[PATCH v2] powernv/opal-prd: Silence memcpy() run-time false positive warnings

[Mahesh Salgaonkar <mahesh@linux.ibm.com>]

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
[...]

Split the copy 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>
---
Change from v1:
- Rework the memcpy to copy message header and rest of the message
  separately without adding flex array.
---
 arch/powerpc/platforms/powernv/opal-prd.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/powernv/opal-prd.c b/arch/powerpc/platforms/powernv/opal-prd.c
index 113bdb151f687..a068cbc4d43c0 100644
--- a/arch/powerpc/platforms/powernv/opal-prd.c
+++ b/arch/powerpc/platforms/powernv/opal-prd.c
@@ -352,7 +352,9 @@ static int opal_prd_msg_notifier(struct notifier_block *nb,
 	if (!item)
 		return -ENOMEM;
 
-	memcpy(&item->msg, msg->params, msg_size);
+	item->msg = *hdr;
+	hdr++;
+	memcpy((char *)&item->msg + sizeof(*hdr), hdr, msg_size - sizeof(*hdr));
 
 	spin_lock_irqsave(&opal_prd_msg_queue_lock, flags);
 	list_add_tail(&item->list, &opal_prd_msg_queue);