[CRIU] [PATCH 2/6] fdstore: add a storage for file descriptors

[Andrei Vagin]

From: Andrei Vagin <avagin at virtuozzo.com>

We need a storage for file descriptors which is shared between processes
and doesn't use a lot of file descriptors. We are going to use it on
restore and if it will use file descriptors, we will have to find
descriptors which don't used by all restored processes to not confilict
with their descriptors.

There are two solutions. The first one is a service (process) which
handles to command push_fd(id, fd) and pop_fd(id, fd).

Another solution is to save descriptros in a unix socket.  It requires
only one extra descriptor which we can register as a service fd. Each
unix socket has a buffer and can fit a number of file descriptros. We
can use SK_PEEK_OFF and MSG_PEEK to get file descriptros from a socket
as many times as we need.

This patch implements the second solution.

Signed-off-by: Andrei Vagin <avagin at virtuozzo.com>
---
 criu/Makefile.crtools    |  1 +
 criu/fdstore.c           | 86 ++++++++++++++++++++++++++++++++++++++++++++++++
 criu/include/fdstore.h   |  8 +++++
 criu/include/servicefd.h |  1 +
 4 files changed, 96 insertions(+)
 create mode 100644 criu/fdstore.c
 create mode 100644 criu/include/fdstore.h

diff --git a/criu/Makefile.crtools b/criu/Makefile.crtools
index 8188129..b85d776 100644
--- a/criu/Makefile.crtools
+++ b/criu/Makefile.crtools
@@ -79,6 +79,7 @@ obj-y			+= uts_ns.o
 obj-y			+= path.o
 obj-y			+= autofs.o
 obj-y			+= uffd.o
+obj-y			+= fdstore.o
 
 ifeq ($(VDSO),y)
 obj-y			+= pie-util-vdso.o
diff --git a/criu/fdstore.c b/criu/fdstore.c
new file mode 100644
index 0000000..f4824ae
--- /dev/null
+++ b/criu/fdstore.c
@@ -0,0 +1,86 @@
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <stdio.h>
+
+#include "common/scm.h"
+#include "servicefd.h"
+#include "fdstore.h"
+#include "xmalloc.h"
+#include "log.h"
+
+static int next_id;
+
+int fdstore_init(void)
+{
+	struct sockaddr_un addr;
+	unsigned int addrlen;
+	struct stat st;
+	int sk, ret;
+
+	sk = socket(AF_UNIX, SOCK_DGRAM | SOCK_NONBLOCK, 0);
+	if (sk < 0) {
+		pr_perror("Unable to create a socket");
+		return -1;
+	}
+
+	if (fstat(sk, &st)) {
+		pr_perror("Unable to stat a file descriptor");
+		close(sk);
+		return -1;
+	}
+
+	addr.sun_family = AF_UNIX;
+	addrlen = snprintf(addr.sun_path, sizeof(addr.sun_path), "X/criu-fdstore-%"PRIx64, st.st_ino);
+	addrlen += sizeof(addr.sun_family);
+
+	addr.sun_path[0] = 0;
+	if (bind(sk, (struct sockaddr *) &addr, addrlen)) {
+		pr_perror("Unable to bind a socket");
+		close(sk);
+		return -1;
+	}
+	if (connect(sk, (struct sockaddr *) &addr, addrlen)) {
+		pr_perror("Unable to connect a socket");
+		close(sk);
+		return -1;
+	}
+
+	ret = install_service_fd(FDSTORE_SK_OFF, sk);
+	close(sk);
+	if (ret < 0)
+		return -1;
+
+	return 0;
+}
+
+int fdstore_add(int fd)
+{
+	int sk = get_service_fd(FDSTORE_SK_OFF);
+
+	if (send_fd(sk, NULL, 0, fd))
+		return -1;
+
+	next_id++;
+
+	return next_id - 1;
+}
+
+int fdstore_get(int id)
+{
+	int sk = get_service_fd(FDSTORE_SK_OFF);
+	socklen_t len = sizeof(id);
+	int fd;
+
+	if (getsockopt(sk, SOL_SOCKET, SO_PEEK_OFF, &id, &len)) {
+		pr_perror("Unable to a peek offset");
+		return -1;
+	}
+
+	fd = recv_fd(sk);
+	if (fd < 0)
+		return -1;
+	return fd;
+}
diff --git a/criu/include/fdstore.h b/criu/include/fdstore.h
new file mode 100644
index 0000000..b406cdd
--- /dev/null
+++ b/criu/include/fdstore.h
@@ -0,0 +1,8 @@
+#ifndef __CRIU_FDSTORE_H__
+#define __CRIU_FDSTORE_H__
+
+int fdstore_init(void);
+int fdstore_add(int fd);
+int fdstore_get(int id);
+
+#endif
diff --git a/criu/include/servicefd.h b/criu/include/servicefd.h
index 1b48e90..3e0e376 100644
--- a/criu/include/servicefd.h
+++ b/criu/include/servicefd.h
@@ -22,6 +22,7 @@ enum sfd_type {
 	TRANSPORT_FD_OFF, /* to transfer file descriptors */
 	LAZY_PAGES_SK_OFF, /* socket for communication with lazy-pages daemon */
 	RPC_SK_OFF,
+	FDSTORE_SK_OFF,
 
 	SERVICE_FD_MAX
 };
-- 
2.7.4