wait_noreap_copyout(): check for ->wo_info != NULL

Current behaviour of sys_waitid() looks odd.  If user passes infop ==
NULL, sys_waitid() returns success.  When user additionally specifies flag
WNOWAIT, sys_waitid() returns -EFAULT on the same conditions.  When user
combines WNOWAIT with WCONTINUED, sys_waitid() again returns success.

This patch adds check for ->wo_info in wait_noreap_copyout().

User-visible change: starting from this commit, sys_waitid() always checks
infop != NULL and does not fail if it is NULL.
Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1140,6 +1140,7 @@ static int wait_noreap_copyout(struct wait_opts *wo, struct task_struct *p,
 
 	put_task_struct(p);
 	infop = wo->wo_info;
+	if (infop) {
 		if (!retval)
 			retval = put_user(SIGCHLD, &infop->si_signo);
 		if (!retval)
@@ -1152,6 +1153,7 @@ static int wait_noreap_copyout(struct wait_opts *wo, struct task_struct *p,
 			retval = put_user(uid, &infop->si_uid);
 		if (!retval)
 			retval = put_user(status, &infop->si_status);
+	}
 	if (!retval)
 		retval = pid;
 	return retval;