linux-kernel - Re: [PATCH v2] Input: try trimming too long modalias strings

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Date: Mon, 29 Apr 2024 21:04:23 -0400
From: Jason Andryuk <jandryuk@...il.com>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc: linux-input@...r.kernel.org, Hans de Goede <hdegoede@...hat.com>, 
	Benjamin Tissoires <benjamin.tissoires@...hat.com>, Peter Hutterer <peter.hutterer@...-t.net>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Input: try trimming too long modalias strings

On Mon, Apr 29, 2024 at 5:50 PM Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
>
> If an input device declares too many capability bits then modalias
> string for such device may become too long and not fit into uevent
> buffer, resulting in failure of sending said uevent. This, in turn,
> may prevent userspace from recognizing existence of such devices.
>
> This is typically not a concern for real hardware devices as they have
> limited number of keys, but happen with synthetic devices such as
> ones created by xen-kbdfront driver, which creates devices as being
> capable of delivering all possible keys, since it doesn't know what
> keys the backend may produce.
>
> To deal with such devices input core will attempt to trim key data,
> in the hope that the rest of modalias string will fit in the given
> buffer. When trimming key data it will indicate that it is not
> complete by placing "+," sign, resulting in conversions like this:
>
> old: k71,72,73,74,78,7A,7B,7C,7D,8E,9E,A4,AD,E0,E1,E4,F8,174,
> new: k71,72,73,74,78,7A,7B,7C,+,
>
> This should allow existing udev rules continue to work with existing
> devices, and will also allow writing more complex rules that would
> recognize trimmed modalias and check input device characteristics by
> other means (for example by parsing KEY= data in uevent or parsing
> input device sysfs attributes).
>
> Note that the driver core may try adding more uevent environment
> variables once input core is done adding its own, so when forming
> modalias we can not use the entire available buffer, so we reduce
> it by somewhat an arbitrary amount (96 bytes).
>
> Reported-by: Jason Andryuk <jandryuk@...il.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>

Tested-by: Jason Andryuk <jandryuk@...il.com>

I don't have the gdm setup available to test, but loginctl looks good
showing the Xen Virtual Keyboard assigned to a seat:
# loginctl seat-status seat0
seat0
         Devices:
                  ├─/sys/devices/LNXSYSTM:00/LNXPWRBN:00/input/input0
                  │ input:input0 "Power Button"
                  ├─/sys/devices/LNXSYSTM:00/LNXSLPBN:00/input/input1
                  │ input:input1 "Sleep Button"
                  ├─/sys/devices/platform/i8042/serio0/input/input2
                  │ input:input2 "AT Translated Set 2 keyboard"
                  ├─/sys/devices/platform/i8042/serio1/input/input4
                  │ input:input4 "ImExPS/2 Generic Explorer Mouse"
                  ├─/sys/devices/virtual/input/input5
                  │ input:input5 "Xen Virtual Keyboard"
                  │ └─/sys/devices/virtual/input/input5/event4
                  │   input:event4
                  └─/sys/devices/virtual/input/input6
                    input:input6 "Xen Virtual Pointer"

Thanks,
Jason