AppArmor should support changing the security context of a threaded process

[Tyler Hicks]

The aa_change_profile() and aa_change_onexec() functions are not safe to use with multi-threaded processes because only the main thread will be transitioned. We should support a way to transition all threads of a multi-threaded process.

One immediate user of the feature would be mapplauncherd in Ubuntu Touch. The QML booster starts the QML engine as part of its pre-caching work. Since the QML engine is multi-threaded, the QML booster cannot be used for confined apps on Ubuntu Touch because there's no way to transition all the threads to the necessary profile for app confinement.

[Andrea Bernabei (faenil)]

This would be immensely beneficial to our platform, it would be great to get some resources assigned to it :)

[Pat McGowan (pat-mcgowan)]

Note that testing results concluded the applauncherd approach did not yield any notable improvement in startup time.

[Tyler Hicks (tyhicks)]

Thanks for the update, Pat. The AppArmor project, in general, wants to support multi threaded security context changes but we can lower the priority of this feature request now that we know it won't benefit app startup times.

[Pat McGowan (pat-mcgowan)]

To clarify the actual benefit, on a first run before the cache is created it was seen to save as much as 1 to 1.5 secs on some apps, which is equivalent to what the cache provides. So providing this would improve the overall experience, and I believe it would allow us to stop using the cache which occasionally causes app starting issues, for example when it somehow becomes corrupted.(see bug #1560020)