[patch] Shiftfs Patch for 5.4.87

5000_shifts-ubuntu-20.04.patch (text/plain), 54.16 KB, created by Mike Pagano on 2021-01-07 13:10:25 UTC

--- /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 <[e-mail]>");
+MODULE_AUTHOR("Christian Brauner <[e-mail]>");
+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"