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

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Date:   Tue, 10 Mar 2020 09:07:21 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.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][next] ocfs2: dlm: Replace zero-length array with
 flexible-array member



On 2020/3/10 04:20, 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>

Acked-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
> ---
>  fs/ocfs2/dlm/dlmcommon.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ocfs2/dlm/dlmcommon.h b/fs/ocfs2/dlm/dlmcommon.h
> index 0463dce65bb2..c8a444622faa 100644
> --- a/fs/ocfs2/dlm/dlmcommon.h
> +++ b/fs/ocfs2/dlm/dlmcommon.h
> @@ -564,7 +564,7 @@ struct dlm_migratable_lockres
>  	// 48 bytes
>  	u8 lvb[DLM_LVB_LEN];
>  	// 112 bytes
> -	struct dlm_migratable_lock ml[0];  // 16 bytes each, begins at byte 112
> +	struct dlm_migratable_lock ml[];  // 16 bytes each, begins at byte 112
>  };
>  #define DLM_MIG_LOCKRES_MAX_LEN  \
>  	(sizeof(struct dlm_migratable_lockres) + \
> @@ -601,7 +601,7 @@ struct dlm_convert_lock
>  
>  	u8 name[O2NM_MAX_NAME_LEN];
>  
> -	s8 lvb[0];
> +	s8 lvb[];
>  };
>  #define DLM_CONVERT_LOCK_MAX_LEN  (sizeof(struct dlm_convert_lock)+DLM_LVB_LEN)
>  
> @@ -616,7 +616,7 @@ struct dlm_unlock_lock
>  
>  	u8 name[O2NM_MAX_NAME_LEN];
>  
> -	s8 lvb[0];
> +	s8 lvb[];
>  };
>  #define DLM_UNLOCK_LOCK_MAX_LEN  (sizeof(struct dlm_unlock_lock)+DLM_LVB_LEN)
>  
> @@ -632,7 +632,7 @@ struct dlm_proxy_ast
>  
>  	u8 name[O2NM_MAX_NAME_LEN];
>  
> -	s8 lvb[0];
> +	s8 lvb[];
>  };
>  #define DLM_PROXY_AST_MAX_LEN  (sizeof(struct dlm_proxy_ast)+DLM_LVB_LEN)
>  
>