[HECnet] DECnet for Linux Bug Fixes

[John Forecast]

Hi David,

> On Jun 29, 2020, at 9:44 PM, David Moylan <djm at wiz.net.au> wrote:
> 
> Hi John,
>  
> This is amazing news. I have been unable to use DECnet under Linux for a very long time as the implementation was broken.
>  
> The last time I saw DECnet for Linux working was back in the Ubuntu 14 timeframe. Nothing modern worked at all with the end result usually being the entire VM locking as soon as you went to initiate any form of DECnet connection (this required a power cycle or reset to recover).
> 

Mostly it seems to have been a case of benign neglect - a new feature/function is added to the kernel and someone modifies the DECnet code to make sure it still compiles but there’s no-one around to do any testing.

> I did some research and it looked like no-one was maintaining the code. I saw a few attempts from a few people to resurge this, but I could never get their code trees to compile.
>  
> I noticed in the readme that you’ve previously used this with Debian Buster 10 which puts this in a similar space to Ubuntu 18.04 Bionic, but your comments said “Due to kernel changes, the above no longer work.”
>  
> So should I be able to use your kernel module code under Ubuntu 18.04 (running kernel 4.15.0 or similar) or has something recently broken?
> 

The code as it stands today will probably not even compile with a 4.15.0 tree - all of my building/testing has been done on 4.19 and 5.4. Since writing the above note about “kernel changes” I’ve had to figure out the Linux versioning macros. It should be pretty easy to back out that particular change and add a version check - it’s only two calls to a kernel routine which added an extra argument which DECnet doesn’t use. The harder problem is finding out which version made the change. With these changes, there’s a good chance that it will work on 4.15 although no guarantees! Let me know if you want me to make those changes.

  John.

> Cheers, Wiz!!
>  
>  
> From: owner-hecnet at Update.UU.SE [mailto:owner-hecnet at Update.UU.SE] On Behalf Of John Forecast
> Sent: Tuesday, 30 June 2020 10:45 AM
> To: hecnet at update.uu.se
> Subject: [HECnet] DECnet for Linux Bug Fixes
>  
> Recently I have found, and fixed, a number of bugs in the DECnet for Linux kernel module. Some of
> these bugs may be relevant for members of this mailing list who are using DECnet for Linux,
> especially on a Raspberry Pi.
>  
> Major bugs fixed:
>  
> - Ethernet Listen Timer was not implemented
>  
>           In addition, the check for an address change for the designated router (DR) was missing
>           although there was a check if the DR’s priority had changed.
>  
>           Note: this code changes the format of /proc/net/decnet_dev to include the listen timer value.
>                     nml/nml2 needs to be rebuilt to understand the changed format.
>  
> - System panic when using the loopback device (lo)
>  
>           The DECnet code was missing a destructor routine which is used to avoid data copying.
>  
> - The neighbour (adjacent node) code could be broken by kernel changes
>  
>           The code made use of a now deprecated feature (zero length array at the end of a structure)
>           in order to access data in a surrounding structure. This happened to work “by chance”
>           until kernel 5.4.42 on 32-bit processors.
>  
> - Interrupt message flow control was broken
>  
>           The flow control was a mixture of using the SEND/DONTSEND status of the data
>           subchannel and a message count. This seems to work between Linux systems but is broken
>           when communicating with other systems - during the life of a logical link, the remote system
>           could only send a single interrupt message while the Linux system could pretty much send
>           as many interrupts as it wanted possibly overrunning the remote systems buffers.
>  
> - Optional data on received connect confirm message was corrupted
>  
>           The code was getting the optional data from the wrong offset in the message.
>  
> - Next hop cache problem
>  
>           30 seconds after a logical link was taken down, the next hop cache entry was deleted. As
>           part of this deletion, the link was taken “down” which caused a neighbour entry to be
>           created for the same node address but accessed via the loopback device. Sometimes this
>           would cause the designated router to become accessible via the loopback device and
>           subsequent connections would fail.
>  
> - Intra-ethernet bit ignored
>  
>           The intra-ethernet bit in the routing flags is ignored on inbound traffic. If there was a neighbour
>           entry for the remote node at connection time, everything would work correctly. If there wad no
>           entry, all outbound  traffic would be sent through the designated router for the duration of
>           the logical link.
>  
> - Promiscuous mode alters DECnet behaviour
>  
>           If the ethernet NIC used for DECnet had promiscuous mode enabled (e.g. using tcpdump
>           for traffic tracing), the DECnet code would start seeing endnode hello’s, populating
>           neighbour structures and causing the problems described above for the intra-ethernet bit
>           to go away.
>  
> New programs:
>  
> DECnet Test Send and DECnet Test Receiver (DTS/DTR)
>  
>           Test programs created via reverse engineering the protocol exchanges. Used to find a
>           number of the bugs described above.
>  
> NML2
>  
>           New implementation of the Network Management Listener. Supports SUMMARY, STATUS
>           and CHARACTERISTICS for NODES, CIRCUITS and AREAS. It does not support LINKS
>           and OBJECTS which were in the old version but are system specific operations which
>           were only visible from DECnet-VMS systems.
>  
>           The old version is still the default during installation. The new version can be installed by:
>  
>                       cd dnprogs/nml2
>                       make
>                       sudo make install
>  
>           which will overwrite the installed executable and man page.
>  
> OS Support:
>  
>           As of 06/29/2020 the code has been tested with:
>  
>                       Raspbian 2020-05-27 release with kernels 4.19.126 and 5.4.44 (32-bit only)
>  
>                       Pi OS 2020-05-27 32-bit release with kernel 4.19.126 and 5.4.44
>  
>                       pI OS 2020-05-27 (fully updated on 06/26/2020) with kernel 5.4.49
>  
>  
> The source code and installation instructions are available at:
>  
>           <https://github.com/JohnForecast/RaspbianDECnet <https://github.com/JohnForecast/RaspbianDECnet>>
>  
>   John.
>  

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