fs/overlayfs/export.c

/*
 * Overlayfs NFS export support.
 *
 * Amir Goldstein <amir73il@gmail.com>
 *
 * Copyright (C) 2017-2018 CTERA Networks. All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 as published by
 * the Free Software Foundation.
 */

#include <linux/fs.h>
#include <linux/cred.h>
#include <linux/mount.h>
#include <linux/namei.h>
#include <linux/xattr.h>
#include <linux/exportfs.h>
#include <linux/ratelimit.h>
#include "overlayfs.h"

/*
 * We only need to encode origin if there is a chance that the same object was
 * encoded pre copy up and then we need to stay consistent with the same
 * encoding also after copy up. If non-pure upper is not indexed, then it was
 * copied up before NFS export was enabled. In that case we don't need to worry
 * about staying consistent with pre copy up encoding and we encode an upper
 * file handle. Overlay root dentry is a private case of non-indexed upper.
 *
 * The following table summarizes the different file handle encodings used for
 * different overlay object types:
 *
 *  Object type		| Encoding
 * --------------------------------
 *  Pure upper		| U
 *  Non-indexed upper	| U
 *  Indexed upper	| L (*)
 *  Non-upper		| L (*)
 *
 * U = upper file handle
 * L = lower file handle
 *
 * (*) Connecting an overlay dir from real lower dentry is not always
 * possible when there are redirects in lower layers. To mitigate this case,
 * we copy up the lower dir first and then encode an upper dir file handle.
 */
static bool ovl_should_encode_origin(struct dentry *dentry)
{
	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;

	if (!ovl_dentry_lower(dentry))
		return false;

	/*
	 * Decoding a merge dir, whose origin's parent is under a redirected
	 * lower dir is not always possible. As a simple aproximation, we do
	 * not encode lower dir file handles when overlay has multiple lower
	 * layers and origin is below the topmost lower layer.
	 *
	 * TODO: copy up only the parent that is under redirected lower.
	 */
	if (d_is_dir(dentry) && ofs->upper_mnt &&
	    OVL_E(dentry)->lowerstack[0].layer->idx > 1)
		return false;

	/* Decoding a non-indexed upper from origin is not implemented */
	if (ovl_dentry_upper(dentry) &&
	    !ovl_test_flag(OVL_INDEX, d_inode(dentry)))
		return false;

	return true;
}

static int ovl_encode_maybe_copy_up(struct dentry *dentry)
{
	int err;

	if (ovl_dentry_upper(dentry))
		return 0;

	err = ovl_want_write(dentry);
	if (err)
		return err;

	err = ovl_copy_up(dentry);

	ovl_drop_write(dentry);
	return err;
}

static int ovl_d_to_fh(struct dentry *dentry, char *buf, int buflen)
{
	struct dentry *upper;
	struct dentry *origin = ovl_dentry_lower(dentry);
	struct ovl_fh *fh = NULL;
	int err;

	/*
	 * If we should not encode a lower dir file handle, copy up and encode
	 * an upper dir file handle.
	 */
	if (!ovl_should_encode_origin(dentry)) {
		err = ovl_encode_maybe_copy_up(dentry);
		if (err)
			goto fail;

		origin = NULL;
	}

	upper = ovl_dentry_upper(dentry);
	err = -EACCES;
	if (!upper || origin)
		goto fail;

	/* TODO: encode non pure-upper by origin */
	fh = ovl_encode_fh(upper, true);

	err = -EOVERFLOW;
	if (fh->len > buflen)
		goto fail;

	memcpy(buf, (char *)fh, fh->len);
	err = fh->len;

out:
	kfree(fh);
	return err;

fail:
	pr_warn_ratelimited("overlayfs: failed to encode file handle (%pd2, err=%i, buflen=%d, len=%d, type=%d)\n",
			    dentry, err, buflen, fh ? (int)fh->len : 0,
			    fh ? fh->type : 0);
	goto out;
}

static int ovl_dentry_to_fh(struct dentry *dentry, u32 *fid, int *max_len)
{
	int res, len = *max_len << 2;

	res = ovl_d_to_fh(dentry, (char *)fid, len);
	if (res <= 0)
		return FILEID_INVALID;

	len = res;

	/* Round up to dwords */
	*max_len = (len + 3) >> 2;
	return OVL_FILEID;
}

static int ovl_encode_inode_fh(struct inode *inode, u32 *fid, int *max_len,
			       struct inode *parent)
{
	struct dentry *dentry;
	int type;

	/* TODO: encode connectable file handles */
	if (parent)
		return FILEID_INVALID;

	dentry = d_find_any_alias(inode);
	if (WARN_ON(!dentry))
		return FILEID_INVALID;

	type = ovl_dentry_to_fh(dentry, fid, max_len);

	dput(dentry);
	return type;
}

/*
 * Find or instantiate an overlay dentry from real dentries.
 */
static struct dentry *ovl_obtain_alias(struct super_block *sb,
				       struct dentry *upper,
				       struct ovl_path *lowerpath)
{
	struct inode *inode;
	struct dentry *dentry;
	struct ovl_entry *oe;
	void *fsdata = &oe;

	/* TODO: obtain non pure-upper */
	if (lowerpath)
		return ERR_PTR(-EIO);

	inode = ovl_get_inode(sb, dget(upper), NULL, NULL, 0);
	if (IS_ERR(inode)) {
		dput(upper);
		return ERR_CAST(inode);
	}

	dentry = d_find_any_alias(inode);
	if (!dentry) {
		dentry = d_alloc_anon(inode->i_sb);
		if (!dentry)
			goto nomem;
		oe = ovl_alloc_entry(0);
		if (!oe)
			goto nomem;

		dentry->d_fsdata = oe;
		ovl_dentry_set_upper_alias(dentry);
	}

	return d_instantiate_anon(dentry, inode);

nomem:
	iput(inode);
	dput(dentry);
	return ERR_PTR(-ENOMEM);
}

/*
 * Lookup a child overlay dentry to get a connected overlay dentry whose real
 * dentry is @real. If @real is on upper layer, we lookup a child overlay
 * dentry with the same name as the real dentry. Otherwise, we need to consult
 * index for lookup.
 */
static struct dentry *ovl_lookup_real_one(struct dentry *connected,
					  struct dentry *real,
					  struct ovl_layer *layer)
{
	struct inode *dir = d_inode(connected);
	struct dentry *this, *parent = NULL;
	struct name_snapshot name;
	int err;

	/* TODO: lookup by lower real dentry */
	if (layer->idx)
		return ERR_PTR(-EACCES);

	/*
	 * Lookup child overlay dentry by real name. The dir mutex protects us
	 * from racing with overlay rename. If the overlay dentry that is above
	 * real has already been moved to a parent that is not under the
	 * connected overlay dir, we return -ECHILD and restart the lookup of
	 * connected real path from the top.
	 */
	inode_lock_nested(dir, I_MUTEX_PARENT);
	err = -ECHILD;
	parent = dget_parent(real);
	if (ovl_dentry_upper(connected) != parent)
		goto fail;

	/*
	 * We also need to take a snapshot of real dentry name to protect us
	 * from racing with underlying layer rename. In this case, we don't
	 * care about returning ESTALE, only from dereferencing a free name
	 * pointer because we hold no lock on the real dentry.
	 */
	take_dentry_name_snapshot(&name, real);
	this = lookup_one_len(name.name, connected, strlen(name.name));
	err = PTR_ERR(this);
	if (IS_ERR(this)) {
		goto fail;
	} else if (!this || !this->d_inode) {
		dput(this);
		err = -ENOENT;
		goto fail;
	} else if (ovl_dentry_upper(this) != real) {
		dput(this);
		err = -ESTALE;
		goto fail;
	}

out:
	release_dentry_name_snapshot(&name);
	dput(parent);
	inode_unlock(dir);
	return this;

fail:
	pr_warn_ratelimited("overlayfs: failed to lookup one by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
			    real, layer->idx, connected, err);
	this = ERR_PTR(err);
	goto out;
}

/*
 * Lookup a connected overlay dentry whose real dentry is @real.
 * If @real is on upper layer, we lookup a child overlay dentry with the same
 * path the real dentry. Otherwise, we need to consult index for lookup.
 */
static struct dentry *ovl_lookup_real(struct super_block *sb,
				      struct dentry *real,
				      struct ovl_layer *layer)
{
	struct dentry *connected;
	int err = 0;

	/* TODO: use index when looking up by lower real dentry */
	if (layer->idx)
		return ERR_PTR(-EACCES);

	connected = dget(sb->s_root);
	while (!err) {
		struct dentry *next, *this;
		struct dentry *parent = NULL;
		struct dentry *real_connected = ovl_dentry_upper(connected);

		if (real_connected == real)
			break;

		/* Find the topmost dentry not yet connected */
		next = dget(real);
		for (;;) {
			parent = dget_parent(next);

			if (parent == real_connected)
				break;

			/*
			 * If real has been moved out of 'real_connected',
			 * we will not find 'real_connected' and hit the layer
			 * root. In that case, we need to restart connecting.
			 * This game can go on forever in the worst case. We
			 * may want to consider taking s_vfs_rename_mutex if
			 * this happens more than once.
			 */
			if (parent == layer->mnt->mnt_root) {
				dput(connected);
				connected = dget(sb->s_root);
				break;
			}

			/*
			 * If real file has been moved out of the layer root
			 * directory, we will eventully hit the real fs root.
			 * This cannot happen by legit overlay rename, so we
			 * return error in that case.
			 */
			if (parent == next) {
				err = -EXDEV;
				break;
			}

			dput(next);
			next = parent;
		}

		if (!err) {
			this = ovl_lookup_real_one(connected, next, layer);
			if (IS_ERR(this))
				err = PTR_ERR(this);

			/*
			 * Lookup of child in overlay can fail when racing with
			 * overlay rename of child away from 'connected' parent.
			 * In this case, we need to restart the lookup from the
			 * top, because we cannot trust that 'real_connected' is
			 * still an ancestor of 'real'.
			 */
			if (err == -ECHILD) {
				this = dget(sb->s_root);
				err = 0;
			}
			if (!err) {
				dput(connected);
				connected = this;
			}
		}

		dput(parent);
		dput(next);
	}

	if (err)
		goto fail;

	return connected;

fail:
	pr_warn_ratelimited("overlayfs: failed to lookup by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
			    real, layer->idx, connected, err);
	dput(connected);
	return ERR_PTR(err);
}

/*
 * Get an overlay dentry from upper/lower real dentries.
 */
static struct dentry *ovl_get_dentry(struct super_block *sb,
				     struct dentry *upper,
				     struct ovl_path *lowerpath)
{
	struct ovl_fs *ofs = sb->s_fs_info;
	struct ovl_layer upper_layer = { .mnt = ofs->upper_mnt };

	/* TODO: get non-upper dentry */
	if (!upper)
		return ERR_PTR(-EACCES);

	/*
	 * Obtain a disconnected overlay dentry from a non-dir real upper
	 * dentry.
	 */
	if (!d_is_dir(upper))
		return ovl_obtain_alias(sb, upper, NULL);

	/* Removed empty directory? */
	if ((upper->d_flags & DCACHE_DISCONNECTED) || d_unhashed(upper))
		return ERR_PTR(-ENOENT);

	/*
	 * If real upper dentry is connected and hashed, get a connected
	 * overlay dentry with the same path as the real upper dentry.
	 */
	return ovl_lookup_real(sb, upper, &upper_layer);
}

static struct dentry *ovl_upper_fh_to_d(struct super_block *sb,
					struct ovl_fh *fh)
{
	struct ovl_fs *ofs = sb->s_fs_info;
	struct dentry *dentry;
	struct dentry *upper;

	if (!ofs->upper_mnt)
		return ERR_PTR(-EACCES);

	upper = ovl_decode_fh(fh, ofs->upper_mnt);
	if (IS_ERR_OR_NULL(upper))
		return upper;

	dentry = ovl_get_dentry(sb, upper, NULL);
	dput(upper);

	return dentry;
}

static struct dentry *ovl_fh_to_dentry(struct super_block *sb, struct fid *fid,
				       int fh_len, int fh_type)
{
	struct dentry *dentry = NULL;
	struct ovl_fh *fh = (struct ovl_fh *) fid;
	int len = fh_len << 2;
	unsigned int flags = 0;
	int err;

	err = -EINVAL;
	if (fh_type != OVL_FILEID)
		goto out_err;

	err = ovl_check_fh_len(fh, len);
	if (err)
		goto out_err;

	/* TODO: decode non-upper */
	flags = fh->flags;
	if (flags & OVL_FH_FLAG_PATH_UPPER)
		dentry = ovl_upper_fh_to_d(sb, fh);
	err = PTR_ERR(dentry);
	if (IS_ERR(dentry) && err != -ESTALE)
		goto out_err;

	return dentry;

out_err:
	pr_warn_ratelimited("overlayfs: failed to decode file handle (len=%d, type=%d, flags=%x, err=%i)\n",
			    len, fh_type, flags, err);
	return ERR_PTR(err);
}

static struct dentry *ovl_fh_to_parent(struct super_block *sb, struct fid *fid,
				       int fh_len, int fh_type)
{
	pr_warn_ratelimited("overlayfs: connectable file handles not supported; use 'no_subtree_check' exportfs option.\n");
	return ERR_PTR(-EACCES);
}

static int ovl_get_name(struct dentry *parent, char *name,
			struct dentry *child)
{
	/*
	 * ovl_fh_to_dentry() returns connected dir overlay dentries and
	 * ovl_fh_to_parent() is not implemented, so we should not get here.
	 */
	WARN_ON_ONCE(1);
	return -EIO;
}

static struct dentry *ovl_get_parent(struct dentry *dentry)
{
	/*
	 * ovl_fh_to_dentry() returns connected dir overlay dentries, so we
	 * should not get here.
	 */
	WARN_ON_ONCE(1);
	return ERR_PTR(-EIO);
}

const struct export_operations ovl_export_operations = {
	.encode_fh	= ovl_encode_inode_fh,
	.fh_to_dentry	= ovl_fh_to_dentry,
	.fh_to_parent	= ovl_fh_to_parent,
	.get_name	= ovl_get_name,
	.get_parent	= ovl_get_parent,
};