Add /dev/kmsg_debug.

Add /dev/kmsg_debug on userdebug devices, to allow crash_dump to log
crashes to dmesg when logd isn't up yet (or is the one crashing).

Bug: http://b/36574794
Test: stop tombstoned; crasher; dmesg
Change-Id: I249e11291c58fee77098dec3fd3271ea23363ac9

private/file_contexts

@@ -104,6 +104,7 @@
 /dev/ptmx		u:object_r:ptmx_device:s0
 /dev/pvrsrvkm		u:object_r:gpu_device:s0
 /dev/kmsg		u:object_r:kmsg_device:s0
+/dev/kmsg_debug	u:object_r:kmsg_debug_device:s0
 /dev/null		u:object_r:null_device:s0
 /dev/nvhdcp1		u:object_r:video_device:s0
 /dev/random		u:object_r:random_device:s0

public/crash_dump.te

@@ -15,6 +15,9 @@ dontaudit crash_dump self:capability { sys_ptrace };
 
 userdebug_or_eng(`
   allow crash_dump logd:process { ptrace signal sigchld sigstop sigkill };
+
+  # Let crash_dump write to /dev/kmsg_debug crashes that happen before logd comes up.
+  allow crash_dump kmsg_debug_device:chr_file { open append };
 ')
 
 # Use inherited file descriptors

public/device.te

@@ -36,6 +36,7 @@ type mtp_device, dev_type, mlstrustedobject;
 type nfc_device, dev_type;
 type ptmx_device, dev_type, mlstrustedobject;
 type kmsg_device, dev_type;
+type kmsg_debug_device, dev_type;
 type null_device, dev_type, mlstrustedobject;
 type random_device, dev_type, mlstrustedobject;
 type sensors_device, dev_type;

public/init.te

@@ -13,6 +13,10 @@ allow init tmpfs:chr_file { create setattr unlink rw_file_perms };
 # /dev/kmsg
 allow init tmpfs:chr_file relabelfrom;
 allow init kmsg_device:chr_file { write relabelto };
+# /dev/kmsg_debug
+userdebug_or_eng(`
+  allow init kmsg_debug_device:chr_file { write relabelto };
+')
 # /dev/__properties__
 allow init properties_device:dir relabelto;
 allow init properties_serial:file { write relabelto };