netdev - [PATCH][next] zd1211rw/zd_usb.h: Replace zero-length array with flexible-array member

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Message-ID: <20200305111216.GA24982@embeddedor>
Date:   Thu, 5 Mar 2020 05:12:16 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Daniel Drake <dsd@...too.org>, Ulrich Kunitz <kune@...ne-taler.de>,
        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] zd1211rw/zd_usb.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/zydas/zd1211rw/zd_usb.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/wireless/zydas/zd1211rw/zd_usb.h b/drivers/net/wireless/zydas/zd1211rw/zd_usb.h
index a52ee323a142..8f03b09a602c 100644
--- a/drivers/net/wireless/zydas/zd1211rw/zd_usb.h
+++ b/drivers/net/wireless/zydas/zd1211rw/zd_usb.h
@@ -69,7 +69,7 @@ enum control_requests {
 
 struct usb_req_read_regs {
 	__le16 id;
-	__le16 addr[0];
+	__le16 addr[];
 } __packed;
 
 struct reg_data {
@@ -79,7 +79,7 @@ struct reg_data {
 
 struct usb_req_write_regs {
 	__le16 id;
-	struct reg_data reg_writes[0];
+	struct reg_data reg_writes[];
 } __packed;
 
 enum {
@@ -95,7 +95,7 @@ struct usb_req_rfwrite {
 	/* 2: other (default) */
 	__le16 bits;
 	/* RF2595: 24 */
-	__le16 bit_values[0];
+	__le16 bit_values[];
 	/* (ZD_CR203 & ~(RF_IF_LE | RF_CLK | RF_DATA)) | (bit ? RF_DATA : 0) */
 } __packed;
 
@@ -118,7 +118,7 @@ struct usb_int_header {
 
 struct usb_int_regs {
 	struct usb_int_header hdr;
-	struct reg_data regs[0];
+	struct reg_data regs[];
 } __packed;
 
 struct usb_int_retry_fail {
-- 
2.25.0