[PATCH][next] chrome: wilco_ec: event: Replace zero-length array with flexible-array member

[PATCH][next] chrome: wilco_ec: event: 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/platform/chrome/wilco_ec/event.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/platform/chrome/wilco_ec/event.c b/drivers/platform/chrome/wilco_ec/event.c
index dba3d445623f..814518509739 100644
--- a/drivers/platform/chrome/wilco_ec/event.c
+++ b/drivers/platform/chrome/wilco_ec/event.c
@@ -79,7 +79,7 @@ static DEFINE_IDA(event_ida);
 struct ec_event {
 	u16 size;
 	u16 type;
-	u16 event[0];
+	u16 event[];
 } __packed;
 
 #define ec_event_num_words(ev) (ev->size - 1)
@@ -96,7 +96,7 @@ struct ec_event_queue {
 	int capacity;
 	int head;
 	int tail;
-	struct ec_event *entries[0];
+	struct ec_event *entries[];
 };
 
 /* Maximum number of events to store in ec_event_queue */
-- 
2.23.0

Re: [PATCH][next] chrome: wilco_ec: event: Replace zero-length array with flexible-array member

[Enric Balletbo i Serra]

Hi Gustavo

On 21/3/20 0:16, 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>


Queued for 5.7. Thanks!

~ Enric

> ---
>  drivers/platform/chrome/wilco_ec/event.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/platform/chrome/wilco_ec/event.c b/drivers/platform/chrome/wilco_ec/event.c
> index dba3d445623f..814518509739 100644
> --- a/drivers/platform/chrome/wilco_ec/event.c
> +++ b/drivers/platform/chrome/wilco_ec/event.c
> @@ -79,7 +79,7 @@ static DEFINE_IDA(event_ida);
>  struct ec_event {
>  	u16 size;
>  	u16 type;
> -	u16 event[0];
> +	u16 event[];
>  } __packed;
>  
>  #define ec_event_num_words(ev) (ev->size - 1)
> @@ -96,7 +96,7 @@ struct ec_event_queue {
>  	int capacity;
>  	int head;
>  	int tail;
> -	struct ec_event *entries[0];
> +	struct ec_event *entries[];
>  };
>  
>  /* Maximum number of events to store in ec_event_queue */
>