Bug#960702: ethtool -m values change when output is redirected

Package: ethtool
Version: 1:4.19-1
Severity: important

I'm facing a very strange behavior. The command ethtool -m <interface> report the transceiver DOM values correctly, but when the command output is redirected to an other program, values change to somthing else.

Here is a transcript:
root@localhost:~# ethtool -m enp10s0; echo -e '\n\n\n'; ethtool -m enp10s0 | cat
Identifier : 0x03 (SFP)
Extended identifier : 0x04 (GBIC/SFP defined by 2-wire interface ID)
Connector : 0x07 (LC)
Transceiver codes : 0x10 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
Transceiver type : 10G Ethernet: 10G Base-SR
Encoding : 0x06 (64B/66B)
BR, Nominal : 10300MBd
Rate identifier : 0x00 (unspecified)
Length (SMF,km) : 0km
Length (SMF) : 0m
Length (50um) : 80m
Length (62.5um) : 20m
Length (Copper) : 0m
Length (OM3) : 300m
Laser wavelength : 850nm
Vendor name : Pureoptics
Vendor OUI : 00:00:00
Vendor PN : EX-SFP-10GE-SR
Vendor rev : B4
Option values : 0x00 0x1a
Option : RX_LOS implemented
Option : TX_FAULT implemented
Option : TX_DISABLE implemented
BR margin, max : 0%
BR margin, min : 0%
Vendor SN : M4351243
Date code : 190515
Optical diagnostics support : Yes
Laser bias current : 16.480 mA
Laser output power : 3.0768 mW / 4.88 dBm
Receiver signal average optical power : 1.2298 mW / 0.90 dBm
Module temperature : 48.47 degrees C / 119.24 degrees F
Module voltage : 1.2336 V
Alarm/warning flags implemented : Yes
Laser bias current high alarm : Off
Laser bias current low alarm : Off
Laser bias current high warning : Off
Laser bias current low warning : Off
Laser output power high alarm : Off
Laser output power low alarm : Off
Laser output power high warning : Off
Laser output power low warning : Off
Module temperature high alarm : Off
Module temperature low alarm : Off
Module temperature high warning : Off
Module temperature low warning : Off
Module voltage high alarm : Off
Module voltage low alarm : Off
Module voltage high warning : Off
Module voltage low warning : Off
Laser rx power high alarm : Off
Laser rx power low alarm : Off
Laser rx power high warning : Off
Laser rx power low warning : Off
Laser bias current high alarm threshold : 4.744 mA
Laser bias current low alarm threshold : 49.896 mA
Laser bias current high warning threshold : 51.776 mA
Laser bias current low warning threshold : 50.910 mA
Laser output power high alarm threshold : 2.5701 mW / 4.10 dBm
Laser output power low alarm threshold : 0.8224 mW / -0.85 dBm
Laser output power high warning threshold : 0.8224 mW / -0.85 dBm
Laser output power low warning threshold : 0.8224 mW / -0.85 dBm
Module temperature high alarm threshold : 0.00 degrees C / 32.00 degrees F
Module temperature low alarm threshold : 0.00 degrees C / 32.00 degrees F
Module temperature high warning threshold : 0.00 degrees C / 32.00 degrees F
Module temperature low warning threshold : 0.00 degrees C / 32.00 degrees F
Module voltage high alarm threshold : 0.4356 V
Module voltage low alarm threshold : 0.0000 V
Module voltage high warning threshold : 0.0000 V
Module voltage low warning threshold : 0.0000 V
Laser rx power high alarm threshold : 0.8224 mW / -0.85 dBm
Laser rx power low alarm threshold : 0.8224 mW / -0.85 dBm
Laser rx power high warning threshold : 0.8224 mW / -0.85 dBm
Laser rx power low warning threshold : 0.8224 mW / -0.85 dBm

Identifier : 0x03 (SFP)
Extended identifier : 0x04 (GBIC/SFP defined by 2-wire interface ID)
Connector : 0x07 (LC)
Transceiver codes : 0x10 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
Transceiver type : 10G Ethernet: 10G Base-SR
Encoding : 0x06 (64B/66B)
BR, Nominal : 10300MBd
Rate identifier : 0x00 (unspecified)
Length (SMF,km) : 0km
Length (SMF) : 0m
Length (50um) : 80m
Length (62.5um) : 20m
Length (Copper) : 0m
Length (OM3) : 300m
Laser wavelength : 850nm
Vendor name : Pureoptics
Vendor OUI : 00:00:00
Vendor PN : EX-SFP-10GE-SR
Vendor rev : B4
Option values : 0x00 0x1a
Option : RX_LOS implemented
Option : TX_FAULT implemented
Option : TX_DISABLE implemented
BR margin, max : 0%
BR margin, min : 0%
Vendor SN : M4351243
Date code : 190515
Optical diagnostics support : Yes
Laser bias current : 16.448 mA
Laser output power : 0.8224 mW / -0.85 dBm
Receiver signal average optical power : 0.8224 mW / -0.85 dBm
Module temperature : 32.12 degrees C / 89.83 degrees F
Module voltage : 0.8224 V
Alarm/warning flags implemented : Yes
Laser bias current high alarm : Off
Laser bias current low alarm : Off
Laser bias current high warning : Off
Laser bias current low warning : Off
Laser output power high alarm : Off
Laser output power low alarm : Off
Laser output power high warning : Off
Laser output power low warning : Off
Module temperature high alarm : Off
Module temperature low alarm : Off
Module temperature high warning : Off
Module temperature low warning : Off
Module voltage high alarm : On
Module voltage low alarm : Off
Module voltage high warning : On
Module voltage low warning : Off
Laser rx power high alarm : Off
Laser rx power low alarm : Off
Laser rx power high warning : Off
Laser rx power low warning : Off
Laser bias current high alarm threshold : 4.754 mA
Laser bias current low alarm threshold : 51.402 mA
Laser bias current high warning threshold : 56.552 mA
Laser bias current low warning threshold : 53.964 mA
Laser output power high alarm threshold : 2.6981 mW / 4.31 dBm
Laser output power low alarm threshold : 2.9216 mW / 4.66 dBm
Laser output power high warning threshold : 0.8224 mW / -0.85 dBm
Laser output power low warning threshold : 0.8224 mW / -0.85 dBm
Module temperature high alarm threshold : 0.00 degrees C / 32.00 degrees F
Module temperature low alarm threshold : 0.00 degrees C / 32.00 degrees F
Module temperature high warning threshold : 0.00 degrees C / 32.00 degrees F
Module temperature low warning threshold : 0.00 degrees C / 32.00 degrees F
Module voltage high alarm threshold : 0.4368 V
Module voltage low alarm threshold : 0.0000 V
Module voltage high warning threshold : 0.0000 V
Module voltage low warning threshold : 0.0000 V
Laser rx power high alarm threshold : 0.8224 mW / -0.85 dBm
Laser rx power low alarm threshold : 0.8224 mW / -0.85 dBm
Laser rx power high warning threshold : 0.8224 mW / -0.85 dBm
Laser rx power low warning threshold : 0.8224 mW / -0.85 dBm

As you can see alsmost all mesuring values (C, V, mA and dBm) change. The values when output is redirected are unreliable which makes it impossible to use in a script.
I am using Debian GNU/Linux 10 (buster), kernel 4.19.0-9-amd64 #1 SMP Debian 4.19.118-2 (2020-04-29) x86_64 GNU/Linux and libc6 2.28-10

FYI, I could not reproduce the bug on Debian Stretch (ethtool 1:4.7-1+b1, kernel 4.9.0-11-amd64 and libc6 2.24-11+deb9u4).

--
Yannis Aribaud