cachefiles: use vfs_tmpfile_open() helper

Use the vfs_tmpfile_open() helper instead of doing tmpfile creation and
opening separately.

The only minor difference is that previously no permission checking was
done, while vfs_tmpfile_open() will call may_open() with zero access mask
(i.e. no access is checked).  Even if this would make a difference with
callers caps (don't see how it could, even in the LSM codepaths) cachfiles
raises caps before performing the tmpfile creation, so this extra
permission check will not result in any regression.
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/cachefiles/namei.c

@@ -442,18 +442,19 @@ struct file *cachefiles_create_tmpfile(struct cachefiles_object *object)
 	const struct cred *saved_cred;
 	struct dentry *fan = volume->fanout[(u8)object->cookie->key_hash];
 	struct file *file;
-	struct path path;
+	const struct path parentpath = { .mnt = cache->mnt, .dentry = fan };
 	uint64_t ni_size;
 	long ret;
 
 
 	cachefiles_begin_secure(cache, &saved_cred);
 
-	path.mnt = cache->mnt;
 	ret = cachefiles_inject_write_error();
 	if (ret == 0) {
-		path.dentry = vfs_tmpfile(&init_user_ns, fan, S_IFREG, O_RDWR);
-		ret = PTR_ERR_OR_ZERO(path.dentry);
+		file = vfs_tmpfile_open(&init_user_ns, &parentpath, S_IFREG,
+					O_RDWR | O_LARGEFILE | O_DIRECT,
+					cache->cache_cred);
+		ret = PTR_ERR_OR_ZERO(file);
 	}
 	if (ret) {
 		trace_cachefiles_vfs_error(object, d_inode(fan), ret,
@@ -463,10 +464,10 @@ struct file *cachefiles_create_tmpfile(struct cachefiles_object *object)
 		goto err;
 	}
 
-	trace_cachefiles_tmpfile(object, d_backing_inode(path.dentry));
+	trace_cachefiles_tmpfile(object, file_inode(file));
 
 	/* This is a newly created file with no other possible user */
-	if (!cachefiles_mark_inode_in_use(object, d_inode(path.dentry)))
+	if (!cachefiles_mark_inode_in_use(object, file_inode(file)))
 		WARN_ON(1);
 
 	ret = cachefiles_ondemand_init_object(object);
@@ -477,27 +478,19 @@ struct file *cachefiles_create_tmpfile(struct cachefiles_object *object)
 	ni_size = round_up(ni_size, CACHEFILES_DIO_BLOCK_SIZE);
 
 	if (ni_size > 0) {
-		trace_cachefiles_trunc(object, d_backing_inode(path.dentry), 0, ni_size,
+		trace_cachefiles_trunc(object, file_inode(file), 0, ni_size,
 				       cachefiles_trunc_expand_tmpfile);
 		ret = cachefiles_inject_write_error();
 		if (ret == 0)
-			ret = vfs_truncate(&path, ni_size);
+			ret = vfs_truncate(&file->f_path, ni_size);
 		if (ret < 0) {
 			trace_cachefiles_vfs_error(
-				object, d_backing_inode(path.dentry), ret,
+				object, file_inode(file), ret,
 				cachefiles_trace_trunc_error);
 			goto err_unuse;
 		}
 	}
 
-	file = open_with_fake_path(&path, O_RDWR | O_LARGEFILE | O_DIRECT,
-				   d_backing_inode(path.dentry), cache->cache_cred);
-	ret = PTR_ERR(file);
-	if (IS_ERR(file)) {
-		trace_cachefiles_vfs_error(object, d_backing_inode(path.dentry),
-					   ret, cachefiles_trace_open_error);
-		goto err_unuse;
-	}
 	ret = -EINVAL;
 	if (unlikely(!file->f_op->read_iter) ||
 	    unlikely(!file->f_op->write_iter)) {
@@ -505,14 +498,13 @@ struct file *cachefiles_create_tmpfile(struct cachefiles_object *object)
 		pr_notice("Cache does not support read_iter and write_iter\n");
 		goto err_unuse;
 	}
-	dput(path.dentry);
 out:
 	cachefiles_end_secure(cache, saved_cred);
 	return file;
 
 err_unuse:
-	cachefiles_do_unmark_inode_in_use(object, d_inode(path.dentry));
-	dput(path.dentry);
+	cachefiles_do_unmark_inode_in_use(object, file_inode(file));
+	fput(file);
 err:
 	file = ERR_PTR(ret);
 	goto out;