[CRIU] [PATCH] net: mount a new tmpfs if it isn't enough rights to open /run/xtables.lock

[Andrei Vagin]

Starting with iptables 1.6.2, we have to use the --wait option,
but it doesn't work properly with userns, because in this case,
we don't have enough rights to open /run/xtables.lock.

(00.174703)      1: 	Running iptables-restore -w for iptables-restore -w Fatal: can't open lock file /run/xtables.lock: Permission denied
(00.192058)      1: Error (criu/util.c:842): exited, status=4
(00.192080)      1: Error (criu/net.c:1738): iptables-restore -w failed
(00.192088)      1: Error (criu/net.c:2389): Can't create net_ns
(00.192131)      1: Error (criu/util.c:1567): Can't wait or bad status: errno=0, status=65280

This patch workarounds this problem by mounting tmpfs into /run.

https://github.com/checkpoint-restore/criu/issues/469

Cc: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Cc: Kirill Tkhai <ktkhai at virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin at openvz.org>
---
 criu/net.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/criu/net.c b/criu/net.c
index 6430bdfcb..27f11179f 100644
--- a/criu/net.c
+++ b/criu/net.c
@@ -1965,11 +1965,54 @@ out:
 	return ret;
 }
 
+/*
+ * iptables-restore is executed from a target userns and it may have not enough
+ * rights to open /run/xtables.lock. Here we try to workaround this problem.
+ */
+static int replace_xtable_lock() { static bool done; int fd;
+
+	if (!(root_ns_mask & CLONE_NEWUSER))
+		return 0;
+
+	/* It is a question what to do when we have userns without mntns */
+	if (!(root_ns_mask & CLONE_NEWNS))
+		return 0;
+
+	if (done)
+		return 0;
+
+	fd = open("/run/xtables.lock", O_RDONLY);
+	if (fd >= 0) {
+		close(fd);
+		done = true;
+		return 0;
+	}
+
+	/*
+	 * /run/xtables.lock may not exist, so we can't just bind-mount a file
+	 * over it.
+	 * A new mount will not be propagated to the host mount namespace,
+	 * because we are in another userns.
+	 */
+
+	if (mount("criu-xtable-lock", "/run", "tmpfs", 0, NULL)) {
+		pr_perror("Unable to mount tmpfs into /run");
+		return -1;
+	}
+
+	done = true;
+
+	return 0;
+}
+
 static inline int restore_iptables(int pid)
 {
 	int ret = -1;
 	struct cr_img *img;
 
+	if (replace_xtable_lock())
+		return -1;
+
 	img = open_image(CR_FD_IPTABLES, O_RSTR, pid);
 	if (img == NULL)
 		return -1;
-- 
2.17.0