Re: [PATCH] powernv/opal-prd: Silence memcpy() run-time false positive warnings

[Jordan Niethe <jniethe5@gmail.com>]

On 26/6/23 5:04 pm, Mahesh Salgaonkar wrote:
> opal_prd_msg_notifier extracts the opal prd message size from the message
> header and uses it for allocating opal_prd_msg_queue_item that includes
> the correct message size to be copied. However, while running under
> CONFIG_FORTIFY_SOURCE=y, it triggers following run-time warning:
> 
> [ 6458.234352] memcpy: detected field-spanning write (size 32) of single field "&item->msg" at arch/powerpc/platforms/powernv/opal-prd.c:355 (size 4)
> [ 6458.234390] WARNING: CPU: 9 PID: 660 at arch/powerpc/platforms/powernv/opal-prd.c:355 opal_prd_msg_notifier+0x174/0x188 [opal_prd]
> [...]
> [ 6458.234709] NIP [c00800000e0c0e6c] opal_prd_msg_notifier+0x174/0x188 [opal_prd]
> [ 6458.234723] LR [c00800000e0c0e68] opal_prd_msg_notifier+0x170/0x188 [opal_prd]
> [ 6458.234736] Call Trace:
> [ 6458.234742] [c0000002acb23c10] [c00800000e0c0e68] opal_prd_msg_notifier+0x170/0x188 [opal_prd] (unreliable)
> [ 6458.234759] [c0000002acb23ca0] [c00000000019ccc0] notifier_call_chain+0xc0/0x1b0
> [ 6458.234774] [c0000002acb23d00] [c00000000019ceac] atomic_notifier_call_chain+0x2c/0x40
> [ 6458.234788] [c0000002acb23d20] [c0000000000d69b4] opal_message_notify+0xf4/0x2c0
> [...]
> 
> Add a flexible array member to avoid false positive run-time warning.
> 
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Signed-off-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> ---
>   arch/powerpc/platforms/powernv/opal-prd.c |    7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/opal-prd.c b/arch/powerpc/platforms/powernv/opal-prd.c
> index 113bdb151f687..9e2c4775f75f5 100644
> --- a/arch/powerpc/platforms/powernv/opal-prd.c
> +++ b/arch/powerpc/platforms/powernv/opal-prd.c
> @@ -30,7 +30,10 @@
>    */
>   struct opal_prd_msg_queue_item {
>   	struct list_head		list;
> -	struct opal_prd_msg_header	msg;
> +	union {
> +		struct opal_prd_msg_header	msg;
> +		DECLARE_FLEX_ARRAY(__u8, msg_flex);
> +	};
>   };
>   
>   static struct device_node *prd_node;
> @@ -352,7 +355,7 @@ static int opal_prd_msg_notifier(struct notifier_block *nb,
>   	if (!item)
>   		return -ENOMEM;
>   
> -	memcpy(&item->msg, msg->params, msg_size);
> +	memcpy(&item->msg_flex, msg->params, msg_size);

This does silence the warning but it seems like we might be able to go a 
little further.

What about not adding that flex array to struct opal_prd_msg_queue_item, 
but adding one to struct opal_prd_msg. That is what the data format 
actually is.

So we'd have something like:


     struct opal_prd_msg  {

         struct opal_prd_msg_header hdr;

         char msg[];

     }


and change things to use that instead?

But that might be more trouble than it is worth, alternatively we can 
just do:

	item->msg = *hdr;
	memcpy((char *)item->msg + sizeof(*hdr), (char *)hdr + sizeof(*hdr), 
msg_size - sizeof(*hdr));


>   
>   	spin_lock_irqsave(&opal_prd_msg_queue_lock, flags);
>   	list_add_tail(&item->list, &opal_prd_msg_queue);
> 
> 
> 
>