[PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

[PATCH] auxdisplay: charlcd: 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
inadvertenly 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/auxdisplay/charlcd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/auxdisplay/charlcd.c b/drivers/auxdisplay/charlcd.c
index 874c259a8829..c0da3820454b 100644
--- a/drivers/auxdisplay/charlcd.c
+++ b/drivers/auxdisplay/charlcd.c
@@ -88,7 +88,7 @@ struct charlcd_priv {
 		int len;
 	} esc_seq;
 
-	unsigned long long drvdata[0];
+	unsigned long long drvdata[];
 };
 
 #define charlcd_to_priv(p)	container_of(p, struct charlcd_priv, lcd)
-- 
2.25.0

Re: [PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

[Miguel Ojeda]

Hi Gustavo,

On Wed, Feb 12, 2020 at 8:49 PM Gustavo A. R. Silva
<gustavo@embeddedor.com> 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
> inadvertenly introduced[3] to the codebase from now on.

s:inadvertenly:inadvertently:

> 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>

I saw the discussion regarding this -- thanks! Do you want me to
handle this or will you push everything centrally? If the latter, have
my

Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

Cheers,
Miguel

Re: [PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

Hi Miguel,

On 2/12/20 14:49, Miguel Ojeda wrote:
> Hi Gustavo,
> 
> On Wed, Feb 12, 2020 at 8:49 PM Gustavo A. R. Silva
> <gustavo@embeddedor.com> 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
>> inadvertenly introduced[3] to the codebase from now on.
> 
> s:inadvertenly:inadvertently:
> 

Thanks for this.

>> 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>
> 
> I saw the discussion regarding this -- thanks! Do you want me to
> handle this or will you push everything centrally? If the latter, have
> my
> 

Please, go ahead and handle it.

> Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
> 

Thanks
--
Gustavo

Re: [PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

[Miguel Ojeda]

Hi Gustavo,

On Thu, Feb 13, 2020 at 7:11 AM Gustavo A. R. Silva
<gustavo@embeddedor.com> wrote:
>
> Please, go ahead and handle it.

Picking it up for next week!

Cheers,
Miguel

Re: [PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

Hi Miguel,

On 3/6/20 15:24, Miguel Ojeda wrote:
> Hi Gustavo,
> 
> On Thu, Feb 13, 2020 at 7:11 AM Gustavo A. R. Silva
> <gustavo@embeddedor.com> wrote:
>>
>> Please, go ahead and handle it.
> 
> Picking it up for next week!
> 

Awesome. :)

Thank you
--
Gustavo