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

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Message-ID: <1b78acd5-2b5f-55f1-5571-73f45d3c87f7@embeddedor.com>
Date:   Mon, 2 Mar 2020 17:57:11 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ocfs2: Replace zero-length array with flexible-array
 member

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)
>