BUG/MEDIUM: sock: Remove FD_POLL_HUP during connect() if FD_POLL_ERR is not set

epoll_wait() may return EPOLLUP and/or EPOLLRDHUP after an asynchronous
connect(), to indicate that the peer accepted the connection then
immediately closed before epoll_wait() returned. When this happens,
sock_conn_check() is called to check whether or not the connection correctly
established, and after that the receive channel of the socket is assumed to
already be closed. This lets haproxy send the request at best (if RDHUP and
not HUP) then immediately close.

Over the last two years, there were a few reports about this spuriously
happening on connections where network captures proved that the server had
not closed at all (and sometimes even received the request and responded to
it after haproxy had closed). The logs show that a successful connection is
immediately reported on error after the request was sent. After
investigations, it appeared that a EPOLLUP, or eventually a EPOLLRDHUP, can
be reported by epool_wait() during the connect() but in sock_conn_check(),
the connect() reports a success. So the connection is validated but the HUP
is handled on the first receive and an error is reported.

The same behavior could be observed on health-checks, leading HAProxy to
consider the server as DOWN while it is not.

The only explanation at this point is that it is a kernel bug, notably
because it does not even match the documentation for connect() nor epoll. In
addition for now it was only observed with Ubuntu kernels 5.4 and 5.15 and
was never reproduced on any other one.

We have no reproducer but here is the typical strace observed:

socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 114
fcntl(114, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
setsockopt(114, SOL_TCP, TCP_NODELAY, [1], 4) = 0
connect(114, {sa_family=AF_INET, sin_port=htons(11000), sin_addr=inet_addr("A.B.C.D")}, 16) = -1 EINPROGRESS (Operation now in progress)
epoll_ctl(19, EPOLL_CTL_ADD, 114, {events=EPOLLIN|EPOLLOUT|EPOLLRDHUP, data={u32=114, u64=114}}) = 0
epoll_wait(19, [{events=EPOLLIN, data={u32=15, u64=15}}, {events=EPOLLIN, data={u32=151, u64=151}}, {events=EPOLLIN, data={u32=59, u64=59}}, {events=EPOLLIN|EPOLLRDHUP, data={u32=114, u64=114}}], 200, 0) = 4
epoll_ctl(19, EPOLL_CTL_MOD, 114, {events=EPOLLOUT, data={u32=114, u64=114}}) = 0
epoll_wait(19, [{events=EPOLLOUT, data={u32=114, u64=114}}, {events=EPOLLIN, data={u32=15, u64=15}}, {events=EPOLLIN, data={u32=10, u64=10}}, {events=EPOLLIN, data={u32=165, u64=165}}], 200, 0) = 4
connect(114, {sa_family=AF_INET, sin_port=htons(11000), sin_addr=inet_addr("A.B.C.D")}, 16) = 0
sendto(114, "POST "..., 1009, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 1009
close(114)                              = 0

Some ressources about this issue:
  - https://www.spinics.net/lists/netdev/msg876470.html
  - haproxy/haproxy#1863
  - haproxy/haproxy#2368

So, to workaround the issue, we have decided to remove FD_POLL_HUP flag on
the FD during the connection establishement if FD_POLL_ERR is not reported
too in sock_conn_check(). This way, the call to connect() is able to
validate or reject the connection. At the end, if the HUP or RDHUP flags
were valid, either connect() would report the error itself, or the next
recv() would return 0 confirming the closure that the poller tried to
report. EPOLL_RDHUP is only an optimization to save a syscall anyway, and
this pattern is so rare that nobody will ever notice the extra call to
recv().

Please note that at least one reporter confirmed that using poll() instead
of epoll() also addressed the problem, so that can also be a temporary
workaround for those discovering the problem without the ability to
immediately upgrade.

The event is accounted via a COUNT_IF(), to be able to spot it in future
issue. Just in case.

This patch should fix the issue #1863 and #2368. It may be related
to #2751. It should be backported as far as 2.4. In 3.0 and below, the
COUNT_IF() must be removed.

Author: capflam

src/sock.c

@@ -945,6 +945,31 @@ int sock_conn_check(struct connection *conn)
 			goto done;
 		if (!(fdtab[fd].state & (FD_POLL_ERR|FD_POLL_HUP)))
 			goto wait;
+
+		/* Removing HUP if there is no ERR reported.
+		 *
+		 * After a first connect() returning EINPROGRESS, it seems
+		 * possible to have EPOLLHUP or EPOLLRDHUP reported by
+		 * epoll_wait() and turned to an error while the following
+		 * connect() will return a success. So the connection is
+		 * validated but the error is saved and reported on the first
+		 * subsequent read.
+		 *
+		 * We have no explanation for now. Why epoll report the
+		 * connection is closed while the connect() it able to validate
+		 * it ? no idea. But, it seems reasonnable in this case, and if
+		 * no error was reported, to remove the the HUP flag. At worst, if
+		 * the connection is really closed, this will be reported later.
+		 *
+		 * Only observed on Ubuntu kernel (5.4/5.15). See:
+		 *   - https://github.com/haproxy/haproxy/issues/1863
+		 *   - https://www.spinics.net/lists/netdev/msg876470.html
+		 */
+		if (unlikely((fdtab[fd].state & (FD_POLL_HUP|FD_POLL_ERR)) == FD_POLL_HUP)) {
+			COUNT_IF(1, "Removing FD_POLL_HUP if no FD_POLL_ERR to let connect() decide");
+			fdtab[fd].state &= ~FD_POLL_HUP;
+		}
+
 		/* error present, fall through common error check path */
 	}