[PATCH] sctp: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
 include/linux/sctp.h |   36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/include/linux/sctp.h b/include/linux/sctp.h
index 8ccd82105de8..76731230bbc5 100644
--- a/include/linux/sctp.h
+++ b/include/linux/sctp.h
@@ -221,7 +221,7 @@ struct sctp_datahdr {
 	__be16 stream;
 	__be16 ssn;
 	__u32 ppid;
-	__u8  payload[0];
+	__u8  payload[];
 };
 
 struct sctp_data_chunk {
@@ -269,7 +269,7 @@ struct sctp_inithdr {
 	__be16 num_outbound_streams;
 	__be16 num_inbound_streams;
 	__be32 initial_tsn;
-	__u8  params[0];
+	__u8  params[];
 };
 
 struct sctp_init_chunk {
@@ -299,13 +299,13 @@ struct sctp_cookie_preserve_param {
 /* Section 3.3.2.1 Host Name Address (11) */
 struct sctp_hostname_param {
 	struct sctp_paramhdr param_hdr;
-	uint8_t hostname[0];
+	uint8_t hostname[];
 };
 
 /* Section 3.3.2.1 Supported Address Types (12) */
 struct sctp_supported_addrs_param {
 	struct sctp_paramhdr param_hdr;
-	__be16 types[0];
+	__be16 types[];
 };
 
 /* ADDIP Section 3.2.6 Adaptation Layer Indication */
@@ -317,25 +317,25 @@ struct sctp_adaptation_ind_param {
 /* ADDIP Section 4.2.7 Supported Extensions Parameter */
 struct sctp_supported_ext_param {
 	struct sctp_paramhdr param_hdr;
-	__u8 chunks[0];
+	__u8 chunks[];
 };
 
 /* AUTH Section 3.1 Random */
 struct sctp_random_param {
 	struct sctp_paramhdr param_hdr;
-	__u8 random_val[0];
+	__u8 random_val[];
 };
 
 /* AUTH Section 3.2 Chunk List */
 struct sctp_chunks_param {
 	struct sctp_paramhdr param_hdr;
-	__u8 chunks[0];
+	__u8 chunks[];
 };
 
 /* AUTH Section 3.3 HMAC Algorithm */
 struct sctp_hmac_algo_param {
 	struct sctp_paramhdr param_hdr;
-	__be16 hmac_ids[0];
+	__be16 hmac_ids[];
 };
 
 /* RFC 2960.  Section 3.3.3 Initiation Acknowledgement (INIT ACK) (2):
@@ -350,7 +350,7 @@ struct sctp_initack_chunk {
 /* Section 3.3.3.1 State Cookie (7) */
 struct sctp_cookie_param {
 	struct sctp_paramhdr p;
-	__u8 body[0];
+	__u8 body[];
 };
 
 /* Section 3.3.3.1 Unrecognized Parameters (8) */
@@ -384,7 +384,7 @@ struct sctp_sackhdr {
 	__be32 a_rwnd;
 	__be16 num_gap_ack_blocks;
 	__be16 num_dup_tsns;
-	union sctp_sack_variable variable[0];
+	union sctp_sack_variable variable[];
 };
 
 struct sctp_sack_chunk {
@@ -436,7 +436,7 @@ struct sctp_shutdown_chunk {
 struct sctp_errhdr {
 	__be16 cause;
 	__be16 length;
-	__u8  variable[0];
+	__u8  variable[];
 };
 
 struct sctp_operr_chunk {
@@ -594,7 +594,7 @@ struct sctp_fwdtsn_skip {
 
 struct sctp_fwdtsn_hdr {
 	__be32 new_cum_tsn;
-	struct sctp_fwdtsn_skip skip[0];
+	struct sctp_fwdtsn_skip skip[];
 };
 
 struct sctp_fwdtsn_chunk {
@@ -611,7 +611,7 @@ struct sctp_ifwdtsn_skip {
 
 struct sctp_ifwdtsn_hdr {
 	__be32 new_cum_tsn;
-	struct sctp_ifwdtsn_skip skip[0];
+	struct sctp_ifwdtsn_skip skip[];
 };
 
 struct sctp_ifwdtsn_chunk {
@@ -658,7 +658,7 @@ struct sctp_addip_param {
 
 struct sctp_addiphdr {
 	__be32	serial;
-	__u8	params[0];
+	__u8	params[];
 };
 
 struct sctp_addip_chunk {
@@ -718,7 +718,7 @@ struct sctp_addip_chunk {
 struct sctp_authhdr {
 	__be16 shkey_id;
 	__be16 hmac_id;
-	__u8   hmac[0];
+	__u8   hmac[];
 };
 
 struct sctp_auth_chunk {
@@ -733,7 +733,7 @@ struct sctp_infox {
 
 struct sctp_reconf_chunk {
 	struct sctp_chunkhdr chunk_hdr;
-	__u8 params[0];
+	__u8 params[];
 };
 
 struct sctp_strreset_outreq {
@@ -741,13 +741,13 @@ struct sctp_strreset_outreq {
 	__be32 request_seq;
 	__be32 response_seq;
 	__be32 send_reset_at_tsn;
-	__be16 list_of_streams[0];
+	__be16 list_of_streams[];
 };
 
 struct sctp_strreset_inreq {
 	struct sctp_paramhdr param_hdr;
 	__be32 request_seq;
-	__be16 list_of_streams[0];
+	__be16 list_of_streams[];
 };
 
 struct sctp_strreset_tsnreq {