parasite: handle errors while a transport socket is being created

Currently if socket() or connect() syscall-s failed, parasite cures itself,
but criu has not got any signals and waits on accept().

This patch adds a futex to synchronize parasite and criu. The server socket
is created with SOCK_NONBLOCK and waits on the futex when a parasite
connects to it, only then criu calls accept() and it returns immediately.
Reported-by: Yohei Kamitsukasa <uhoidx@gmail.com>
Cc: Yohei Kamitsukasa <uhoidx@gmail.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/parasite.h

@@ -15,6 +15,7 @@
 
 #include "image.h"
 #include "util-pie.h"
+#include "lock.h"
 
 #include "images/vma.pb-c.h"
 #include "images/tty.pb-c.h"
@@ -72,6 +73,7 @@ struct parasite_init_args {
 	struct rt_sigframe	*sigframe;
 
 	void			*sigreturn_addr;
+	futex_t			daemon_connected;
 };
 
 struct parasite_unmap_args {

criu/net.c

@@ -1678,7 +1678,7 @@ static int prep_ns_sockets(struct ns_id *ns, bool for_dump)
 	} else
 		ns->net.nlsk = -1;
 
-	ret = ns->net.seqsk = socket(PF_UNIX, SOCK_SEQPACKET, 0);
+	ret = ns->net.seqsk = socket(PF_UNIX, SOCK_SEQPACKET | SOCK_NONBLOCK, 0);
 	if (ret < 0) {
 		pr_perror("Can't create seqsk for parasite");
 		goto err_sq;

criu/parasite-syscall.c

@@ -509,6 +509,8 @@ static int parasite_init_daemon(struct parasite_ctl *ctl, struct ns_id *net)
 	args->sigframe = ctl->rsigframe;
 	args->log_level = log_get_loglevel();
 
+	futex_set(&args->daemon_connected, 0);
+
 	if (prepare_tsock(ctl, pid, args, net))
 		goto err;
 
@@ -520,6 +522,13 @@ static int parasite_init_daemon(struct parasite_ctl *ctl, struct ns_id *net)
 	if (parasite_run(pid, PTRACE_CONT, ctl->parasite_ip, ctl->rstack, &regs, &ctl->orig))
 		goto err;
 
+	futex_wait_while_eq(&args->daemon_connected, 0);
+	if (futex_get(&args->daemon_connected) != 1) {
+		errno = -(int)futex_get(&args->daemon_connected);
+		pr_perror("Unable to connect a transport socket");
+		goto err;
+	}
+
 	if (accept_tsock(ctl) < 0)
 		goto err;
 

criu/pie/parasite.c

@@ -735,7 +735,7 @@ static noinline __used int parasite_init_daemon(void *data)
 	args->sigreturn_addr = fini_sigreturn;
 	sigframe = args->sigframe;
 
-	tsock = sys_socket(PF_UNIX, SOCK_SEQPACKET, 0);
+	ret = tsock = sys_socket(PF_UNIX, SOCK_SEQPACKET, 0);
 	if (tsock < 0) {
 		pr_err("Can't create socket: %d\n", tsock);
 		goto err;
@@ -747,6 +747,8 @@ static noinline __used int parasite_init_daemon(void *data)
 		goto err;
 	}
 
+	futex_set_and_wake(&args->daemon_connected, 1);
+
 	ret = recv_fd(tsock);
 	if (ret >= 0) {
 		log_set_fd(ret);
@@ -758,6 +760,7 @@ static noinline __used int parasite_init_daemon(void *data)
 	parasite_daemon(data);
 
 err:
+	futex_set_and_wake(&args->daemon_connected, ret);
 	fini();
 	BUG();