The write is here: https://android.googlesource.com/platform/system/core/+/master/rootdir/init.rc#257.

Denials (on a device with the sysfs_vibrator label properly applied):
denied { write } for name="vibrator" dev="sysfs" ino=49613 scontext=u:r:init:s0 tcontext=u:object_r:sysfs_vibrator:s0 tclass=dir
denied { write } for name="trigger" dev="sysfs" ino=49620 scontext=u:r:init:s0 tcontext=u:object_r:sysfs_vibrator:s0 tclass=file
denied { open } for path="/sys/devices/<redacted>/vibrator/trigger" dev="sysfs" ino=49620 scontext=u:r:init:s0 tcontext=u:object_r:sysfs_vibrator:s0 tclass=file

Bug: b/72643420
Test: Device boots, denials gone

Change-Id: Ib50d9a8533303daccb1330685e3204bea3fbd8a8