linux-kernel - [PATCH][next] ndisc: Replace zero-length array with flexible-array member

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Message-ID: <20200229005311.GA8953@embeddedor>
Date:   Fri, 28 Feb 2020 18:53:11 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] ndisc: 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@...eddedor.com>
---
 include/net/ndisc.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index b5ebeb3b0de0..1c61aeb3a1c0 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -80,12 +80,12 @@ extern struct neigh_table nd_tbl;
 struct nd_msg {
         struct icmp6hdr	icmph;
         struct in6_addr	target;
-	__u8		opt[0];
+	__u8		opt[];
 };
 
 struct rs_msg {
 	struct icmp6hdr	icmph;
-	__u8		opt[0];
+	__u8		opt[];
 };
 
 struct ra_msg {
@@ -98,7 +98,7 @@ struct rd_msg {
 	struct icmp6hdr icmph;
 	struct in6_addr	target;
 	struct in6_addr	dest;
-	__u8		opt[0];
+	__u8		opt[];
 };
 
 struct nd_opt_hdr {
-- 
2.25.0