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

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

[Gustavo A. R. Silva]

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

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

[Ilya Dryomov]

On Fri, May 14, 2021 at 11:51 PM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
>
> 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 {

Hi Gustavo,

I went ahead and removed reply_buf.  We never receive authorizer
replies in auth_none mode, so patching it to be a flexible array
is rather pointless.

Thanks,

                Ilya

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

[Gustavo A. R. Silva]

On 5/15/21 05:42, Ilya Dryomov wrote:
> 
> Hi Gustavo,
> 
> I went ahead and removed reply_buf.  We never receive authorizer
> replies in auth_none mode, so patching it to be a flexible array
> is rather pointless.

Sounds great. :)

Thanks, Ilya.
--
Gustavo