util: add a function to run an action is a child process

The action is run in a very lightweight process.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/include/util.h

@@ -344,4 +344,6 @@ extern int epoll_del_rfd(int epfd, struct epoll_rfd *rfd);
 extern int epoll_run_rfds(int epfd, struct epoll_event *evs, int nr_fds, int tmo);
 extern int epoll_prepare(int nr_events, struct epoll_event **evs);
 
+extern int call_in_child_process(int (*fn)(void *), void *arg);
+
 #endif /* __CR_UTIL_H__ */

criu/util.c

@@ -47,6 +47,7 @@
 #include "namespaces.h"
 #include "criu-log.h"
 
+#include "clone-noasan.h"
 #include "cr_options.h"
 #include "servicefd.h"
 #include "cr-service.h"
@@ -1381,3 +1382,32 @@ free_events:
 	xfree(*events);
 	return -1;
 }
+
+int call_in_child_process(int (*fn)(void *), void *arg)
+{
+	int status, ret = -1;
+	pid_t pid;
+	/*
+	 * Parent freezes till child exit, so child may use the same stack.
+	 * No SIGCHLD flag, so it's not need to block signal.
+	 */
+	pid = clone_noasan(fn, CLONE_VFORK | CLONE_VM | CLONE_FILES |
+			   CLONE_IO | CLONE_SIGHAND | CLONE_SYSVSEM, arg);
+	if (pid == -1) {
+		pr_perror("Can't clone");
+		return -1;
+	}
+	errno = 0;
+	if (waitpid(pid, &status, __WALL) != pid || !WIFEXITED(status) || WEXITSTATUS(status)) {
+		pr_err("Can't wait or bad status: errno=%d, status=%d\n", errno, status);
+		goto out;
+	}
+	ret = 0;
+	/*
+	 * Child opened PROC_SELF for pid. If we create one more child
+	 * with the same pid later, it will try to reuse this /proc/self.
+	 */
+out:
+	close_pid_proc();
+	return ret;
+}