net/af_iucv: Use struct_group() to zero struct iucv_sock region

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Add struct_group() to mark the region of struct iucv_sock that gets
initialized to zero. Avoid the future warning:

In function 'fortify_memset_chk',
    inlined from 'iucv_sock_alloc' at net/iucv/af_iucv.c:476:2:
./include/linux/fortify-string.h:199:4: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  199 |    __write_overflow_field(p_size_field, size);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Acked-by: Karsten Graul <kgraul@linux.ibm.com>
Link: https://lore.kernel.org/lkml/19ff61a0-0cda-6000-ce56-dc6b367c00d6@linux.ibm.com/Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/iucv/af_iucv.h

@@ -112,10 +112,12 @@ enum iucv_tx_notify {
 
 struct iucv_sock {
 	struct sock		sk;
-	char			src_user_id[8];
-	char			src_name[8];
-	char			dst_user_id[8];
-	char			dst_name[8];
+	struct_group(init,
+		char		src_user_id[8];
+		char		src_name[8];
+		char		dst_user_id[8];
+		char		dst_name[8];
+	);
 	struct list_head	accept_q;
 	spinlock_t		accept_q_lock;
 	struct sock		*parent;

net/iucv/af_iucv.c

@@ -473,7 +473,7 @@ static struct sock *iucv_sock_alloc(struct socket *sock, int proto, gfp_t prio,
 	atomic_set(&iucv->msg_recv, 0);
 	iucv->path = NULL;
 	iucv->sk_txnotify = afiucv_hs_callback_txnotify;
-	memset(&iucv->src_user_id , 0, 32);
+	memset(&iucv->init, 0, sizeof(iucv->init));
 	if (pr_iucv)
 		iucv->transport = AF_IUCV_TRANS_IUCV;
 	else