ovl: implement tmpfile

Combine inode creation with opening a file.

There are six separate objects that are being set up: the backing inode,
dentry and file, and the overlay inode, dentry and file.  Cleanup in case
of an error is a bit of a challenge and is difficult to test, so careful
review is needed.

All tmpfile testcases except generic/509 now run/pass, and no regressions
are observed with full xfstests.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>

fs/backing-file.c

@@ -52,6 +52,29 @@ struct file *backing_file_open(const struct path *user_path, int flags,
 }
 EXPORT_SYMBOL_GPL(backing_file_open);
 
+struct file *backing_tmpfile_open(const struct path *user_path, int flags,
+				  const struct path *real_parentpath,
+				  umode_t mode, const struct cred *cred)
+{
+	struct mnt_idmap *real_idmap = mnt_idmap(real_parentpath->mnt);
+	struct file *f;
+	int error;
+
+	f = alloc_empty_backing_file(flags, cred);
+	if (IS_ERR(f))
+		return f;
+
+	path_get(user_path);
+	*backing_file_user_path(f) = *user_path;
+	error = vfs_tmpfile(real_idmap, real_parentpath, f, mode);
+	if (error) {
+		fput(f);
+		f = ERR_PTR(error);
+	}
+	return f;
+}
+EXPORT_SYMBOL(backing_tmpfile_open);
+
 struct backing_aio {
 	struct kiocb iocb;
 	refcount_t ref;

fs/internal.h

@@ -62,6 +62,9 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode);
 int do_symlinkat(struct filename *from, int newdfd, struct filename *to);
 int do_linkat(int olddfd, struct filename *old, int newdfd,
 			struct filename *new, int flags);
+int vfs_tmpfile(struct mnt_idmap *idmap,
+		const struct path *parentpath,
+		struct file *file, umode_t mode);
 
 /*
  * namespace.c

fs/namei.c

@@ -3668,7 +3668,7 @@ static int do_open(struct nameidata *nd,
  * On non-idmapped mounts or if permission checking is to be performed on the
  * raw inode simply pass @nop_mnt_idmap.
  */
-static int vfs_tmpfile(struct mnt_idmap *idmap,
+int vfs_tmpfile(struct mnt_idmap *idmap,
 		const struct path *parentpath,
 		struct file *file, umode_t mode)
 {

fs/overlayfs/dir.c

@@ -14,6 +14,7 @@
 #include <linux/posix_acl_xattr.h>
 #include <linux/atomic.h>
 #include <linux/ratelimit.h>
+#include <linux/backing-file.h>
 #include "overlayfs.h"
 
 static unsigned short ovl_redirect_max = 256;
@@ -260,14 +261,13 @@ static int ovl_set_opaque(struct dentry *dentry, struct dentry *upperdentry)
  * may not use to instantiate the new dentry.
  */
 static int ovl_instantiate(struct dentry *dentry, struct inode *inode,
-			   struct dentry *newdentry, bool hardlink)
+			   struct dentry *newdentry, bool hardlink, struct file *tmpfile)
 {
 	struct ovl_inode_params oip = {
 		.upperdentry = newdentry,
 		.newinode = inode,
 	};
 
-	ovl_dir_modified(dentry->d_parent, false);
 	ovl_dentry_set_upper_alias(dentry);
 	ovl_dentry_init_reval(dentry, newdentry, NULL);
 
@@ -295,6 +295,9 @@ static int ovl_instantiate(struct dentry *dentry, struct inode *inode,
 		inc_nlink(inode);
 	}
 
+	if (tmpfile)
+		d_mark_tmpfile(tmpfile, inode);
+
 	d_instantiate(dentry, inode);
 	if (inode != oip.newinode) {
 		pr_warn_ratelimited("newly created inode found in cache (%pd2)\n",
@@ -345,7 +348,8 @@ static int ovl_create_upper(struct dentry *dentry, struct inode *inode,
 		ovl_set_opaque(dentry, newdentry);
 	}
 
-	err = ovl_instantiate(dentry, inode, newdentry, !!attr->hardlink);
+	ovl_dir_modified(dentry->d_parent, false);
+	err = ovl_instantiate(dentry, inode, newdentry, !!attr->hardlink, NULL);
 	if (err)
 		goto out_cleanup;
 out_unlock:
@@ -529,7 +533,8 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 		if (err)
 			goto out_cleanup;
 	}
-	err = ovl_instantiate(dentry, inode, newdentry, hardlink);
+	ovl_dir_modified(dentry->d_parent, false);
+	err = ovl_instantiate(dentry, inode, newdentry, hardlink, NULL);
 	if (err) {
 		ovl_cleanup(ofs, udir, newdentry);
 		dput(newdentry);
@@ -551,12 +556,35 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 	goto out_dput;
 }
 
+static int ovl_setup_cred_for_create(struct dentry *dentry, struct inode *inode,
+				     umode_t mode, const struct cred *old_cred)
+{
+	int err;
+	struct cred *override_cred;
+
+	override_cred = prepare_creds();
+	if (!override_cred)
+		return -ENOMEM;
+
+	override_cred->fsuid = inode->i_uid;
+	override_cred->fsgid = inode->i_gid;
+	err = security_dentry_create_files_as(dentry, mode, &dentry->d_name,
+					      old_cred, override_cred);
+	if (err) {
+		put_cred(override_cred);
+		return err;
+	}
+	put_cred(override_creds(override_cred));
+	put_cred(override_cred);
+
+	return 0;
+}
+
 static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 			      struct ovl_cattr *attr, bool origin)
 {
 	int err;
 	const struct cred *old_cred;
-	struct cred *override_cred;
 	struct dentry *parent = dentry->d_parent;
 
 	old_cred = ovl_override_creds(dentry->d_sb);
@@ -572,10 +600,6 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 	}
 
 	if (!attr->hardlink) {
-		err = -ENOMEM;
-		override_cred = prepare_creds();
-		if (!override_cred)
-			goto out_revert_creds;
 		/*
 		 * In the creation cases(create, mkdir, mknod, symlink),
 		 * ovl should transfer current's fs{u,g}id to underlying
@@ -589,18 +613,10 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 		 * create a new inode, so just use the ovl mounter's
 		 * fs{u,g}id.
 		 */
-		override_cred->fsuid = inode->i_uid;
-		override_cred->fsgid = inode->i_gid;
-		err = security_dentry_create_files_as(dentry,
-				attr->mode, &dentry->d_name, old_cred,
-				override_cred);
-		if (err) {
-			put_cred(override_cred);
+		err = ovl_setup_cred_for_create(dentry, inode, attr->mode, old_cred);
+		if (err)
 			goto out_revert_creds;
 	}
-		put_cred(override_creds(override_cred));
-		put_cred(override_cred);
-	}
 
 	if (!ovl_dentry_is_whiteout(dentry))
 		err = ovl_create_upper(dentry, inode, attr);
@@ -1290,6 +1306,100 @@ static int ovl_rename(struct mnt_idmap *idmap, struct inode *olddir,
 	return err;
 }
 
+static int ovl_create_tmpfile(struct file *file, struct dentry *dentry,
+			      struct inode *inode, umode_t mode)
+{
+	const struct cred *old_cred;
+	struct path realparentpath;
+	struct file *realfile;
+	struct dentry *newdentry;
+	/* It's okay to set O_NOATIME, since the owner will be current fsuid */
+	int flags = file->f_flags | OVL_OPEN_FLAGS;
+	int err;
+
+	err = ovl_copy_up(dentry->d_parent);
+	if (err)
+		return err;
+
+	old_cred = ovl_override_creds(dentry->d_sb);
+	err = ovl_setup_cred_for_create(dentry, inode, mode, old_cred);
+	if (err)
+		goto out_revert_creds;
+
+	ovl_path_upper(dentry->d_parent, &realparentpath);
+	realfile = backing_tmpfile_open(&file->f_path, flags, &realparentpath,
+					mode, current_cred());
+	err = PTR_ERR_OR_ZERO(realfile);
+	pr_debug("tmpfile/open(%pd2, 0%o) = %i\n", realparentpath.dentry, mode, err);
+	if (err)
+		goto out_revert_creds;
+
+	/* ovl_instantiate() consumes the newdentry reference on success */
+	newdentry = dget(realfile->f_path.dentry);
+	err = ovl_instantiate(dentry, inode, newdentry, false, file);
+	if (!err) {
+		file->private_data = realfile;
+	} else {
+		dput(newdentry);
+		fput(realfile);
+	}
+out_revert_creds:
+	revert_creds(old_cred);
+	return err;
+}
+
+static int ovl_dummy_open(struct inode *inode, struct file *file)
+{
+	return 0;
+}
+
+static int ovl_tmpfile(struct mnt_idmap *idmap, struct inode *dir,
+		       struct file *file, umode_t mode)
+{
+	int err;
+	struct dentry *dentry = file->f_path.dentry;
+	struct inode *inode;
+
+	if (!OVL_FS(dentry->d_sb)->tmpfile)
+		return -EOPNOTSUPP;
+
+	err = ovl_want_write(dentry);
+	if (err)
+		return err;
+
+	err = -ENOMEM;
+	inode = ovl_new_inode(dentry->d_sb, mode, 0);
+	if (!inode)
+		goto drop_write;
+
+	inode_init_owner(&nop_mnt_idmap, inode, dir, mode);
+	err = ovl_create_tmpfile(file, dentry, inode, inode->i_mode);
+	if (err)
+		goto put_inode;
+
+	/*
+	 * Check if the preallocated inode was actually used.  Having something
+	 * else assigned to the dentry shouldn't happen as that would indicate
+	 * that the backing tmpfile "leaked" out of overlayfs.
+	 */
+	err = -EIO;
+	if (WARN_ON(inode != d_inode(dentry)))
+		goto put_realfile;
+
+	/* inode reference was transferred to dentry */
+	inode = NULL;
+	err = finish_open(file, dentry, ovl_dummy_open);
+put_realfile:
+	/* Without FMODE_OPENED ->release() won't be called on @file */
+	if (!(file->f_mode & FMODE_OPENED))
+		fput(file->private_data);
+put_inode:
+	iput(inode);
+drop_write:
+	ovl_drop_write(dentry);
+	return err;
+}
+
 const struct inode_operations ovl_dir_inode_operations = {
 	.lookup		= ovl_lookup,
 	.mkdir		= ovl_mkdir,
@@ -1310,4 +1420,5 @@ const struct inode_operations ovl_dir_inode_operations = {
 	.update_time	= ovl_update_time,
 	.fileattr_get	= ovl_fileattr_get,
 	.fileattr_set	= ovl_fileattr_set,
+	.tmpfile	= ovl_tmpfile,
 };

fs/overlayfs/file.c

@@ -24,9 +24,6 @@ static char ovl_whatisit(struct inode *inode, struct inode *realinode)
 		return 'm';
 }
 
-/* No atime modification on underlying */
-#define OVL_OPEN_FLAGS (O_NOATIME)
-
 static struct file *ovl_open_realfile(const struct file *file,
 				      const struct path *realpath)
 {

fs/overlayfs/overlayfs.h

@@ -175,6 +175,9 @@ static inline int ovl_metadata_digest_size(const struct ovl_metacopy *metacopy)
 	return (int)metacopy->len - OVL_METACOPY_MIN_SIZE;
 }
 
+/* No atime modification on underlying */
+#define OVL_OPEN_FLAGS (O_NOATIME)
+
 extern const char *const ovl_xattr_table[][2];
 static inline const char *ovl_xattr(struct ovl_fs *ofs, enum ovl_xattr ox)
 {

include/linux/backing-file.h

@@ -22,6 +22,9 @@ struct backing_file_ctx {
 struct file *backing_file_open(const struct path *user_path, int flags,
 			       const struct path *real_path,
 			       const struct cred *cred);
+struct file *backing_tmpfile_open(const struct path *user_path, int flags,
+				  const struct path *real_parentpath,
+				  umode_t mode, const struct cred *cred);
 ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
 			       struct kiocb *iocb, int flags,
 			       struct backing_file_ctx *ctx);