From: Mattias Nissler <mnissler@chromium.org>
To: openssh-unix-dev@mindrot.org
Subject: [PATCH] Add POLLOUT when connect()ing in non-blocking mode.
Date: Thu, 21 Nov 2019 00:34:50 +0100

With the current POLLIN as the only requested event, there won't be a
poll event reported when the TCP connection has been established
successfully, but only after receiving data from the other side. This
is a problem when connecting to servers that don't send their
identification string immediately, e.g. the sslh multiplexer waits for
the first client packet to identify the requested service. To make
this work better and be consistent with blocking connect(), also
request POLLOUT events such that poll() returns once the TCP
connection has come up.
---
 misc.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

 Here is a reproduction of the issue:

  (cr) mnissler@toroa ~ $ uname -r
  4.19.67-2rodete2-amd64
  (cr) mnissler@toroa ~ $ /usr/sbin/sslh -V
  sslh-fork v1.18
  (cr) mnissler@toroa ~ $ ssh -V
  OpenSSH_8.1p1-hpn14v16, OpenSSL 1.0.2t  10 Sep 2019

  Start sslh with a timeout of 2 seconds:

  (cr) mnissler@toroa ~ $ /usr/sbin/sslh -p localhost:2222 -t 2 --ssh localhost:22 -f &
  [1] 251851
  sslh-fork v1.18 started

  When passing ConnectTimeout=2, the client doesn't notice the TCP
  connection coming up so the connection times out after 2 seconds:

  (cr) mnissler@toroa ~ $ ssh -v -o ConnectTimeout=2 -p 2222 localhost
  OpenSSH_8.1p1-hpn14v16, OpenSSL 1.0.2t  10 Sep 2019
  debug1: Reading configuration data /home/mnissler/.ssh/config
  debug1: Reading configuration data /etc/ssh/ssh_config
  debug1: Connecting to localhost [::1] port 2222.
  debug1: connect to address ::1 port 2222: Connection timed out
  debug1: Connecting to localhost [127.0.0.1] port 2222.
  debug1: connect to address 127.0.0.1 port 2222: Connection timed out
  ssh: connect to host localhost port 2222: Connection timed out
  ssh:connection from localhost:37766 to localhost:2222 forwarded from localhost:41614 to localhost:ssh

  Increasing ConnectTimeout to 3, the connection comes up
  successfully, but only after the sslh timeout expires, which causes
  unnecessary delay:

  (cr) mnissler@toroa ~ $ ssh -v -o ConnectTimeout=3 -p 2222 localhost
  OpenSSH_8.1p1-hpn14v16, OpenSSL 1.0.2t  10 Sep 2019
  debug1: Reading configuration data /home/mnissler/.ssh/config
  debug1: Reading configuration data /etc/ssh/ssh_config
  debug1: Connecting to localhost [::1] port 2222.
  <<< hangs for 2 seconds here >>>
  debug1: fd 3 clearing O_NONBLOCK
  debug1: Connection established.

--- a/misc.c
+++ b/misc.c
@@ -238,12 +238,12 @@ set_rdomain(int fd, const char *name)
 }
 
 /*
- * Wait up to *timeoutp milliseconds for fd to be readable. Updates
+ * Wait up to *timeoutp milliseconds for events on fd. Updates
  * *timeoutp with time remaining.
  * Returns 0 if fd ready or -1 on timeout or error (see errno).
  */
-int
-waitrfd(int fd, int *timeoutp)
+static int
+waitfd(int fd, int *timeoutp, short events)
 {
 	struct pollfd pfd;
 	struct timeval t_start;
@@ -251,7 +251,7 @@ waitrfd(int fd, int *timeoutp)
 
 	monotime_tv(&t_start);
 	pfd.fd = fd;
-	pfd.events = POLLIN;
+	pfd.events = events;
 	for (; *timeoutp >= 0;) {
 		r = poll(&pfd, 1, *timeoutp);
 		oerrno = errno;
@@ -269,6 +269,16 @@ waitrfd(int fd, int *timeoutp)
 	return -1;
 }
 
+/*
+ * Wait up to *timeoutp milliseconds for fd to be readable. Updates
+ * *timeoutp with time remaining.
+ * Returns 0 if fd ready or -1 on timeout or error (see errno).
+ */
+int
+waitrfd(int fd, int *timeoutp) {
+	return waitfd(fd, timeoutp, POLLIN);
+}
+
 /*
  * Attempt a non-blocking connect(2) to the specified address, waiting up to
  * *timeoutp milliseconds for the connection to complete. If the timeout is
@@ -295,7 +305,7 @@ timeout_connect(int sockfd, const struct sockaddr *serv_addr,
 	} else if (errno != EINPROGRESS)
 		return -1;
 
-	if (waitrfd(sockfd, timeoutp) == -1)
+	if (waitfd(sockfd, timeoutp, POLLIN | POLLOUT) == -1)
 		return -1;
 
 	/* Completed or failed */
-- 
2.24.0.432.g9d3f5f5b63-goog