[guardian-dev] Orbot v14.1.4: The Battle for the Background!

[Michael Rogers]

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After a bit of googling it seems to be handled by a kernel module...
or at least it was in Android 3.10, I can't find the kernel sources
after that.

https://android.googlesource.com/kernel/common.git/+/android-3.10/drivers/staging/android/lowmemorykiller.c

Userland declares a policy by setting memory thresholds and attaching
oom_adj scores to processes, and the driver SIGKILLs processes without
(as far as I can see) calling back into userland first.

See also:
http://www.programering.com/a/MjNzADMwATE.html

Cheers,
Michael

On 01/12/14 16:37, Tim Bray wrote:
> When I was in the Android group at Google, the dogma was always
> that you can't count on on destroy() being called. The actual code
> that does this should be part of AOSP...
> 
> On Dec 1, 2014 8:33 AM, "Michael Rogers" <michael at briarproject.org 
> <mailto:michael at briarproject.org>> wrote:
> 
> On 01/12/14 14:16, Nathan of Guardian wrote:
>> In addition, there was an interesting phenomenon we took
>> advantage of, which was that even if the TorService instance was
>> killed, the separate processes launched via Runtime.exec() for
>> the tor and privoxy/polipo binaries were not killed. This often
>> caused problems on upgrades, where the entire app was killed or
>> uninstalled, and yet /tor and /polipo kept running. This was
>> definitely a bug, but it also allowed us to be a bit lazy,
>> because as long as these background processes kept running, the
>> lifecycle of the TorService didn't matter so much. Ultimately,
>> this caused many problems with orphaned processees and multiple
>> instaces of /tor running, that caused all sorts of crazy.
> 
> We used to have a lot of code in Briar's TorPlugin for detecting
> and killing orphaned processes, but Yaron Goland pointed out that
> by using __OwningControllerProcess and TAKEOWNERSHIP, the Tor
> process can clean itself up when the controlling process dies. So
> TorPlugin is a lot simpler now.
> 
> https://code.briarproject.org/akwizgran/briar/blob/master/briar-android/src/org/briarproject/plugins/tor/TorPlugin.java
>
>  This requires a small patch to jtorctl to add support for the 
> TAKEOWNERSHIP command.
> 
>> And another thing (i am running out of ways to say there is
>> more), on early builds of Lollipop, this whole issue was really 
>> exacerbated with foreground apps that use a lot of memory, like 
>> Chrome browser. Using Chrome with Orbot (via Wifi or APN proxy 
>> settings on), caused almost an instant low memory warning in 
>> TorService, and then a kill -9. Often, Chrome browser itself
>> would be killed, too. Fortunately, an upgrade to the final
>> release ROM (on my Nexus 7 2013), stopped this behavior, proving
>> that this extreme memory management was actually a bug and not a
>> feature.
> 
> This is good to know, thanks!
> 
>> Not to be forgotten, there was also a bug in TorService where 
>> setForeground() was not always being called, but since we call 
>> setOngoing(true) in our NotificationBuilder, it made it seem
>> like it was from the UI perspective (you couldn't swipe away the 
>> service). I noticed this by running a task manager app, which 
>> indicated which services were foreground, and I noticed
>> sometimes TorService was not being called. There is also a known
>> issue with ICS, where receiving/registering for certain Broadcast
>> types, can cause you Foreground=true Service to be reset to
>> Foreground=false. I can't find the exact ticket, but is out
>> there, and Google knows about it.
> 
> Argh, it never ends, does it? I'll have a look for the ticket and
> see whether BriarService is affected.
> 
> Cheers, Michael _______________________________________________ 
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