arcnet: Replace zero-length array with flexible-array member

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]

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 <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/uapi/linux/if_arcnet.h b/include/uapi/linux/if_arcnet.h
index 683878036d76d7ccdd6fe7a6e907a7977a40067b..b122cfac71288c35e8c18c60744a7ff54d5c8b90 100644 (file)
--- a/include/uapi/linux/if_arcnet.h
+++ b/include/uapi/linux/if_arcnet.h
@@ -60,7 +60,7 @@ struct arc_rfc1201 {
        __u8  proto;            /* protocol ID field - varies           */
        __u8  split_flag;       /* for use with split packets           */
        __be16   sequence;      /* sequence number                      */
-       __u8  payload[0];       /* space remaining in packet (504 bytes)*/
+       __u8  payload[];        /* space remaining in packet (504 bytes)*/
 };
 #define RFC1201_HDR_SIZE 4
 
@@ -69,7 +69,7 @@ struct arc_rfc1201 {
  */
 struct arc_rfc1051 {
        __u8 proto;             /* ARC_P_RFC1051_ARP/RFC1051_IP */
-       __u8 payload[0];        /* 507 bytes                    */
+       __u8 payload[]; /* 507 bytes                    */
 };
 #define RFC1051_HDR_SIZE 1
 
@@ -80,7 +80,7 @@ struct arc_rfc1051 {
 struct arc_eth_encap {
        __u8 proto;             /* Always ARC_P_ETHER                   */
        struct ethhdr eth;      /* standard ethernet header (yuck!)     */
-       __u8 payload[0];        /* 493 bytes                            */
+       __u8 payload[]; /* 493 bytes                            */
 };
 #define ETH_ENCAP_HDR_SIZE 14