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

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 12, 2024 at 09:52:51PM -0400, Jason Andryuk wrote:
> From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> 
> commit 0774d19038c496f0c3602fb505c43e1b2d8eed85 upstream.
> 
> 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@gmail.com>
> Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
> Tested-by: Jason Andryuk <jandryuk@gmail.com>
> Link: https://lore.kernel.org/r/ZjAWMQCJdrxZkvkB@google.com
> Cc: stable@vger.kernel.org
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> [ Apply to linux-5.15.y ]
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> ---
> Built with 5.15 and 4.19.  Tested on 5.15.
> 
> Drop const from struct input_dev *id
> Declare i outside loop:
> 
> drivers/input/input.c: In function ‘input_print_modalias_parts’:
> drivers/input/input.c:1393:25: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
>  1393 |                         for (int i = size - 1 - remainder - 3; i >= 0; i--) {
>       |                         ^~~
> ---
>  drivers/input/input.c | 105 ++++++++++++++++++++++++++++++++++++------
>  1 file changed, 90 insertions(+), 15 deletions(-)

Both now queued up, thanks.

greg k-h