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Date:   Fri, 14 Feb 2020 11:21:32 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] Input: cyapa - 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/input/mouse/cyapa_gen5.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/input/mouse/cyapa_gen5.c b/drivers/input/mouse/cyapa_gen5.c
index 14239fbd72cf..7f012bfa2658 100644
--- a/drivers/input/mouse/cyapa_gen5.c
+++ b/drivers/input/mouse/cyapa_gen5.c
@@ -250,7 +250,7 @@ struct cyapa_tsg_bin_image_data_record {
 
 struct cyapa_tsg_bin_image {
 	struct cyapa_tsg_bin_image_head image_head;
-	struct cyapa_tsg_bin_image_data_record records[0];
+	struct cyapa_tsg_bin_image_data_record records[];
 } __packed;
 
 struct pip_bl_packet_start {
@@ -271,7 +271,7 @@ struct pip_bl_cmd_head {
 	u8 report_id;  /* Bootloader output report id, must be 40h */
 	u8 rsvd;  /* Reserved, must be 0 */
 	struct pip_bl_packet_start packet_start;
-	u8 data[0];  /* Command data variable based on commands */
+	u8 data[];  /* Command data variable based on commands */
 } __packed;
 
 /* Initiate bootload command data structure. */
@@ -300,7 +300,7 @@ struct tsg_bl_metadata_row_params {
 struct tsg_bl_flash_row_head {
 	u8 flash_array_id;
 	__le16 flash_row_id;
-	u8 flash_data[0];
+	u8 flash_data[];
 } __packed;
 
 struct pip_app_cmd_head {
@@ -314,7 +314,7 @@ struct pip_app_cmd_head {
 	 * Bit 6-0: command code.
 	 */
 	u8 cmd_code;
-	u8 parameter_data[0];  /* Parameter data variable based on cmd_code */
+	u8 parameter_data[];  /* Parameter data variable based on cmd_code */
 } __packed;
 
 /* Application get/set parameter command data structure */
-- 
2.25.0

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