Subject: Re: [PATCH v2 2/5] treewide: Replace open-coded flex arrays in unions

From: Kees Cook <[email-masked]>
Date: Fri, 27 Aug 2021 09:17:37 -0700

On Thu, Aug 26, 2021 at 08:24:52AM +0200, Marc Kleine-Budde wrote:
> [...]
> BTW: Is there opportunity for conversion, too?
> 
> | drivers/net/can/peak_canfd/peak_pciefd_main.c:146:32: warning: array of flexible structures

Untested potential solution:

diff --git a/drivers/net/can/peak_canfd/peak_pciefd_main.c b/drivers/net/can/peak_canfd/peak_pciefd_main.c
index 1df3c4b54f03..efa2b5a52bd7 100644
--- a/drivers/net/can/peak_canfd/peak_pciefd_main.c
+++ b/drivers/net/can/peak_canfd/peak_pciefd_main.c
@@ -143,7 +143,11 @@ struct pciefd_rx_dma {
 	__le32 irq_status;
 	__le32 sys_time_low;
 	__le32 sys_time_high;
-	struct pucan_rx_msg msg[];
+	/*
+	 * with "msg" being pciefd_irq_rx_cnt(priv->irq_status)-many
+	 * variable-sized struct pucan_rx_msg following.
+	 */
+	__le32 msg[];
 } __packed __aligned(4);
 
 /* Tx Link record */
@@ -327,7 +331,7 @@ static irqreturn_t pciefd_irq_handler(int irq, void *arg)
 
 	/* handle rx messages (if any) */
 	peak_canfd_handle_msgs_list(&priv->ucan,
-				    rx_dma->msg,
+				    (struct pucan_rx_msg *)rx_dma->msg,
 				    pciefd_irq_rx_cnt(priv->irq_status));
 
 	/* handle tx link interrupt (if any) */

It's not great, but it's also not strictly a flex array, in the sense
that since struct pucan_rx_msg is a variable size, the compiler cannot
reason about the size of struct pciefd_rx_dma based only on the
irq_status encoding...

-- 
Kees Cook