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

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Message-ID: <202003091446.6B5E42EF@keescook>
Date:   Mon, 9 Mar 2020 14:46:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] pstore: ram_core: Replace zero-length array with
 flexible-array member

On Mon, Mar 09, 2020 at 03:23:27PM -0500, 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>

Thanks! Applied to for-next/pstore.

-Kees

> ---
>  fs/pstore/ram_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index 1f4d8c06f9be..c917c191e78c 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -34,7 +34,7 @@ struct persistent_ram_buffer {
>  	uint32_t    sig;
>  	atomic_t    start;
>  	atomic_t    size;
> -	uint8_t     data[0];
> +	uint8_t     data[];
>  };
>  
>  #define PERSISTENT_RAM_SIG (0x43474244) /* DBGC */
> -- 
> 2.25.0
> 

-- 
Kees Cook