On Sun, Apr 28, 2024 at 07:43:52PM -0400, Jason Andryuk wrote: > Hi Dmitry, > > On Sat, Apr 27, 2024 at 10:06 PM Dmitry Torokhov > <dmitry.torokhov@xxxxxxxxx> 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). > > I think adding these links may be useful for cross referencing: > [1] https://github.com/systemd/systemd/issues/22944 > [2] https://lore.kernel.org/xen-devel/87o8dw52jc.fsf@xxxxxxxxxxxxxxxx/T/ > > > Reported-by: Jason Andryuk <jandryuk@xxxxxxxxx> > > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> > > Thank you for looking at this. I think this idea of truncating the > modalias is better than just dropping keys. > > cat-ing the individual sysfs files works, but there is still an issue: > > # sudo udevadm trigger --action=change > [ 601.379977] ------------[ cut here ]------------ > [ 601.395959] add_uevent_var: buffer size too small > [ 601.412009] WARNING: CPU: 0 PID: 630 at lib/kobject_uevent.c:671 > add_uevent_var+0x11c/0x130 ... > Another path needs to truncate the buffer? Or the problem is that the > total uevent buffer size is what matters - not just the keys modalias? Yes, exactly right - the driver core may add more environment variables, such as SEQNUM=, HOME=, PATH=. I created v2 of the patch that tries to leave some space at the end for these additional variables. > > My other thought is wondering whether the presence of '+' will cause > parsing errors? Has '+' been used before - or will it be an > unexpected character? I hope not. It is after "," and not a hex digit, so old code parsing keys hopefully considers it as a separate part of modalias. I.e. it was coping with: ...-e0,1,2,4,k110,111,112,113,114,115,r0,1,8,... so I believe it should cope with ...-e0,1,2,4,k110,111,112,113,114,115,+,r0,1,8,... But let's see Peter and Benjamin will say... Thanks. -- Dmitry
