buglwIP - A Lightweight TCP/IP stack - Bugs: bug #43739, Accept not reporting errors about...


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bug #43739: Accept not reporting errors about aborted connections

Submitted by:  Joel Cunningham <jcunningham>
Submitted on:  Tue 02 Dec 2014 05:18:26 PM UTC  
Category:  sockets/netconn Severity:  2 - Minor
Item Group:  Change Request Status:  Fixed
Privacy:  Public Assigned to:  Simon Goldschmidt <goldsimon>
Open/Closed:  Closed Planned Release:  None
lwIP version:  1.4.1

Tue 05 Apr 2016 07:43:52 PM UTC, comment #5: 

Fixed: netconn_accept() returns ERR_ABRT (sockets: ECONNABORTED) for aborted connections, ERR_CLSD (sockets: EINVAL) if the listening netconn is closed, which better seems to follow the standard.
The only downside is that ERR_ABRT is not stored in conn->last_err (since that would block further acceptes: ERR_ABRT is fatal) -> task #13922 Remove fatal error handling

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.
Wed 23 Mar 2016 08:59:54 PM UTC, comment #4: 

I've started to work on this by passing 'tcp_alloc()' failures on listen-input to the accept-callback (this is the first time the accept callback is called with err != ERR_OK).

ATTENTION: accept callbacks now have to handle NULL pointers for pcb, but any well-written code would already have checked this :-)

Next step is to fix netconn/socket API to handle these errors when reading 'acceptmbox' and returning an appropriate error (ERR_ABRT would be the correct but this is still marked as FATAL...)

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.
Wed 25 Feb 2015 09:09:53 PM UTC, comment #3: 

> After looking up the opengroup specification for accept(), that lead me to consider this a bug.

Well, that spec also says it may fail if 'There was insufficient memory available to complete the operation', so this might be optional as well...

Anyway, I'd gladly accept a patch for this :-)

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.
Wed 03 Dec 2014 03:46:10 PM UTC, comment #2: 

I would agree out-of-pcbs should be included in this failure (also out-of-sockets).  The accept() call in sockets.c will return ENFILE if out of sockets.

From the application point of view, if a pending connection can't be handed off due to failure to allocate connection state (either socket, netconn, or pcb) it would make sense to be consistent with error handling.

When I encountered this, I had no idea why the connection was being aborted on the server.  It took considerable debugging in api_lib.c, api_msg.c, and tcp_in.c to pinpoint the failure.  There are no LWIP_ASSERTs for this failure.

After looking up the opengroup specification for accept(), that lead me to consider this a bug.  I would consider it low priority since it just makes life easier for application developers.

I'm not sure how the backlog should play into this since a full backlog usually doesn't result in a RST, but ignoring the SYN or ignoring resulting ACK from listener's SYN+ACK.  The Linux backlog behavior is an interesting data point: http://veithen.github.io/2014/01/01/how-tcp-backlog-works-in-linux.html

Joel Cunningham <jcunningham>
Project Member
Tue 02 Dec 2014 08:45:29 PM UTC, comment #1: 

Well... what's the big difference between "out-of-netconns" (internally: connection is established and then aborted) and "out-of-pcbs" (internally: SYN is RST directly)?

Whend fixing the "out-of-netconns", the "out-of-pcbs" should be fixed, too.

In any case, when returning an error here, it should be enforced to use a listen-backlog. Or maybe only return an error when a backlog is used?

After all, I'm not really sure whether returning an error is the correct thing to do here... What do you need this error for, anyway?

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.
Tue 02 Dec 2014 05:18:26 PM UTC, original submission:  

I've discovered what appears to be a bug in accept's handling of aborted connections due to exhaustion of netconns.

As per the Open group spec, I would expect an error to be returned when a new connection fails the accept process:


I'm not sure if the correct return value should be ECONNABORTED or ENFILE (or even ENOMEM)

In my particular case, the listener is establishing a new connection, but the final step in tcp_process(), where the PCB is put into ESTABLISHED state, TCP_EVENT_ACCEPT() is called and that returns ERR_ABRT because allocation of a netconn failed (accept_function() returns ERR_MEM).  After that, the new connection is aborted, but no error is reported on the listening netconn/socket.

It looks like errors from within TCP are reported via the tcp_err callback, but this would be hard to do in tcp_process() because there is no linkage between the new connected PCB and the listener PCB.  This makes me think that maybe the error should be reported in accept_function() where the allocation failure happens.  Thoughts?

Joel Cunningham <jcunningham>
Project Member


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    Follow 6 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2016-04-05 goldsimon StatusIn Progress => Fixed
        Open/ClosedOpen => Closed
    2016-03-23 goldsimon StatusNone => In Progress
        Assigned toNone => goldsimon
    2015-02-25 goldsimon Severity3 - Normal => 2 - Minor
        Item GroupFaulty Behaviour => Change Request

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