[PATCH][next] ceph: Replace zero-length array with flexible array member

From: "Gustavo A. R. Silva" <gustavoars@kernel.org>
To: Ilya Dryomov <idryomov@gmail.com>,
	Jeff Layton <jlayton@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>
Cc: ceph-devel@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	linux-hardening@vger.kernel.org
Subject: [PATCH][next] ceph: Replace zero-length array with flexible array member
Date: Fri, 14 May 2021 16:52:09 -0500	[thread overview]
Message-ID: <20210514215209.GA33310@embeddedor> (raw)

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].

Notice that, in this case, sizeof(au->reply_buf) translates to zero,
becase in the original code reply_buf is a zero-length array. Now that
reply_buf is transformed into a flexible array, the mentioned line of
code is now replaced by a literal 0.

Also, as a safeguard, explicitly assign NULL to
auth->authorizer_reply_buf, as no heap is allocated for it, therefore
it should not be accessible.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 net/ceph/auth_none.c | 4 ++--
 net/ceph/auth_none.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ceph/auth_none.c b/net/ceph/auth_none.c
index 70e86e462250..10ee16d2cbf0 100644
--- a/net/ceph/auth_none.c
+++ b/net/ceph/auth_none.c
@@ -111,8 +111,8 @@ static int ceph_auth_none_create_authorizer(
 	auth->authorizer = (struct ceph_authorizer *) au;
 	auth->authorizer_buf = au->buf;
 	auth->authorizer_buf_len = au->buf_len;
-	auth->authorizer_reply_buf = au->reply_buf;
-	auth->authorizer_reply_buf_len = sizeof (au->reply_buf);
+	auth->authorizer_reply_buf_len = 0;
+	auth->authorizer_reply_buf = NULL;
 
 	return 0;
 }
diff --git a/net/ceph/auth_none.h b/net/ceph/auth_none.h
index 4158f064302e..3c68c0ee3dab 100644
--- a/net/ceph/auth_none.h
+++ b/net/ceph/auth_none.h
@@ -16,7 +16,7 @@ struct ceph_none_authorizer {
 	struct ceph_authorizer base;
 	char buf[128];
 	int buf_len;
-	char reply_buf[0];
+	char reply_buf[];
 };
 
 struct ceph_auth_none_info {
-- 
2.27.0

