How to remap mouse button not identified by xev

Question

The Logitech M720 has a thumb button which does not show up with a button identifier in xev:

This thumb button results in Ctrl+Alt+Tab.

I am using Fedora 28, though the same issue is reported here for Ubuntu: https://askubuntu.com/questions/956484/logitech-mouse-thumb-button-not-showing-in-xev

There are also some related questions for difference Logitech mice: MX Master question and MX Anywhere 2S, which seems to have gotten some button names with evtest, but my evtest output for the thumb button doesn't look like it has any button names:

Event: time 1538277331.280194, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e0
Event: time 1538277331.280194, type 1 (EV_KEY), code 29 (KEY_LEFTCTRL), value 1
Event: time 1538277331.280194, -------------- SYN_REPORT ------------
Event: time 1538277331.288214, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2
Event: time 1538277331.288214, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 1
Event: time 1538277331.288214, -------------- SYN_REPORT ------------
Event: time 1538277331.296210, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002b
Event: time 1538277331.296210, type 1 (EV_KEY), code 15 (KEY_TAB), value 1
Event: time 1538277331.296210, -------------- SYN_REPORT ------------
Event: time 1538277331.304218, type 4 (EV_MSC), code 4 (MSC_SCAN), value 7002b
Event: time 1538277331.304218, type 1 (EV_KEY), code 15 (KEY_TAB), value 0
Event: time 1538277331.304218, -------------- SYN_REPORT ------------
Event: time 1538277331.312212, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2
Event: time 1538277331.312212, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 0
Event: time 1538277331.312212, -------------- SYN_REPORT ------------
Event: time 1538277331.320218, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e0
Event: time 1538277331.320218, type 1 (EV_KEY), code 29 (KEY_LEFTCTRL), value 0
Event: time 1538277331.320218, -------------- SYN_REPORT ------------

Is that button really hardwired to emit CTRL ALT TAB ?

Is it possible to remap the thumb button?

Answer 1

The button produces the sequence Left Ctrl Left Alt Tab on the kernel input layer level, which means it's the device itself that produces that sequence. That this is different for other Logitech devices means nothing.

I don't know how "hardwired" that sequence is, it may be possible to change it Logitech tools specific for that device. That won't be easy, you'll have to find such tools (likely for Windows), then you need to log the USB commands that Windows sends and receives with a USB sniffer, then you have to write a small program to imitate them on Linux. The good thing about that is if you make it open source, other people with this device are also able to use it, so maybe someone has already done it (spend a bit of time googling to find out).

The other option is that you catch this particular sequence on the X level (with your WM, or xbindkeys etc.), and attach your own event etc. to it. Though of course this will also detect the same sequence on your keyboard.

A third option is to detect the sequence on the input layer level. For this, you'll need to be able to write a C program that "grabs" the corresponding /dev/input/eventX, translates that particular key sequence and passes through everything else, using an uinput device as a new input layer source.

A fourth option could be to use hwdb to do some kind of translation, but AFAIK it doesn't translate sequences (but I haven't looked at that in detail).