linux-kernel - Re: [PATCH] tty: n_hdlc: Use flexible-array member

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Date:   Tue, 21 Jan 2020 06:54:22 +0100
From:   Jiri Slaby <jslaby@...e.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: n_hdlc: Use flexible-array member

On 21. 01. 20, 0:45, Gustavo A. R. Silva wrote:
> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> presence of a "variable length array":
> 
> struct something {
>     int length;
>     u8 data[1];
> };
> 
> struct something *instance;
> 
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
> 
> There is also 0-byte arrays. Both cases pose confusion for things like
> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
> to declare variable-length types such as the one above is a flexible array
> member[2] which need to be the last member of a structure and empty-sized:
> 
> struct something {
>         int stuff;
>         u8 data[];
> };
> 
> Also, 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
> unadvertenly introduced[3] to the codebase from now on.
> 
> [1] https://github.com/KSPP/linux/issues/21
> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  drivers/tty/n_hdlc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
> index 98361acd3053..b5499ca8757e 100644
> --- a/drivers/tty/n_hdlc.c
> +++ b/drivers/tty/n_hdlc.c
> @@ -115,7 +115,7 @@
>  struct n_hdlc_buf {
>  	struct list_head  list_item;
>  	int		  count;
> -	char		  buf[1];
> +	char		  buf[];
>  };
>  
>  #define	N_HDLC_BUF_SIZE	(sizeof(struct n_hdlc_buf) + maxframe)

Have you checked, that you don't have to "+ 1" here now?

Other than that:
Acked-by: Jiri Slaby <jslaby@...e.cz>

thanks,
-- 
js
suse labs