linux-kernel - [PATCH] mwl8k: Replace zero-length array with flexible-array

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Message-ID: <20200507185914.GA15124@embeddedor>
Date:   Thu, 7 May 2020 13:59:14 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Lennert Buytenhek <buytenh@...tstofly.org>
Cc:     linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] mwl8k: Replace zero-length array with flexible-array

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@...nel.org>
---
 drivers/net/wireless/marvell/mwl8k.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/marvell/mwl8k.c b/drivers/net/wireless/marvell/mwl8k.c
index 47fb4b3ea004..97f23f93f6e7 100644
--- a/drivers/net/wireless/marvell/mwl8k.c
+++ b/drivers/net/wireless/marvell/mwl8k.c
@@ -2668,7 +2668,7 @@ struct mwl8k_cmd_mac_multicast_adr {
 	struct mwl8k_cmd_pkt header;
 	__le16 action;
 	__le16 numaddr;
-	__u8 addr[0][ETH_ALEN];
+	__u8 addr[][ETH_ALEN];
 };

 #define MWL8K_ENABLE_RX_DIRECTED	0x0001