[PATCH] Input: cyapa - replace zero-length array with flexible-array member

[PATCH] Input: cyapa - replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

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@embeddedor.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

Re: [PATCH] Input: cyapa - replace zero-length array with flexible-array member

[Dmitry Torokhov]

On Fri, Feb 14, 2020 at 11:21:32AM -0600, Gustavo A. R. Silva wrote:
> 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@embeddedor.com>

Applied, thank you.

> ---
>  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
> 

-- 
Dmitry