Date: Sun, 28 Apr 2024 19:43:52 -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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: try trimming too long modalias strings

Hi Dmitry,

On Sat, Apr 27, 2024 at 10:06 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).

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@vps.thesusis.net/T/

> Reported-by: Jason Andryuk <jandryuk@...il.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>

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
[  601.440379] Modules linked in: xen_kbdfront xen_blkfront xen_netfront
[  601.462078] CPU: 0 PID: 630 Comm: udevadm Tainted: G        W
   6.8.7+ #2
[  601.486003] Hardware name: Xen HVM domU, BIOS 4.19-unstable 03/09/2024
[  601.504867] RIP: 0010:add_uevent_var+0x11c/0x130
[  601.527988] Code: 5b 41 5c 5d c3 cc cc cc cc 48 c7 c7 c0 3c 4d 9e
e8 49 4c 1c ff 0f 0b b8 f4 ff ff ff eb ce 48 c7 c7 e8 3c 4d 9e e8 34
4c 1c ff <0f> 0b eb e9 e8 eb e0 02 00 66 66 2e 0f 1f 84 00 00 00 00 00
90 90
[  601.590038] RSP: 0018:ffffadc60053bcf0 EFLAGS: 00010282
[  601.612133] RAX: 0000000000000000 RBX: ffff96f943c0a000 RCX: 0000000000000000
[  601.634794] RDX: ffff96f9bd428cd0 RSI: ffff96f9bd41d740 RDI: ffff96f9bd41d740
[  601.651867] RBP: ffffadc60053bd50 R08: 00000000ffffdfff R09: 0000000000000001
[  601.677718] R10: 00000000ffffdfff R11: ffffffff9e65cc00 R12: 0000000000000003
[  601.699194] R13: ffff96f943c0a000 R14: ffffffff9e0db1d0 R15: 0000000000000000
[  601.718038] FS:  00007fa9a1084d40(0000) GS:ffff96f9bd400000(0000)
knlGS:0000000000000000
[  601.741494] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  601.761050] CR2: 00007fff40d8bd58 CR3: 0000000002712001 CR4: 00000000000606f0
[  601.783095] Call Trace:
[  601.791569]  <TASK>
[  601.798207]  ? add_uevent_var+0x11c/0x130
[  601.810481]  ? __warn+0x7c/0x130
[  601.822437]  ? add_uevent_var+0x11c/0x130
[  601.833016]  ? report_bug+0x171/0x1a0
[  601.848035]  ? handle_bug+0x3c/0x80
[  601.858013]  ? exc_invalid_op+0x17/0x70
[  601.873026]  ? asm_exc_invalid_op+0x1a/0x20
[  601.888058]  ? add_uevent_var+0x11c/0x130
[  601.899042]  kobject_uevent_env+0x28e/0x510
[  601.916043]  kobject_synth_uevent+0x326/0x330
[  601.927373]  uevent_store+0x19/0x50
[  601.940381]  kernfs_fop_write_iter+0x122/0x1b0
[  601.952343]  vfs_write+0x299/0x470
[  601.961853]  ksys_write+0x6a/0xf0
[  601.975611]  do_syscall_64+0x52/0x120
[  601.985363]  entry_SYSCALL_64_after_hwframe+0x78/0x80
[  602.001942] RIP: 0033:0x7fa9a18693b4
[  602.058258] Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b3 0f
1f 80 00 00 00 00 48 8d 05 49 53 0d 00 8b 00 85 c0 75 13 b8 01 00 00
00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 41 54 49 89 d4 55 48 89
f5 53
[  602.107044] RSP: 002b:00007ffc204b01e8 EFLAGS: 00000246 ORIG_RAX:
0000000000000001
[  602.125072] RAX: ffffffffffffffda RBX: 0000000000000007 RCX: 00007fa9a18693b4
[  602.144401] RDX: 0000000000000007 RSI: 00007ffc204b02d0 RDI: 0000000000000003
[  602.161037] RBP: 00007ffc204b02d0 R08: 00005641874fbcd0 R09: 0000564187578700
[  602.183535] R10: 0000000000000000 R11: 0000000000000246 R12: 00005641874fbbf0
[  602.201557] R13: 0000000000000007 R14: 00007fa9a1935760 R15: 0000000000000007
[  602.220506]  </TASK>
[  602.227408] ---[ end trace 0000000000000000 ]---
[  602.239544] synth uevent: /devices/virtual/input/input5: failed to
send uevent
[  602.260848] input input5: uevent: failed to send synthetic uevent: -12

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?

My other thought is wondering whether the presence of '+' will cause
parsing errors?  Has '+' been used before - or will it be an
unexpected character?

Thanks,
Jason

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