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

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Date: Tue, 30 Apr 2024 18:25:13 -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 9:04 PM Jason Andryuk <jandryuk@...il.com> wrote:
>
> 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"

What do you think about Cc: stable@...r.kernel.org?  I'd like to get
the Xen Keyboard working as widely as possible, so I'd like it
backported if possible.

Thanks,
Jason