nsexec: retry unshare on EINVAL

Older kernels may return EINVAL on unshare when a process is reading
runc's /proc/$PID/status or /proc/$PID/maps. This was fixed by kernel
commit 12c641ab8270f ("unshare: Unsharing a thread does not require
unsharing a vm") in Linuxt  v4.3.

For CentOS 7, the fix was backported to CentOS 7.7 (kernel 3.10.0-1062).

To work around this kernel bug, let's retry on EINVAL a few times.

Reported-by: zzyyzte <zhang.yu58@zte.com.cn>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/nsenter/nsexec.c

@@ -833,6 +833,25 @@ void send_mountsources(int sockfd, pid_t child, char *mountsources, size_t mount
 		bail("failed to close container mount namespace fd %d", container_mntns_fd);
 }
 
+void try_unshare(int flags, const char *msg)
+{
+	write_log(DEBUG, "unshare %s", msg);
+	/*
+	 * Kernels prior to v4.3 may return EINVAL on unshare when another process
+	 * reads runc's /proc/$PID/status or /proc/$PID/maps. To work around this,
+	 * retry on EINVAL a few times.
+	 */
+	int retries = 5;
+	for (; retries > 0; retries--) {
+		if (unshare(flags) == 0) {
+			return;
+		}
+		if (errno != EINVAL)
+			break;
+	}
+	bail("failed to unshare %s", msg);
+}
+
 void nsexec(void)
 {
 	int pipenum;
@@ -1171,9 +1190,7 @@ void nsexec(void)
 			 * problem.
 			 */
 			if (config.cloneflags & CLONE_NEWUSER) {
-				write_log(DEBUG, "unshare user namespace");
-				if (unshare(CLONE_NEWUSER) < 0)
-					bail("failed to unshare user namespace");
+				try_unshare(CLONE_NEWUSER, "user namespace");
 				config.cloneflags &= ~CLONE_NEWUSER;
 
 				/*
@@ -1225,9 +1242,7 @@ void nsexec(void)
 			 * some old kernel versions where clone(CLONE_PARENT | CLONE_NEWPID)
 			 * was broken, so we'll just do it the long way anyway.
 			 */
-			write_log(DEBUG, "unshare remaining namespace (except cgroupns)");
-			if (unshare(config.cloneflags & ~CLONE_NEWCGROUP) < 0)
-				bail("failed to unshare remaining namespaces (except cgroupns)");
+			try_unshare(config.cloneflags & ~CLONE_NEWCGROUP, "remaining namespaces (except cgroupns)");
 
 			/* Ask our parent to send the mount sources fds. */
 			if (config.mountsources) {
@@ -1340,8 +1355,7 @@ void nsexec(void)
 			}
 
 			if (config.cloneflags & CLONE_NEWCGROUP) {
-				if (unshare(CLONE_NEWCGROUP) < 0)
-					bail("failed to unshare cgroup namespace");
+				try_unshare(CLONE_NEWCGROUP, "cgroup namespace");
 			}
 
 			write_log(DEBUG, "signal completion to stage-0");