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

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Message-ID: <20200228132615.GA21172@embeddedor>
Date:   Fri, 28 Feb 2020 07:26:15 -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] net: mpls: 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/mpls_iptunnel.h | 2 +-
 net/mpls/internal.h         | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/net/mpls_iptunnel.h b/include/net/mpls_iptunnel.h
index 6b4759eae158..9deb3a3735da 100644
--- a/include/net/mpls_iptunnel.h
+++ b/include/net/mpls_iptunnel.h
@@ -11,7 +11,7 @@ struct mpls_iptunnel_encap {
 	u8	ttl_propagate;
 	u8	default_ttl;
 	u8	reserved1;
-	u32	label[0];
+	u32	label[];
 };
 
 static inline struct mpls_iptunnel_encap *mpls_lwtunnel_encap(struct lwtunnel_state *lwtstate)
diff --git a/net/mpls/internal.h b/net/mpls/internal.h
index 768a302879b4..0e9aa94adc07 100644
--- a/net/mpls/internal.h
+++ b/net/mpls/internal.h
@@ -98,7 +98,7 @@ struct mpls_nh { /* next hop label forwarding entry */
 	u8			nh_via_table;
 	u8			nh_reserved1;
 
-	u32			nh_label[0];
+	u32			nh_label[];
 };
 
 /* offset of via from beginning of mpls_nh */
@@ -154,7 +154,7 @@ struct mpls_route { /* next hop label forwarding entry */
 	u8			rt_nh_size;
 	u8			rt_via_offset;
 	u8			rt_reserved1;
-	struct mpls_nh		rt_nh[0];
+	struct mpls_nh		rt_nh[];
 };
 
 #define for_nexthops(rt) {						\
-- 
2.25.0