Value too small or too large

[tonyskalski]

Since updating to Zabbix 4.0.6 I have been getting values too small or too large from a Campbell Scientific data logger. libzbxmodbus is working for a dozen other meters I am monitoring. There have been no changes to the data loggers for many years. I have been successfully retrieving data with libzbxmodbus for 18 months, prior to the upgrade.

Checking with modpoll -m tcp -t 3:float -a 1 -r 33 -c 1 -1 <ip_addr> I get 57.0025 (the measurement is relative humidity). In Zabbix I am using modbus_read_registers[<ip_addr>, 1, 32, 4, float, LE] and I see 'Value 13364801568768.00000 is too small or too large'.

I've taken packet captures and decoded the modbus messages for both modpoll and libzbxmodbus. I see the same data for each method (more or less - it varies by ~1% each reading).

I have tried all combinations of data type and endian-ness without luck. I have recompiled and reinstalled (via git) twice. What am I missing?

[i-ky]

According to this tool, binary representation of 13364801568768.00000 is 0x55427bc4. And according to another tool we can get to a value close to what you expect (53.4418755, actually) by changing byte order from ABCD to BADC. Have you tried MLE instead of LE?

As far as I remember, there were issues in libmodbus regarding byte order (e.g. stephane/libmodbus/issues/358 and stephane/libmodbus/pull/441). This is one of the reasons libzbxmodbus has its own patched libmodbus. Unfortunately, it is indistinguishable from mainstream libmodbus (#22).

My best guess at this point is that during upgrade of Zabbix you accidentally upgraded libmodbus as well and mainstream version has overwritten the patched one.

[tonyskalski]

I had the same thought (an unpatched libmodbus). So before the last re-install of libzbxmodbus, I removed the libmodbus files. For the reinstall I did git clone --recursive https://github.com/v-zhuravlev/libzbxmodbus.git (and autogen and config and make). This should get the patched version of libmodbus right?

[i-ky]

Yes, it should. I would check the output of

$ ldd libzbxmodbus.so

and verify the modification time of libmodbus.so it points to. I hope you have restarted Zabbix after recompiling libs.

[tonyskalski]

While I would swear on stack of holy texts that I tried "0" for the endian-ness, I just re-tried and it is working. I did discover that I need to specify "float" and not "f" for the data type. So I will switch all my keys to use "float, MLE" so that it is more explicit. Thanks for the help!

[i-ky]

I believe with your test command modpoll will assume little endian byte order. The fact that you need MLE in Zabbix may be a hint that you have unpatched libmodbus.

[tonyskalski]

It certainly would seem that way, but:

[root@mon lib]# ldd libzbxmodbus.so
	linux-vdso.so.1 =>  (0x00007ffeb2ff2000)
	libmodbus.so.5 => /etc/zabbix/lib/libmodbus.so.5 (0x00007f076a9fe000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f076a631000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f076ae10000)
[root@mon lib]# ls -l /etc/zabbix/lib/libmodbus.so.5
lrwxrwxrwx 1 root root 18 Apr 18 12:06 /etc/zabbix/lib/libmodbus.so.5 -> libmodbus.so.5.1.0

which is what I compiled via git around lunchtime. I am still confused, but glad it is working.