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

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Date:   Mon, 2 Mar 2020 20:04:49 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Mark Fasheh <mark@...heh.com>,
        Joel Becker <jlbec@...lplan.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ocfs2: Replace zero-length array with flexible-array
 member



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@...ux.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@...eddedor.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)
>>>