ipc,msg: drop dst nil validation in copy_msg

d0edd8528362 (ipc: convert invalid scenarios to use WARN_ON) relaxed the nil dst parameter check, originally being a full BUG_ON. However, this check seems quite unnecessary when the only purpose is for ceckpoint/restore (MSG_COPY flag):

o The copy variable is set initially to nil, apparently as a way of ensuring that prepare_copy is previously called. Which is in fact done, unconditionally at the beginning of do_msgrcv.

o There is no concurrency with 'copy' (stack allocated in do_msgrcv).

Furthermore, any errors in 'copy' (and thus prepare_copy/copy_msg) should always handled by IS_ERR() family. Therefore remove this check altogether as it can never occur with the current users.

---
 ipc/msgutil.c | 1 -
 1 file changed, 1 deletion(-)

index 71f448e..ed81aaf 100644
@@ -123,7 +123,6 @@ struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst)

	size_t len = src->m_ts;
	size_t alen;

-	WARN_ON(dst == NULL);
	if (src->m_ts > dst->m_ts)
		return ERR_PTR(-EINVAL);

--
2.1.4