netdev - [PATCH][next] hostap: Replace zero-length array with flexible-array member

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Message-ID: <20200225011151.GA30675@embeddedor>
Date:   Mon, 24 Feb 2020 19:11:51 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Jouni Malinen <j@...fi>, 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] hostap: 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/intersil/hostap/hostap_common.h | 2 +-
 drivers/net/wireless/intersil/hostap/hostap_wlan.h   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/intersil/hostap/hostap_common.h b/drivers/net/wireless/intersil/hostap/hostap_common.h
index 22543538239b..dd29a8e8d349 100644
--- a/drivers/net/wireless/intersil/hostap/hostap_common.h
+++ b/drivers/net/wireless/intersil/hostap/hostap_common.h
@@ -322,7 +322,7 @@ struct prism2_download_param {
 		u32 addr; /* wlan card address */
 		u32 len;
 		void __user *ptr; /* pointer to data in user space */
-	} data[0];
+	} data[];
 };
 
 #define PRISM2_MAX_DOWNLOAD_AREA_LEN 131072
diff --git a/drivers/net/wireless/intersil/hostap/hostap_wlan.h b/drivers/net/wireless/intersil/hostap/hostap_wlan.h
index 487883fbb58c..dd2603d9b5d3 100644
--- a/drivers/net/wireless/intersil/hostap/hostap_wlan.h
+++ b/drivers/net/wireless/intersil/hostap/hostap_wlan.h
@@ -615,7 +615,7 @@ struct prism2_download_data {
 		u32 addr; /* wlan card address */
 		u32 len;
 		u8 *data; /* allocated data */
-	} data[0];
+	} data[];
 };
 
 
-- 
2.25.0