staging: erofs: fix an error handling in erofs_readdir()

commit acb383f1dcb4f1e79b66d4be3a0b6f519a957b0d upstream.

Richard observed a forever loop of erofs_read_raw_page() [1]
which can be generated by forcely setting ->u.i_blkaddr
to 0xdeadbeef (as my understanding block layer can
handle access beyond end of device correctly).

After digging into that, it seems the problem is highly
related with directories and then I found the root cause
is an improper error handling in erofs_readdir().

Let's fix it now.

[1] https://lore.kernel.org/r/1163995781.68824.1566084358245.JavaMail.zimbra@nod.at/Reported-by: NRichard Weinberger <richard@nod.at>
Fixes: 3aa8ec71 ("staging: erofs: add directory operations")
Cc: <stable@vger.kernel.org> # 4.19+
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NGao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190818125457.25906-1-hsiangkao@aol.com
[ Gao Xiang: Since earlier kernels don't define EFSCORRUPTED,
             let's use original error code instead. ]
Signed-off-by: NGao Xiang <gaoxiang25@huawei.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/dir.c

@@ -100,8 +100,15 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
 		unsigned nameoff, maxsize;
 
 		dentry_page = read_mapping_page(mapping, i, NULL);
-		if (IS_ERR(dentry_page))
-			continue;
+		if (dentry_page == ERR_PTR(-ENOMEM)) {
+			err = -ENOMEM;
+			break;
+		} else if (IS_ERR(dentry_page)) {
+			errln("fail to readdir of logical block %u of nid %llu",
+			      i, EROFS_V(dir)->nid);
+			err = PTR_ERR(dentry_page);
+			break;
+		}
 
 		lock_page(dentry_page);
 		de = (struct erofs_dirent *)kmap(dentry_page);