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@xxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxx>
>> ---
>>  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)
>>