linux-kernel - Re: [PATCH] ext4: namei: Replace zero-length array with flexible-array member

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Date:   Mon, 2 Mar 2020 17:57:59 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: namei: Replace zero-length array with
 flexible-array member

Hi all,

Friendly ping: who can take this?

Thanks
--
Gustavo

On 2/13/20 10:06, 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/ext4/namei.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index 129d2ebae00d..77c327d904bf 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -233,13 +233,13 @@ struct dx_root
>  		u8 unused_flags;
>  	}
>  	info;
> -	struct dx_entry	entries[0];
> +	struct dx_entry	entries[];
>  };
>  
>  struct dx_node
>  {
>  	struct fake_dirent fake;
> -	struct dx_entry	entries[0];
> +	struct dx_entry	entries[];
>  };
>  
>  
>