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Date:   Fri, 14 Feb 2020 16:49:02 -0500
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH AUTOSEL 5.5 451/542] tty: n_hdlc: Use flexible-array
 member and struct_size() helper

On Fri, Feb 14, 2020 at 10:47:23AM -0500, Sasha Levin wrote:
> From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
> 
> [ Upstream commit 85f4c95172d606dd66f7ee1fa50c45a245535ffd ]
> 
> 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
> inadvertenly introduced[3] to the codebase from now on.
> 
> Lastly, make use of the struct_size() helper to safely calculate the
> allocation size for instances of struct n_hdlc_buf and avoid any potential
> type mistakes[4][5].
> 
> [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")
> [4] https://lore.kernel.org/lkml/60e14fb7-8596-e21c-f4be-546ce39e7bdb@embeddedor.com/
> [5] commit 553d66cb1e86 ("iommu/vt-d: Use struct_size() helper")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> Reviewed-by: Jiri Slaby <jslaby@...e.cz>
> Link: https://lore.kernel.org/r/20200121172138.GA3162@embeddedor
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  drivers/tty/n_hdlc.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
> index 98361acd3053f..27b506bf03ced 100644
> --- a/drivers/tty/n_hdlc.c
> +++ b/drivers/tty/n_hdlc.c
> @@ -115,11 +115,9 @@
>  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)
> -
>  struct n_hdlc_buf_list {
>  	struct list_head  list;
>  	int		  count;
> @@ -524,7 +522,8 @@ static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *data,
>  		/* no buffers in free list, attempt to allocate another rx buffer */
>  		/* unless the maximum count has been reached */
>  		if (n_hdlc->rx_buf_list.count < MAX_RX_BUF_COUNT)
> -			buf = kmalloc(N_HDLC_BUF_SIZE, GFP_ATOMIC);
> +			buf = kmalloc(struct_size(buf, buf, maxframe),
> +				      GFP_ATOMIC);
>  	}
>  	
>  	if (!buf) {
> @@ -853,7 +852,7 @@ static struct n_hdlc *n_hdlc_alloc(void)
>  
>  	/* allocate free rx buffer list */
>  	for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
> -		buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
> +		buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
>  		if (buf)
>  			n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,buf);
>  		else if (debuglevel >= DEBUG_LEVEL_INFO)	
> @@ -862,7 +861,7 @@ static struct n_hdlc *n_hdlc_alloc(void)
>  	
>  	/* allocate free tx buffer list */
>  	for(i=0;i<DEFAULT_TX_BUF_COUNT;i++) {
> -		buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
> +		buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
>  		if (buf)
>  			n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,buf);
>  		else if (debuglevel >= DEBUG_LEVEL_INFO)	
> -- 
> 2.20.1
> 

There's no need for any of these variable length array patches to be
backported anywhere.

thanks,

greg k-h

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