libct/nsenter: refactor ipc funcs for reusability

Modify receive_fd() and send_fd() so they can be more readily reused in
cloned_binary.c. Change receive_fd() to have a single responsibility:
receiving and returning a single file descriptor over a UNIX domain
socket. Make send_fd() useable in precarious execution contexts such as
a clone(CLONE_VFORK|CLONE_VM) "thread" where allocating heap memory or
calling exit() would be dangerous.

Signed-off-by: Cory Snider <csnider@mirantis.com>

libcontainer/nsenter/ipc.c

@@ -1,13 +1,12 @@
 #define _GNU_SOURCE
-#include <fcntl.h>
+#include <alloca.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/socket.h>
-#include <unistd.h>
 #include "ipc.h"
 #include "log.h"
 
-void receive_fd(int sockfd, int new_fd)
+int receive_fd(int sockfd)
 {
 	int bytes_read;
 	struct msghdr msg = { };
@@ -16,7 +15,6 @@ void receive_fd(int sockfd, int new_fd)
 	char null_byte = '\0';
 	int ret;
 	int fd_count;
-	int *fd_payload;
 
 	iov.iov_base = &null_byte;
 	iov.iov_len = 1;
@@ -52,21 +50,13 @@ void receive_fd(int sockfd, int new_fd)
 	if (fd_count != 1)
 		bail("received control message from unix socket %d with too many fds: %d", sockfd, fd_count);
 
-	fd_payload = (int *)CMSG_DATA(cmsg);
-	ret = dup3(*fd_payload, new_fd, O_CLOEXEC);
-	if (ret < 0)
-		bail("cannot dup3 fd %d to %d", *fd_payload, new_fd);
-
+	ret = *(int *)CMSG_DATA(cmsg);
 	free(msg.msg_control);
-
-	ret = close(*fd_payload);
-	if (ret < 0)
-		bail("cannot close fd %d", *fd_payload);
+	return ret;
 }
 
-void send_fd(int sockfd, int fd)
+int send_fd(int sockfd, int fd)
 {
-	int bytes_written;
 	struct msghdr msg = { };
 	struct cmsghdr *cmsg;
 	struct iovec iov[1] = { };
@@ -81,11 +71,7 @@ void send_fd(int sockfd, int fd)
 	/* We send only one fd as specified by cmsg->cmsg_len below, even
 	 * though msg.msg_controllen might have more space due to alignment. */
 	msg.msg_controllen = CMSG_SPACE(sizeof(int));
-	msg.msg_control = malloc(msg.msg_controllen);
-	if (msg.msg_control == NULL) {
-		bail("Can't allocate memory to send fd.");
-	}
-
+	msg.msg_control = alloca(msg.msg_controllen);
 	memset(msg.msg_control, 0, msg.msg_controllen);
 
 	cmsg = CMSG_FIRSTHDR(&msg);
@@ -94,10 +80,5 @@ void send_fd(int sockfd, int fd)
 	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
 	memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
 
-	bytes_written = sendmsg(sockfd, &msg, 0);
-
-	free(msg.msg_control);
-
-	if (bytes_written != 1)
-		bail("failed to send fd %d via unix socket %d", fd, sockfd);
+	return sendmsg(sockfd, &msg, 0);
 }

libcontainer/nsenter/ipc.h

@@ -1,7 +1,12 @@
 #ifndef NSENTER_IPC_H
 #define NSENTER_IPC_H
 
-void receive_fd(int sockfd, int new_fd);
-void send_fd(int sockfd, int fd);
+int receive_fd(int sockfd);
+
+/*
+ * send_fd passes the open file descriptor fd to another process via the UNIX
+ * domain socket sockfd. The return value of the sendmsg(2) call is returned.
+ */
+int send_fd(int sockfd, int fd);
 
 #endif /* NSENTER_IPC_H */

libcontainer/nsenter/nsexec.c

@@ -552,7 +552,13 @@ void receive_mountsources(int sockfd)
 			bail("malformed _LIBCONTAINER_MOUNT_FDS env var: fds out of range");
 		}
 
-		receive_fd(sockfd, new_fd);
+		int recv_fd = receive_fd(sockfd);
+		if (dup3(recv_fd, new_fd, O_CLOEXEC) < 0) {
+			bail("cannot dup3 fd %d to %ld", recv_fd, new_fd);
+		}
+		if (close(recv_fd) < 0) {
+			bail("cannot close fd %d", recv_fd);
+		}
 	}
 }
 
@@ -595,7 +601,8 @@ void send_mountsources(int sockfd, pid_t child, char *mountsources, size_t mount
 			bail("failed to open mount source %s", mountsources);
 
 		write_log(DEBUG, "~> sending fd for: %s", mountsources);
-		send_fd(sockfd, fd);
+		if (send_fd(sockfd, fd) < 0)
+			bail("failed to send fd %d via unix socket %d", fd, sockfd);
 
 		ret = close(fd);
 		if (ret != 0)