Bug(s) with creating large numbers of sockets

Bug(s) with creating large numbers of sockets

[Erik Bray]

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Hi all,

I found a few bugs in Cygwin w.r.t. creating large numbers of sockets.
For example, Cygwin will gladly let you create up to RLIMIT_NOFILE
sockets (examples in Python, where I found this problem):

>>> import resource
>>> import socket
>>> resource.getrlimit(resource.RLIMIT_NOFILE)
(256, 3200)
>>> resource.setrlimit(resource.RLIMIT_NOFILE, (3200, 3200))
>>> socks = [socket.socket() for _ in range(3000)]  # A bit fewer than the max but it doesn't matter

However, if I try to do anything interesting with those sockets, such
as poll on them, I get a rather unexpected error:

>>> import select
>>> poll = select.poll()
>>> for sock in socks:
...     poll.register(sock, select.POLLOUT)
...
>>> poll.poll()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 14] Bad address

After some playing around I found that I could make up to exactly 1365
sockets and use them without error.  At 1366 I get the error.  A very
strange and arbitrary number.  It turns out this is limited in Cygwin
by the array in fhandler_socket.cc:

 496 /* Maximum number of concurrently opened sockets from all Cygwin processes
 497    per session.  Note that shared sockets (through dup/fork/exec) are
 498    counted as one socket. */
 499 #define NUM_SOCKS       (32768 / sizeof (wsa_event))
...
 510 static wsa_event wsa_events[NUM_SOCKS] __attribute__((section
(".cygwin_dll     _common"), shared));

This choice for NUM_SOCKS is still seemingly small and pretty
arbitrary, but at least it's a choice, and probably well-motivated.
However, I think it's a problem that it's defined in terms of
sizeof(wsa_event).  On 32-bit Cygwin this is 16, so NUM_SOCKS is 2048
(a less strange number), whereas on 64-bit Cygwin sizeof(wsa_event) ==
24 (due to sizeof(long) == 8, plus alignment), so we are limited
to...1365 sockets.

If we have to set a limit I would just hard-code it to 2048 exactly.
I understand that the overhead associated with sockets in Cygwin
probably limits us from having 10s of thousands (much less millions)
and that's OK--I'm not trying to run some kind of C10K challenge on
Cygwin :)

The other problem, then, seems to be a bug in
fhandler_socket::init_events().  It doesn't check the return value of
search_wsa_event_slot(), which returns NULL if the wsa_events array is
full (and the socket is not a shared socket).  There's not a great
choice here for error code, but setting ENOBUF seems like the best
option.

Please see attached patch.

Best,
Erik

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From 960e402c14db43130e6054d0fe6a4d0af71b4acf Mon Sep 17 00:00:00 2001
From: "Erik M. Bray" <erik.m.bray@gmail.com>
Date: Fri, 3 Nov 2017 15:33:17 +0100
Subject: [PATCH] Fix two bugs in the limit of large numbers of sockets:

* Fix the maximum number of sockets allowed in the session to 2048,
  instead of making it relative to sizeof(wsa_event).

* Return an error and set errno=ENOBUF if a socket can't be created
  due to this limit being reached.
---
 winsup/cygwin/fhandler_socket.cc | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/fhandler_socket.cc b/winsup/cygwin/fhandler_socket.cc
index 7a6dbdc..b8eda57 100644
--- a/winsup/cygwin/fhandler_socket.cc
+++ b/winsup/cygwin/fhandler_socket.cc
@@ -496,7 +496,7 @@ fhandler_socket::af_local_set_secret (char *buf)
 /* Maximum number of concurrently opened sockets from all Cygwin processes
    per session.  Note that shared sockets (through dup/fork/exec) are
    counted as one socket. */
-#define NUM_SOCKS       (32768 / sizeof (wsa_event))
+#define NUM_SOCKS       ((unsigned int) 2048)
 
 #define LOCK_EVENTS	\
   if (wsock_mtx && \
@@ -623,7 +623,14 @@ fhandler_socket::init_events ()
       NtClose (wsock_mtx);
       return false;
     }
-  wsock_events = search_wsa_event_slot (new_serial_number);
+  if (!(wsock_events = search_wsa_event_slot (new_serial_number)));
+    {
+      set_errno (ENOBUFS);
+      NtClose (wsock_evt);
+      NtClose (wsock_mtx);
+      return false;
+    }
+
   /* sock type not yet set here. */
   if (pc.dev == FH_UDP || pc.dev == FH_DGRAM)
     wsock_events->events = FD_WRITE;
-- 
2.8.3


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Re: Bug(s) with creating large numbers of sockets

[Corinna Vinschen]

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Hi Erik,

why didn't you send this to cygwin-patches?  Not much of a problem,
just wondering...

On Nov  3 15:39, Erik Bray wrote:
> [...]
> After some playing around I found that I could make up to exactly 1365
> sockets and use them without error.  At 1366 I get the error.  A very
> strange and arbitrary number.  It turns out this is limited in Cygwin
> by the array in fhandler_socket.cc:
> 
>  496 /* Maximum number of concurrently opened sockets from all Cygwin processes
>  497    per session.  Note that shared sockets (through dup/fork/exec) are
>  498    counted as one socket. */
>  499 #define NUM_SOCKS       (32768 / sizeof (wsa_event))
> ...
>  510 static wsa_event wsa_events[NUM_SOCKS] __attribute__((section
> (".cygwin_dll     _common"), shared));
> 
> This choice for NUM_SOCKS is still seemingly small and pretty
> arbitrary, but at least it's a choice, and probably well-motivated.

This obviously dates back to pre-64 bit times.  The idea at the time was
that the .cygwin_dll_common section fits into a single 64K allocation
chunk.  Every section takes at least this 64K chunk, so we get away with
only one of them for this section.

Turns out, shared sections have a few issues so we even reduced the
usage and only kept stuff in which doesn't pose much of a problem.

Right now .cygwin_dll_common has a size of 0x8220 / 0x8200 of a max.
of 0x10000, so we have still lots of room and nothing really in need
of using a shared section.

> However, I think it's a problem that it's defined in terms of
> sizeof(wsa_event).  On 32-bit Cygwin this is 16, so NUM_SOCKS is 2048
> (a less strange number), whereas on 64-bit Cygwin sizeof(wsa_event) ==
> 24 (due to sizeof(long) == 8, plus alignment), so we are limited
> to...1365 sockets.
> 
> If we have to set a limit I would just hard-code it to 2048 exactly.
> I understand that the overhead associated with sockets in Cygwin
> probably limits us from having 10s of thousands (much less millions)
> and that's OK--I'm not trying to run some kind of C10K challenge on
> Cygwin :)

We only need another 0x220 bytes so we have a theoretical 0xfde0 / 64992
bytes or 2708 sockets for wsa_events on 64 bit.

So, yeah, I think you're right, 2048 is good number.  As I wrote in the
comment preceeding NUM_SOCKS, these are 2048 independent sockets per
user session in parallel.  Sockets created by dup or shared via
fork/exec only take one slot anyway.  2048 independent sockets per user
session should really suffice outside of testcases.

> The other problem, then, seems to be a bug in
> fhandler_socket::init_events().  It doesn't check the return value of
> search_wsa_event_slot(), which returns NULL if the wsa_events array is
> full (and the socket is not a shared socket).  There's not a great
> choice here for error code, but setting ENOBUF seems like the best
> option.

Yep, bug.  Thanks for catching.

> Please see attached patch.

Can you please send it to cygwin-patches with a bit of additional
comment (just a couple of words) why we choose 2048 here?


Thanks,
Corinna


-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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Re: Bug(s) with creating large numbers of sockets

[Erik Bray]

On Nov 3, 2017 9:36 PM, "Corinna Vinschen" wrote:

Hi Erik,

why didn't you send this to cygwin-patches?  Not much of a problem,
just wondering...


Will follow up more later, but just to clarify on this a bit I thought the
issue was just interesting, and that the  question(s) of the upper limit of
sockets allowed by Cygwin was (maybe?) interesting for
discussion/clarification.

But I also meant to send the patch to cygwin-patches and just forgot before
I dashed out the door to go home!

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