[PATCH] ocfs2: Replace zero-length array with flexible-array member

[PATCH] ocfs2: 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>
---
 fs/ocfs2/journal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index 68ba354cf361..b425f0b01dce 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -91,7 +91,7 @@ enum ocfs2_replay_state {
 struct ocfs2_replay_map {
 	unsigned int rm_slots;
 	enum ocfs2_replay_state rm_state;
-	unsigned char rm_replay_slots[0];
+	unsigned char rm_replay_slots[];
 };
 
 static void ocfs2_replay_map_set_state(struct ocfs2_super *osb, int state)
-- 
2.25.0

Re: [PATCH] ocfs2: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

Hi all,

Friendly ping: Who can take this?

Thanks
--
Gustavo

On 2/13/20 10:02, 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>
> ---
>  fs/ocfs2/journal.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
> index 68ba354cf361..b425f0b01dce 100644
> --- a/fs/ocfs2/journal.c
> +++ b/fs/ocfs2/journal.c
> @@ -91,7 +91,7 @@ enum ocfs2_replay_state {
>  struct ocfs2_replay_map {
>  	unsigned int rm_slots;
>  	enum ocfs2_replay_state rm_state;
> -	unsigned char rm_replay_slots[0];
> +	unsigned char rm_replay_slots[];
>  };
>  
>  static void ocfs2_replay_map_set_state(struct ocfs2_super *osb, int state)
> 

Re: [PATCH] ocfs2: Replace zero-length array with flexible-array member

[Joseph Qi]

Sorry for the late replay since I've missed the original mail.
This patch looks good to me.

Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>

On 2020/3/3 07:57, Gustavo A. R. Silva wrote:
> Hi all,
> 
> Friendly ping: Who can take this?
> 
> Thanks
> --
> Gustavo
> 
> On 2/13/20 10:02, 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>
>> ---
>>  fs/ocfs2/journal.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
>> index 68ba354cf361..b425f0b01dce 100644
>> --- a/fs/ocfs2/journal.c
>> +++ b/fs/ocfs2/journal.c
>> @@ -91,7 +91,7 @@ enum ocfs2_replay_state {
>>  struct ocfs2_replay_map {
>>  	unsigned int rm_slots;
>>  	enum ocfs2_replay_state rm_state;
>> -	unsigned char rm_replay_slots[0];
>> +	unsigned char rm_replay_slots[];
>>  };
>>  
>>  static void ocfs2_replay_map_set_state(struct ocfs2_super *osb, int state)
>>

Re: [PATCH] ocfs2: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

On 3/2/20 19:17, Joseph Qi wrote:
> Sorry for the late replay since I've missed the original mail.
> This patch looks good to me.
> 
> Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> 

Thanks, Joseph.
--
Gustavo

> On 2020/3/3 07:57, Gustavo A. R. Silva wrote:
>> Hi all,
>>
>> Friendly ping: Who can take this?
>>
>> Thanks
>> --
>> Gustavo
>>
>> On 2/13/20 10:02, 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>
>>> ---
>>>  fs/ocfs2/journal.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
>>> index 68ba354cf361..b425f0b01dce 100644
>>> --- a/fs/ocfs2/journal.c
>>> +++ b/fs/ocfs2/journal.c
>>> @@ -91,7 +91,7 @@ enum ocfs2_replay_state {
>>>  struct ocfs2_replay_map {
>>>  	unsigned int rm_slots;
>>>  	enum ocfs2_replay_state rm_state;
>>> -	unsigned char rm_replay_slots[0];
>>> +	unsigned char rm_replay_slots[];
>>>  };
>>>  
>>>  static void ocfs2_replay_map_set_state(struct ocfs2_super *osb, int state)
>>>