linux-kernel - [PATCH][next] cw1200/wsm.h: Replace zero-length array with flexible-array member

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Message-ID: <20200305111401.GA25126@embeddedor>
Date:   Thu, 5 Mar 2020 05:14:01 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Solomon Peachy <pizza@...ftnet.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] cw1200/wsm.h: 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>
---
 drivers/net/wireless/st/cw1200/wsm.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/st/cw1200/wsm.h b/drivers/net/wireless/st/cw1200/wsm.h
index ddea57f8c8ab..1ffa47994bb9 100644
--- a/drivers/net/wireless/st/cw1200/wsm.h
+++ b/drivers/net/wireless/st/cw1200/wsm.h
@@ -1623,7 +1623,7 @@ struct wsm_p2p_device_info {
 	u8 local_devname[D11_MAX_SSID_LEN];
 	u8 reserved2[3];
 	u8 num_secdev_supported;
-	struct wsm_p2p_device_type secdevs[0];
+	struct wsm_p2p_device_type secdevs[];
 } __packed;
 
 /* 4.36 SetWCDMABand - WO */
-- 
2.25.0