (cmd == BTRFS_IOC_SNAP_DESTROY) {
++		kuid_t kuid_root = make_kuid(sb->s_user_ns, 0);
++		/*
++		 * Allow the root user in the container to remove subvolumes
++		 * from other users.
++		 */
++		if (uid_valid(kuid_root) && uid_eq(fsuid, kuid_root))
++			cap_raise((*newcred)->cap_effective, CAP_DAC_OVERRIDE);
++	}
++
++	put_cred(override_creds(*newcred));
++	return 0;
++}
++
++static inline void shiftfs_revert_ioctl_creds(const struct cred *oldcred,
++					      struct cred *newcred)
++{
++	return shiftfs_revert_object_creds(oldcred, newcred);
++}
++
++static inline bool is_btrfs_snap_ioctl(int cmd)
++{
++	if ((cmd == BTRFS_IOC_SNAP_CREATE) || (cmd == BTRFS_IOC_SNAP_CREATE_V2))
++		return true;
++
++	return false;
++}
++
++static int shiftfs_btrfs_ioctl_fd_restore(int cmd, int fd, void __user *arg,
++					  struct btrfs_ioctl_vol_args *v1,
++					  struct btrfs_ioctl_vol_args_v2 *v2)
++{
++	int ret;
++
++	if (!is_btrfs_snap_ioctl(cmd))
++		return 0;
++
++	if (cmd == BTRFS_IOC_SNAP_CREATE)
++		ret = copy_to_user(arg, v1, sizeof(*v1));
++	else
++		ret = copy_to_user(arg, v2, sizeof(*v2));
++
++	__close_fd(current->files, fd);
++	kfree(v1);
++	kfree(v2);
++
++	return ret;
++}
++
++static int shiftfs_btrfs_ioctl_fd_replace(int cmd, void __user *arg,
++					  struct btrfs_ioctl_vol_args **b1,
++					  struct btrfs_ioctl_vol_args_v2 **b2,
++					  int *newfd)
++{
++	int oldfd, ret;
++	struct fd src;
++	struct fd lfd = {};
++	struct btrfs_ioctl_vol_args *v1 = NULL;
++	struct btrfs_ioctl_vol_args_v2 *v2 = NULL;
++
++	if (!is_btrfs_snap_ioctl(cmd))
++		return 0;
++
++	if (cmd == BTRFS_IOC_SNAP_CREATE) {
++		v1 = memdup_user(arg, sizeof(*v1));
++		if (IS_ERR(v1))
++			return PTR_ERR(v1);
++		oldfd = v1->fd;
++		*b1 = v1;
++	} else {
++		v2 = memdup_user(arg, sizeof(*v2));
++		if (IS_ERR(v2))
++			return PTR_ERR(v2);
++		oldfd = v2->fd;
++		*b2 = v2;
++	}
++
++	src = fdget(oldfd);
++	if (!src.file)
++		return -EINVAL;
++
++	ret = shiftfs_real_fdget(src.file, &lfd);
++	if (ret) {
++		fdput(src);
++		return ret;
++	}
++
++	/*
++	 * shiftfs_real_fdget() does not take a reference to lfd.file, so
++	 * take a reference here to offset the one which will be put by
++	 * __close_fd(), and make sure that reference is put on fdput(lfd).
++	 */
++	get_file(lfd.file);
++	lfd.flags |= FDPUT_FPUT;
++	fdput(src);
++
++	*newfd = get_unused_fd_flags(lfd.file->f_flags);
++	if (*newfd < 0) {
++		fdput(lfd);
++		return *newfd;
++	}
++
++	fd_install(*newfd, lfd.file);
++
++	if (cmd == BTRFS_IOC_SNAP_CREATE) {
++		v1->fd = *newfd;
++		ret = copy_to_user(arg, v1, sizeof(*v1));
++		v1->fd = oldfd;
++	} else {
++		v2->fd = *newfd;
++		ret = copy_to_user(arg, v2, sizeof(*v2));
++		v2->fd = oldfd;
++	}
++
++	if (ret)
++		shiftfs_btrfs_ioctl_fd_restore(cmd, *newfd, arg, v1, v2);
++
++	return ret;
++}
++
++static long shiftfs_real_ioctl(struct file *file, unsigned int cmd,
++			       unsigned long arg)
++{
++	struct fd lowerfd;
++	struct cred *newcred;
++	const struct cred *oldcred;
++	int newfd = -EBADF;
++	long err = 0, ret = 0;
++	void __user *argp = (void __user *)arg;
++	struct super_block *sb = file->f_path.dentry->d_sb;
++	struct btrfs_ioctl_vol_args *btrfs_v1 = NULL;
++	struct btrfs_ioctl_vol_args_v2 *btrfs_v2 = NULL;
++
++	ret = shiftfs_btrfs_ioctl_fd_replace(cmd, argp, &btrfs_v1, &btrfs_v2,
++					     &newfd);
++	if (ret < 0)
++		return ret;
++
++	ret = shiftfs_real_fdget(file, &lowerfd);
++	if (ret)
++		goto out_restore;
++
++	ret = shiftfs_override_ioctl_creds(cmd, sb, &oldcred, &newcred);
++	if (ret)
++		goto out_fdput;
++
++	ret = vfs_ioctl(lowerfd.file, cmd, arg);
++
++	shiftfs_revert_ioctl_creds(oldcred, newcred);
++
++	shiftfs_copyattr(file_inode(lowerfd.file), file_inode(file));
++	shiftfs_copyflags(file_inode(lowerfd.file), file_inode(file));
++
++out_fdput:
++	fdput(lowerfd);
++
++out_restore:
++	err = shiftfs_btrfs_ioctl_fd_restore(cmd, newfd, argp,
++					     btrfs_v1, btrfs_v2);
++	if (!ret)
++		ret = err;
++
++	return ret;
++}
++
++static bool in_ioctl_whitelist(int flag, unsigned long arg)
++{
++	void __user *argp = (void __user *)arg;
++	u64 flags = 0;
++
++	switch (flag) {
++	case BTRFS_IOC_FS_INFO:
++		return true;
++	case BTRFS_IOC_SNAP_CREATE:
++		return true;
++	case BTRFS_IOC_SNAP_CREATE_V2:
++		return true;
++	case BTRFS_IOC_SUBVOL_CREATE:
++		return true;
++	case BTRFS_IOC_SUBVOL_CREATE_V2:
++		return true;
++	case BTRFS_IOC_SUBVOL_GETFLAGS:
++		return true;
++	case BTRFS_IOC_SUBVOL_SETFLAGS:
++		if (copy_from_user(&flags, argp, sizeof(flags)))
++			return false;
++
++		if (flags & ~BTRFS_SUBVOL_RDONLY)
++			return false;
++
++		return true;
++	case BTRFS_IOC_SNAP_DESTROY:
++		return true;
++	}
++
++	return false;
++}
++
++static long shiftfs_ioctl(struct file *file, unsigned int cmd,
++			  unsigned long arg)
++{
++	switch (cmd) {
++	case FS_IOC_GETVERSION:
++		/* fall through */
++	case FS_IOC_GETFLAGS:
++		/* fall through */
++	case FS_IOC_SETFLAGS:
++		break;
++	default:
++		if (!in_ioctl_whitelist(cmd, arg) ||
++		    !shiftfs_passthrough_ioctls(file->f_path.dentry->d_sb->s_fs_info))
++			return -ENOTTY;
++	}
++
++	return shiftfs_real_ioctl(file, cmd, arg);
++}
++
++static long shiftfs_compat_ioctl(struct file *file, unsigned int cmd,
++				 unsigned long arg)
++{
++	switch (cmd) {
++	case FS_IOC32_GETVERSION:
++		/* fall through */
++	case FS_IOC32_GETFLAGS:
++		/* fall through */
++	case FS_IOC32_SETFLAGS:
++		break;
++	default:
++		if (!in_ioctl_whitelist(cmd, arg) ||
++		    !shiftfs_passthrough_ioctls(file->f_path.dentry->d_sb->s_fs_info))
++			return -ENOIOCTLCMD;
++	}
++
++	return shiftfs_real_ioctl(file, cmd, arg);
++}
++
++enum shiftfs_copyop {
++	SHIFTFS_COPY,
++	SHIFTFS_CLONE,
++	SHIFTFS_DEDUPE,
++};
++
++static ssize_t shiftfs_copyfile(struct file *file_in, loff_t pos_in,
++				struct file *file_out, loff_t pos_out, u64 len,
++				unsigned int flags, enum shiftfs_copyop op)
++{
++	ssize_t ret;
++	struct fd real_in, real_out;
++	const struct cred *oldcred;
++	struct inode *inode_out = file_inode(file_out);
++	struct inode *loweri = inode_out->i_private;
++
++	ret = shiftfs_real_fdget(file_out, &real_out);
++	if (ret)
++		return ret;
++
++	ret = shiftfs_real_fdget(file_in, &real_in);
++	if (ret) {
++		fdput(real_out);
++		return ret;
++	}
++
++	oldcred = shiftfs_override_creds(inode_out->i_sb);
++	switch (op) {
++	case SHIFTFS_COPY:
++		ret = vfs_copy_file_range(real_in.file, pos_in, real_out.file,
++					  pos_out, len, flags);
++		break;
++
++	case SHIFTFS_CLONE:
++		ret = vfs_clone_file_range(real_in.file, pos_in, real_out.file,
++					   pos_out, len, flags);
++		break;
++
++	case SHIFTFS_DEDUPE:
++		ret = vfs_dedupe_file_range_one(real_in.file, pos_in,
++						real_out.file, pos_out, len,
++						flags);
++		break;
++	}
++	revert_creds(oldcred);
++
++	/* Update size */
++	shiftfs_copyattr(loweri, inode_out);
++
++	fdput(real_in);
++	fdput(real_out);
++
++	return ret;
++}
++
++static ssize_t shiftfs_copy_file_range(struct file *file_in, loff_t pos_in,
++				       struct file *file_out, loff_t pos_out,
++				       size_t len, unsigned int flags)
++{
++	return shiftfs_copyfile(file_in, pos_in, file_out, pos_out, len, flags,
++				SHIFTFS_COPY);
++}
++
++static loff_t shiftfs_remap_file_range(struct file *file_in, loff_t pos_in,
++				       struct file *file_out, loff_t pos_out,
++				       loff_t len, unsigned int remap_flags)
++{
++	enum shiftfs_copyop op;
++
++	if (remap_flags & ~(REMAP_FILE_DEDUP | REMAP_FILE_ADVISORY))
++		return -EINVAL;
++
++	if (remap_flags & REMAP_FILE_DEDUP)
++		op = SHIFTFS_DEDUPE;
++	else
++		op = SHIFTFS_CLONE;
++
++	return shiftfs_copyfile(file_in, pos_in, file_out, pos_out, len,
++				remap_flags, op);
++}
++
++static int shiftfs_iterate_shared(struct file *file, struct dir_context *ctx)
++{
++	const struct cred *oldcred;
++	int err = -ENOTDIR;
++	struct file *realfile = file->private_data;
++
++	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
++	err = iterate_dir(realfile, ctx);
++	revert_creds(oldcred);
++
++	return err;
++}
++
++const struct file_operations shiftfs_file_operations = {
++	.open			= shiftfs_open,
++	.release		= shiftfs_release,
++	.llseek			= shiftfs_file_llseek,
++	.read_iter		= shiftfs_read_iter,
++	.write_iter		= shiftfs_write_iter,
++	.fsync			= shiftfs_fsync,
++	.mmap			= shiftfs_mmap,
++	.fallocate		= shiftfs_fallocate,
++	.fadvise		= shiftfs_fadvise,
++	.unlocked_ioctl		= shiftfs_ioctl,
++	.compat_ioctl		= shiftfs_compat_ioctl,
++	.copy_file_range	= shiftfs_copy_file_range,
++	.remap_file_range	= shiftfs_remap_file_range,
++};
++
++const struct file_operations shiftfs_dir_operations = {
++	.open			= shiftfs_dir_open,
++	.release		= shiftfs_dir_release,
++	.compat_ioctl		= shiftfs_compat_ioctl,
++	.fsync			= shiftfs_fsync,
++	.iterate_shared		= shiftfs_iterate_shared,
++	.llseek			= shiftfs_dir_llseek,
++	.read			= generic_read_dir,
++	.unlocked_ioctl		= shiftfs_ioctl,
++};
++
++static const struct address_space_operations shiftfs_aops = {
++	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
++	.direct_IO	= noop_direct_IO,
++};
++
++static void shiftfs_fill_inode(struct inode *inode, unsigned long ino,
++			       umode_t mode, dev_t dev, struct dentry *dentry)
++{
++	struct inode *loweri;
++
++	inode->i_ino = ino;
++	inode->i_flags |= S_NOCMTIME;
++
++	mode &= S_IFMT;
++	inode->i_mode = mode;
++	switch (mode & S_IFMT) {
++	case S_IFDIR:
++		inode->i_op = &shiftfs_dir_inode_operations;
++		inode->i_fop = &shiftfs_dir_operations;
++		break;
++	case S_IFLNK:
++		inode->i_op = &shiftfs_symlink_inode_operations;
++		break;
++	case S_IFREG:
++		inode->i_op = &shiftfs_file_inode_operations;
++		inode->i_fop = &shiftfs_file_operations;
++		inode->i_mapping->a_ops = &shiftfs_aops;
++		break;
++	default:
++		inode->i_op = &shiftfs_special_inode_operations;
++		init_special_inode(inode, mode, dev);
++		break;
++	}
++
++	if (!dentry)
++		return;
++
++	loweri = dentry->d_inode;
++	if (!loweri->i_op->get_link)
++		inode->i_opflags |= IOP_NOFOLLOW;
++
++	shiftfs_copyattr(loweri, inode);
++	shiftfs_copyflags(loweri, inode);
++	set_nlink(inode, loweri->i_nlink);
++}
++
++static int shiftfs_show_options(struct seq_file *m, struct dentry *dentry)
++{
++	struct super_block *sb = dentry->d_sb;
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++
++	if (sbinfo->mark)
++		seq_show_option(m, "mark", NULL);
++
++	if (sbinfo->passthrough)
++		seq_printf(m, ",passthrough=%u", sbinfo->passthrough);
++
++	return 0;
++}
++
++static int shiftfs_statfs(struct dentry *dentry, struct kstatfs *buf)
++{
++	struct super_block *sb = dentry->d_sb;
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++	struct dentry *root = sb->s_root;
++	struct dentry *realroot = root->d_fsdata;
++	struct path realpath = { .mnt = sbinfo->mnt, .dentry = realroot };
++	int err;
++
++	err = vfs_statfs(&realpath, buf);
++	if (err)
++		return err;
++
++	if (!shiftfs_passthrough_statfs(sbinfo))
++		buf->f_type = sb->s_magic;
++
++	return 0;
++}
++
++static void shiftfs_evict_inode(struct inode *inode)
++{
++	struct inode *loweri = inode->i_private;
++
++	clear_inode(inode);
++
++	if (loweri)
++		iput(loweri);
++}
++
++static void shiftfs_put_super(struct super_block *sb)
++{
++	struct shiftfs_super_info *sbinfo = sb->s_fs_info;
++
++	if (sbinfo) {
++		mntput(sbinfo->mnt);
++		put_cred(sbinfo->creator_cred);
++		kfree(sbinfo);
++	}
++}
++
++static const struct xattr_handler shiftfs_xattr_handler = {
++	.prefix = "",
++	.get    = shiftfs_xattr_get,
++	.set    = shiftfs_xattr_set,
++};
++
++const struct xattr_handler *shiftfs_xattr_handlers[] = {
++#ifdef CONFIG_SHIFT_FS_POSIX_ACL
++	&shiftfs_posix_acl_access_xattr_handler,
++	&shiftfs_posix_acl_default_xattr_handler,
++#endif
++	&shiftfs_xattr_handler,
++	NULL
++};
++
++static inline bool passthrough_is_subset(int old_flags, int new_flags)
++{
++	if ((new_flags & old_flags) != new_flags)
++		return false;
++
++	return true;
++}
++
++static int shiftfs_super_check_flags(unsigned long old_flags,
++				     unsigned long new_flags)
++{
++	if ((old_flags & SB_RDONLY) && !(new_flags & SB_RDONLY))
++		return -EPERM;
++
++	if ((old_flags & SB_NOSUID) && !(new_flags & SB_NOSUID))
++		return -EPERM;
++
++	if ((old_flags & SB_NODEV) && !(new_flags & SB_NODEV))
++		return -EPERM;
++
++	if ((old_flags & SB_NOEXEC) && !(new_flags & SB_NOEXEC))
++		return -EPERM;
++
++	if ((old_flags & SB_NOATIME) && !(new_flags & SB_NOATIME))
++		return -EPERM;
++
++	if ((old_flags & SB_NODIRATIME) && !(new_flags & SB_NODIRATIME))
++		return -EPERM;
++
++	if (!(old_flags & SB_POSIXACL) && (new_flags & SB_POSIXACL))
++		return -EPERM;
++
++	return 0;
++}
++
++static int shiftfs_remount(struct super_block *sb, int *flags, char *data)
++{
++	int err;
++	struct shiftfs_super_info new = {};
++	struct shiftfs_super_info *info = sb->s_fs_info;
++
++	err = shiftfs_parse_mount_options(&new, data);
++	if (err)
++		return err;
++
++	err = shiftfs_super_check_flags(sb->s_flags, *flags);
++	if (err)
++		return err;
++
++	/* Mark mount option cannot be changed. */
++	if (info->mark || (info->mark != new.mark))
++		return -EPERM;
++
++	if (info->passthrough != new.passthrough) {
++		/* Don't allow exceeding passthrough options of mark mount. */
++		if (!passthrough_is_subset(info->passthrough_mark,
++					   info->passthrough))
++			return -EPERM;
++
++		info->passthrough = new.passthrough;
++	}
++
++	return 0;
++}
++
++static const struct super_operations shiftfs_super_ops = {
++	.put_super	= shiftfs_put_super,
++	.show_options	= shiftfs_show_options,
++	.statfs		= shiftfs_statfs,
++	.remount_fs	= shiftfs_remount,
++	.evict_inode	= shiftfs_evict_inode,
++};
++
++struct shiftfs_data {
++	void *data;
++	const char *path;
++};
++
++static void shiftfs_super_force_flags(struct super_block *sb,
++				      unsigned long lower_flags)
++{
++	sb->s_flags |= lower_flags & (SB_RDONLY | SB_NOSUID | SB_NODEV |
++				      SB_NOEXEC | SB_NOATIME | SB_NODIRATIME);
++
++	if (!(lower_flags & SB_POSIXACL))
++		sb->s_flags &= ~SB_POSIXACL;
++}
++
++static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
++			      int silent)
++{
++	int err;
++	struct path path = {};
++	struct shiftfs_super_info *sbinfo_mp;
++	char *name = NULL;
++	struct inode *inode = NULL;
++	struct dentry *dentry = NULL;
++	struct shiftfs_data *data = raw_data;
++	struct shiftfs_super_info *sbinfo = NULL;
++
++	if (!data->path)
++		return -EINVAL;
++
++	sb->s_fs_info = kzalloc(sizeof(*sbinfo), GFP_KERNEL);
++	if (!sb->s_fs_info)
++		return -ENOMEM;
++	sbinfo = sb->s_fs_info;
++
++	err = shiftfs_parse_mount_options(sbinfo, data->data);
++	if (err)
++		return err;
++
++	/* to mount a mark, must be userns admin */
++	if (!sbinfo->mark && !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
++		return -EPERM;
++
++	name = kstrdup(data->path, GFP_KERNEL);
++	if (!name)
++		return -ENOMEM;
++
++	err = kern_path(name, LOOKUP_FOLLOW, &path);
++	if (err)
++		goto out_free_name;
++
++	if (!S_ISDIR(path.dentry->d_inode->i_mode)) {
++		err = -ENOTDIR;
++		goto out_put_path;
++	}
++
++	sb->s_flags |= SB_POSIXACL;
++
++	if (sbinfo->mark) {
++		struct cred *cred_tmp;
++		struct super_block *lower_sb = path.mnt->mnt_sb;
++
++		/* to mark a mount point, must root wrt lower s_user_ns */
++		if (!ns_capable(lower_sb->s_user_ns, CAP_SYS_ADMIN)) {
++			err = -EPERM;
++			goto out_put_path;
++		}
++
++		/*
++		 * this part is visible unshifted, so make sure no
++		 * executables that could be used to give suid
++		 * privileges
++		 */
++		sb->s_iflags = SB_I_NOEXEC;
++
++		shiftfs_super_force_flags(sb, lower_sb->s_flags);
++
++		/*
++		 * Handle nesting of shiftfs mounts by referring this mark
++		 * mount back to the original mark mount. This is more
++		 * efficient and alleviates concerns about stack depth.
++		 */
++		if (lower_sb->s_magic == SHIFTFS_MAGIC) {
++			sbinfo_mp = lower_sb->s_fs_info;
++
++			/* Doesn't make sense to mark a mark mount */
++			if (sbinfo_mp->mark) {
++				err = -EINVAL;
++				goto out_put_path;
++			}
++
++			if (!passthrough_is_subset(sbinfo_mp->passthrough,
++						   sbinfo->passthrough)) {
++				err = -EPERM;
++				goto out_put_path;
++			}
++
++			sbinfo->mnt = mntget(sbinfo_mp->mnt);
++			dentry = dget(path.dentry->d_fsdata);
++			/*
++			 * Copy up the passthrough mount options from the
++			 * parent mark mountpoint.
++			 */
++			sbinfo->passthrough_mark = sbinfo_mp->passthrough_mark;
++			sbinfo->creator_cred = get_cred(sbinfo_mp->creator_cred);
++		} else {
++			sbinfo->mnt = mntget(path.mnt);
++			dentry = dget(path.dentry);
++			/*
++			 * For a new mark passthrough_mark and passthrough
++			 * are identical.
++			 */
++			sbinfo->passthrough_mark = sbinfo->passthrough;
++
++			cred_tmp = prepare_creds();
++			if (!cred_tmp) {
++				err = -ENOMEM;
++				goto out_put_path;
++			}
++			/* Don't override disk quota limits or use reserved space. */
++			cap_lower(cred_tmp->cap_effective, CAP_SYS_RESOURCE);
++			sbinfo->creator_cred = cred_tmp;
++		}
++	} else {
++		/*
++		 * This leg executes if we're admin capable in the namespace,
++		 * so be very careful.
++		 */
++		err = -EPERM;
++		if (path.dentry->d_sb->s_magic != SHIFTFS_MAGIC)
++			goto out_put_path;
++
++		sbinfo_mp = path.dentry->d_sb->s_fs_info;
++		if (!sbinfo_mp->mark)
++			goto out_put_path;
++
++		if (!passthrough_is_subset(sbinfo_mp->passthrough,
++					   sbinfo->passthrough))
++			goto out_put_path;
++
++		sbinfo->mnt = mntget(sbinfo_mp->mnt);
++		sbinfo->creator_cred = get_cred(sbinfo_mp->creator_cred);
++		dentry = dget(path.dentry->d_fsdata);
++		/*
++		 * Copy up passthrough settings from mark mountpoint so we can
++		 * verify when the overlay wants to remount with different
++		 * passthrough settings.
++		 */
++		sbinfo->passthrough_mark = sbinfo_mp->passthrough;
++		shiftfs_super_force_flags(sb, path.mnt->mnt_sb->s_flags);
++	}
++
++	sb->s_stack_depth = 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_path;
++	}
++
++	inode = new_inode(sb);
++	if (!inode) {
++		err = -ENOMEM;
++		goto out_put_path;
++	}
++	shiftfs_fill_inode(inode, dentry->d_inode->i_ino, S_IFDIR, 0, dentry);
++
++	ihold(dentry->d_inode);
++	inode->i_private = dentry->d_inode;
++
++	sb->s_magic = SHIFTFS_MAGIC;
++	sb->s_maxbytes = MAX_LFS_FILESIZE;
++	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(inode);
++	if (!sb->s_root) {
++		err = -ENOMEM;
++		goto out_put_path;
++	}
++
++	sb->s_root->d_fsdata = dentry;
++	sbinfo->userns = get_user_ns(dentry->d_sb->s_user_ns);
++	shiftfs_copyattr(dentry->d_inode, sb->s_root->d_inode);
++
++	dentry = NULL;
++	err = 0;
++
++out_put_path:
++	path_put(&path);
++
++out_free_name:
++	kfree(name);
++
++	dput(dentry);
++
++	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_AUTHOR("Seth Forshee <seth.forshee@canonical.com>");
++MODULE_AUTHOR("Christian Brauner <christian.brauner@ubuntu.com>");
++MODULE_DESCRIPTION("id shifting filesystem");
++MODULE_LICENSE("GPL v2");
++module_init(shiftfs_init)
++module_exit(shiftfs_exit)
+--- a/include/uapi/linux/magic.h	2021-01-06 19:08:45.234777659 -0500
++++ b/include/uapi/linux/magic.h	2021-01-06 19:09:53.900375394 -0500
+@@ -96,4 +96,6 @@
+ #define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
+ #define Z3FOLD_MAGIC		0x33
+ 
++#define SHIFTFS_MAGIC         0x6a656a62
++
+ #endif /* __LINUX_MAGIC_H__ */
+--- a/fs/Makefile	2021-01-06 19:10:56.009918778 -0500
++++ b/fs/Makefile	2021-01-06 19:11:55.632442564 -0500
+@@ -132,3 +132,4 @@ obj-$(CONFIG_CEPH_FS)		+= ceph/
+ obj-$(CONFIG_PSTORE)		+= pstore/
+ obj-$(CONFIG_EFIVAR_FS)		+= efivarfs/
+ obj-$(CONFIG_EROFS_FS)		+= erofs/
++obj-$(CONFIG_SHIFT_FS)    += shiftfs.o
+--- a/fs/Kconfig	2021-01-06 19:14:17.709697891 -0500
++++ b/fs/Kconfig	2021-01-06 19:15:23.413281282 -0500
+@@ -122,6 +122,24 @@ source "fs/autofs/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.
++
++config SHIFT_FS_POSIX_ACL
++  bool "shiftfs POSIX Access Control Lists"
++  depends on SHIFT_FS
++  select FS_POSIX_ACL
++  help
++    POSIX Access Control Lists (ACLs) support permissions for users and
++    groups beyond the owner/group/world scheme.
++
++    If you don't know what Access Control Lists are, say N.
++
+ menu "Caches"
+ 
+ source "fs/fscache/Kconfig"