runtime: android logging corruption on Android 5.1

[crawshaw]

I was debugging over someone's shoulder today and their panic as written to logcat was corrupt:

W/logd    (26732): Dropped 6
?/o       (26732): unexpected fault address 0x0
W/logd    (26732): Dropped 6
?/o       (26732): fatal error: fault
W/logd    (26732): Dropped 6
?/o       (26732): [signal 0x7 code=0x1 addr=0x0 pc=0xa5b5cd30]
W/logd    (26732): Dropped 6
W/logd    (26732): Dropped 6
?/o       (26732): goroutine 32 [running]:

There may be a change the logdw format, or we may have always had it slightly wrong but it worked with older Android versions.

I see some suggestions on the internet that the wire format is the layout of struct logger_entry_v3, which looks similar to but not the same as what we are writing:

https://android.googlesource.com/platform/system/core/+/master/include/log/logger.h

/cc @hyangah

[hyangah]

I believe log_entry and (logger_entry_*) are for reading log data.
For writing this is the relevant part:
https://github.com/android/platform_system_core/blob/master/liblog/logd_write.c

Looking at the history, indeed there are some changes we have to pick up (those checked in after March).

[hyangah]

BTW, for long term, we need to find a better solution. Currently we are relying on Android's private api (protocol between liblog and logger) which means it can break any time.

[crawshaw]

Further investigation suggests this is a very isolated behavior. The exact device I saw it on was a T-Mobile Moto X (2nd gen) running Android 5.1.

I could not replicate the problem on a Nexus 7 (2nd gen) running Android 5.1.1.

[rsc]

Seems like we can't do anything unless this can be reproduced.

[xiam]

Just in case you came here looking for the reason why Android Studio is full of "Dropped: 6" lines, try using "adb logcat" instead.