VFS/nfsd/ovl: introduce start_renaming() and end_renaming()

start_renaming() combines name lookup and locking to prepare for rename.
It is used when two names need to be looked up as in nfsd and overlayfs -
cases where one or both dentries are already available will be handled
separately.

__start_renaming() avoids the inode_permission check and hash
calculation and is suitable after filename_parentat() in do_renameat2().
It subsumes quite a bit of code from that function.

start_renaming() does calculate the hash and check X permission and is
suitable elsewhere:
- nfsd_rename()
- ovl_rename()

In ovl, ovl_do_rename_rd() is factored out of ovl_do_rename(), which
itself will be gone by the end of the series.

Acked-by: Chuck Lever <chuck.lever@oracle.com> (for nfsd parts)
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: NeilBrown <neil@brown.name>

--
Changes since v3:
 - added missig dput() in ovl_rename when "whiteout" is not-NULL.

Changes since v2:
 - in __start_renaming() some label have been renamed, and err
   is always set before a "goto out_foo" rather than passing the
   error in a dentry*.
 - ovl_do_rename() changed to call the new ovl_do_rename_rd() rather
   than keeping duplicate code
 - code around ovl_cleanup() call in ovl_rename() restructured.

Link: https://patch.msgid.link/20251113002050.676694-11-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: neilbrown

fs/namei.c

@@ -3667,6 +3667,129 @@ void unlock_rename(struct dentry *p1, struct dentry *p2)
 }
 EXPORT_SYMBOL(unlock_rename);
 
+/**
+ * __start_renaming - lookup and lock names for rename
+ * @rd:           rename data containing parent and flags, and
+ *                for receiving found dentries
+ * @lookup_flags: extra flags to pass to ->lookup (e.g. LOOKUP_REVAL,
+ *                LOOKUP_NO_SYMLINKS etc).
+ * @old_last:     name of object in @rd.old_parent
+ * @new_last:     name of object in @rd.new_parent
+ *
+ * Look up two names and ensure locks are in place for
+ * rename.
+ *
+ * On success the found dentries are stored in @rd.old_dentry,
+ * @rd.new_dentry.  These references and the lock are dropped by
+ * end_renaming().
+ *
+ * The passed in qstrs must have the hash calculated, and no permission
+ * checking is performed.
+ *
+ * Returns: zero or an error.
+ */
+static int
+__start_renaming(struct renamedata *rd, int lookup_flags,
+		 struct qstr *old_last, struct qstr *new_last)
+{
+	struct dentry *trap;
+	struct dentry *d1, *d2;
+	int target_flags = LOOKUP_RENAME_TARGET | LOOKUP_CREATE;
+	int err;
+
+	if (rd->flags & RENAME_EXCHANGE)
+		target_flags = 0;
+	if (rd->flags & RENAME_NOREPLACE)
+		target_flags |= LOOKUP_EXCL;
+
+	trap = lock_rename(rd->old_parent, rd->new_parent);
+	if (IS_ERR(trap))
+		return PTR_ERR(trap);
+
+	d1 = lookup_one_qstr_excl(old_last, rd->old_parent,
+				  lookup_flags);
+	err = PTR_ERR(d1);
+	if (IS_ERR(d1))
+		goto out_unlock;
+
+	d2 = lookup_one_qstr_excl(new_last, rd->new_parent,
+				  lookup_flags | target_flags);
+	err = PTR_ERR(d2);
+	if (IS_ERR(d2))
+		goto out_dput_d1;
+
+	if (d1 == trap) {
+		/* source is an ancestor of target */
+		err = -EINVAL;
+		goto out_dput_d2;
+	}
+
+	if (d2 == trap) {
+		/* target is an ancestor of source */
+		if (rd->flags & RENAME_EXCHANGE)
+			err = -EINVAL;
+		else
+			err = -ENOTEMPTY;
+		goto out_dput_d2;
+	}
+
+	rd->old_dentry = d1;
+	rd->new_dentry = d2;
+	return 0;
+
+out_dput_d2:
+	dput(d2);
+out_dput_d1:
+	dput(d1);
+out_unlock:
+	unlock_rename(rd->old_parent, rd->new_parent);
+	return err;
+}
+
+/**
+ * start_renaming - lookup and lock names for rename with permission checking
+ * @rd:           rename data containing parent and flags, and
+ *                for receiving found dentries
+ * @lookup_flags: extra flags to pass to ->lookup (e.g. LOOKUP_REVAL,
+ *                LOOKUP_NO_SYMLINKS etc).
+ * @old_last:     name of object in @rd.old_parent
+ * @new_last:     name of object in @rd.new_parent
+ *
+ * Look up two names and ensure locks are in place for
+ * rename.
+ *
+ * On success the found dentries are stored in @rd.old_dentry,
+ * @rd.new_dentry.  These references and the lock are dropped by
+ * end_renaming().
+ *
+ * The passed in qstrs need not have the hash calculated, and basic
+ * eXecute permission checking is performed against @rd.mnt_idmap.
+ *
+ * Returns: zero or an error.
+ */
+int start_renaming(struct renamedata *rd, int lookup_flags,
+		   struct qstr *old_last, struct qstr *new_last)
+{
+	int err;
+
+	err = lookup_one_common(rd->mnt_idmap, old_last, rd->old_parent);
+	if (err)
+		return err;
+	err = lookup_one_common(rd->mnt_idmap, new_last, rd->new_parent);
+	if (err)
+		return err;
+	return __start_renaming(rd, lookup_flags, old_last, new_last);
+}
+EXPORT_SYMBOL(start_renaming);
+
+void end_renaming(struct renamedata *rd)
+{
+	unlock_rename(rd->old_parent, rd->new_parent);
+	dput(rd->old_dentry);
+	dput(rd->new_dentry);
+}
+EXPORT_SYMBOL(end_renaming);
+
 /**
  * vfs_prepare_mode - prepare the mode to be used for a new inode
  * @idmap:	idmap of the mount the inode was found from
@@ -5504,14 +5627,11 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 		 struct filename *to, unsigned int flags)
 {
 	struct renamedata rd;
-	struct dentry *old_dentry, *new_dentry;
-	struct dentry *trap;
 	struct path old_path, new_path;
 	struct qstr old_last, new_last;
 	int old_type, new_type;
 	struct inode *delegated_inode = NULL;
-	unsigned int lookup_flags = 0, target_flags =
-		LOOKUP_RENAME_TARGET | LOOKUP_CREATE;
+	unsigned int lookup_flags = 0;
 	bool should_retry = false;
 	int error = -EINVAL;
 
@@ -5522,11 +5642,6 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 	    (flags & RENAME_EXCHANGE))
 		goto put_names;
 
-	if (flags & RENAME_EXCHANGE)
-		target_flags = 0;
-	if (flags & RENAME_NOREPLACE)
-		target_flags |= LOOKUP_EXCL;
-
 retry:
 	error = filename_parentat(olddfd, from, lookup_flags, &old_path,
 				  &old_last, &old_type);
@@ -5556,66 +5671,40 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 		goto exit2;
 
 retry_deleg:
-	trap = lock_rename(new_path.dentry, old_path.dentry);
-	if (IS_ERR(trap)) {
-		error = PTR_ERR(trap);
+	rd.old_parent	   = old_path.dentry;
+	rd.mnt_idmap	   = mnt_idmap(old_path.mnt);
+	rd.new_parent	   = new_path.dentry;
+	rd.delegated_inode = &delegated_inode;
+	rd.flags	   = flags;
+
+	error = __start_renaming(&rd, lookup_flags, &old_last, &new_last);
+	if (error)
 		goto exit_lock_rename;
-	}
 
-	old_dentry = lookup_one_qstr_excl(&old_last, old_path.dentry,
-					  lookup_flags);
-	error = PTR_ERR(old_dentry);
-	if (IS_ERR(old_dentry))
-		goto exit3;
-	new_dentry = lookup_one_qstr_excl(&new_last, new_path.dentry,
-					  lookup_flags | target_flags);
-	error = PTR_ERR(new_dentry);
-	if (IS_ERR(new_dentry))
-		goto exit4;
 	if (flags & RENAME_EXCHANGE) {
-		if (!d_is_dir(new_dentry)) {
+		if (!d_is_dir(rd.new_dentry)) {
 			error = -ENOTDIR;
 			if (new_last.name[new_last.len])
-				goto exit5;
+				goto exit_unlock;
 		}
 	}
 	/* unless the source is a directory trailing slashes give -ENOTDIR */
-	if (!d_is_dir(old_dentry)) {
+	if (!d_is_dir(rd.old_dentry)) {
 		error = -ENOTDIR;
 		if (old_last.name[old_last.len])
-			goto exit5;
+			goto exit_unlock;
 		if (!(flags & RENAME_EXCHANGE) && new_last.name[new_last.len])
-			goto exit5;
-	}
-	/* source should not be ancestor of target */
-	error = -EINVAL;
-	if (old_dentry == trap)
-		goto exit5;
-	/* target should not be an ancestor of source */
-	if (!(flags & RENAME_EXCHANGE))
-		error = -ENOTEMPTY;
-	if (new_dentry == trap)
-		goto exit5;
+			goto exit_unlock;
+	}
 
-	error = security_path_rename(&old_path, old_dentry,
-				     &new_path, new_dentry, flags);
+	error = security_path_rename(&old_path, rd.old_dentry,
+				     &new_path, rd.new_dentry, flags);
 	if (error)
-		goto exit5;
+		goto exit_unlock;
 
-	rd.old_parent	   = old_path.dentry;
-	rd.old_dentry	   = old_dentry;
-	rd.mnt_idmap	   = mnt_idmap(old_path.mnt);
-	rd.new_parent	   = new_path.dentry;
-	rd.new_dentry	   = new_dentry;
-	rd.delegated_inode = &delegated_inode;
-	rd.flags	   = flags;
 	error = vfs_rename(&rd);
-exit5:
-	dput(new_dentry);
-exit4:
-	dput(old_dentry);
-exit3:
-	unlock_rename(new_path.dentry, old_path.dentry);
+exit_unlock:
+	end_renaming(&rd);
 exit_lock_rename:
 	if (delegated_inode) {
 		error = break_deleg_wait(&delegated_inode);

fs/nfsd/vfs.c

@@ -1885,11 +1885,12 @@ __be32
 nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 			    struct svc_fh *tfhp, char *tname, int tlen)
 {
-	struct dentry	*fdentry, *tdentry, *odentry, *ndentry, *trap;
+	struct dentry	*fdentry, *tdentry;
 	int		type = S_IFDIR;
+	struct renamedata rd = {};
 	__be32		err;
 	int		host_err;
-	bool		close_cached = false;
+	struct dentry	*close_cached;
 
 	trace_nfsd_vfs_rename(rqstp, ffhp, tfhp, fname, flen, tname, tlen);
 
@@ -1915,15 +1916,22 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 		goto out;
 
 retry:
+	close_cached = NULL;
 	host_err = fh_want_write(ffhp);
 	if (host_err) {
 		err = nfserrno(host_err);
 		goto out;
 	}
 
-	trap = lock_rename(tdentry, fdentry);
-	if (IS_ERR(trap)) {
-		err = nfserr_xdev;
+	rd.mnt_idmap	= &nop_mnt_idmap;
+	rd.old_parent	= fdentry;
+	rd.new_parent	= tdentry;
+
+	host_err = start_renaming(&rd, 0, &QSTR_LEN(fname, flen),
+				  &QSTR_LEN(tname, tlen));
+
+	if (host_err) {
+		err = nfserrno(host_err);
 		goto out_want_write;
 	}
 	err = fh_fill_pre_attrs(ffhp);
@@ -1933,48 +1941,23 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 	if (err != nfs_ok)
 		goto out_unlock;
 
-	odentry = lookup_one(&nop_mnt_idmap, &QSTR_LEN(fname, flen), fdentry);
-	host_err = PTR_ERR(odentry);
-	if (IS_ERR(odentry))
-		goto out_nfserr;
+	type = d_inode(rd.old_dentry)->i_mode & S_IFMT;
+
+	if (d_inode(rd.new_dentry))
+		type = d_inode(rd.new_dentry)->i_mode & S_IFMT;
 
-	host_err = -ENOENT;
-	if (d_really_is_negative(odentry))
-		goto out_dput_old;
-	host_err = -EINVAL;
-	if (odentry == trap)
-		goto out_dput_old;
-	type = d_inode(odentry)->i_mode & S_IFMT;
-
-	ndentry = lookup_one(&nop_mnt_idmap, &QSTR_LEN(tname, tlen), tdentry);
-	host_err = PTR_ERR(ndentry);
-	if (IS_ERR(ndentry))
-		goto out_dput_old;
-	if (d_inode(ndentry))
-		type = d_inode(ndentry)->i_mode & S_IFMT;
-	host_err = -ENOTEMPTY;
-	if (ndentry == trap)
-		goto out_dput_new;
-
-	if ((ndentry->d_sb->s_export_op->flags & EXPORT_OP_CLOSE_BEFORE_UNLINK) &&
-	    nfsd_has_cached_files(ndentry)) {
-		close_cached = true;
-		goto out_dput_old;
+	if ((rd.new_dentry->d_sb->s_export_op->flags & EXPORT_OP_CLOSE_BEFORE_UNLINK) &&
+	    nfsd_has_cached_files(rd.new_dentry)) {
+		close_cached = dget(rd.new_dentry);
+		goto out_unlock;
 	} else {
-		struct renamedata rd = {
-			.mnt_idmap	= &nop_mnt_idmap,
-			.old_parent	= fdentry,
-			.old_dentry	= odentry,
-			.new_parent	= tdentry,
-			.new_dentry	= ndentry,
-		};
 		int retries;
 
 		for (retries = 1;;) {
 			host_err = vfs_rename(&rd);
 			if (host_err != -EAGAIN || !retries--)
 				break;
-			if (!nfsd_wait_for_delegreturn(rqstp, d_inode(odentry)))
+			if (!nfsd_wait_for_delegreturn(rqstp, d_inode(rd.old_dentry)))
 				break;
 		}
 		if (!host_err) {
@@ -1983,11 +1966,6 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 				host_err = commit_metadata(ffhp);
 		}
 	}
- out_dput_new:
-	dput(ndentry);
- out_dput_old:
-	dput(odentry);
- out_nfserr:
 	if (host_err == -EBUSY) {
 		/*
 		 * See RFC 8881 Section 18.26.4 para 1-3: NFSv4 RENAME
@@ -2006,7 +1984,7 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 		fh_fill_post_attrs(tfhp);
 	}
 out_unlock:
-	unlock_rename(tdentry, fdentry);
+	end_renaming(&rd);
 out_want_write:
 	fh_drop_write(ffhp);
 
@@ -2017,9 +1995,8 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 	 * until this point and then reattempt the whole shebang.
 	 */
 	if (close_cached) {
-		close_cached = false;
-		nfsd_close_cached_files(ndentry);
-		dput(ndentry);
+		nfsd_close_cached_files(close_cached);
+		dput(close_cached);
 		goto retry;
 	}
 out: