restore: block sigchld to remap task_entries

Currently we remap task_entries but it can be used from a sigchld hanler.
We need to block sigchld to remap task_entries and unlock it
when a restorer sigchld handler is set.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cr-restore.c

@@ -2754,6 +2754,8 @@ static int sigreturn_restore(pid_t pid, struct task_restore_args *task_args, uns
 	unsigned long creds_pos = 0;
 	unsigned long creds_pos_next;
 
+	sigset_t blockmask;
+
 	pr_info("Restore via sigreturn\n");
 
 	/* pr_info_vma_list(&self_vma_list); */
@@ -2881,6 +2883,15 @@ static int sigreturn_restore(pid_t pid, struct task_restore_args *task_args, uns
 	}
 
 	task_args->breakpoint = &rsti(current)->breakpoint;
+
+	sigemptyset(&blockmask);
+	sigaddset(&blockmask, SIGCHLD);
+
+	if (sigprocmask(SIG_BLOCK, &blockmask, NULL) == -1) {
+		pr_perror("Can not set mask of blocked signals");
+		return -1;
+	}
+
 	task_args->task_entries = rst_mem_remap_ptr(task_entries_pos, RM_SHREMAP);
 
 	task_args->premmapped_addr = (unsigned long)rsti(current)->premmapped_addr;

criu/pie/restorer.c

@@ -1054,6 +1054,10 @@ long __export_restore_task(struct task_restore_args *args)
 	act.rt_sa_restorer = cr_restore_rt;
 	sys_sigaction(SIGCHLD, &act, NULL, sizeof(k_rtsigset_t));
 
+	ksigfillset(&to_block);
+	ksigaddset(&to_block, SIGCHLD);
+	ret = sys_sigprocmask(SIG_UNBLOCK, &to_block, NULL, sizeof(k_rtsigset_t));
+
 	log_set_fd(args->logfd);
 	log_set_loglevel(args->loglevel);