Userfaultfd does not seem to catch faults on Android

[Panagiotis Apostolou]

I've been trying to incorporate userfaultfd into a library in order to track accesses to specific memory region that I register with the userfaultfd mechanism but I have some problems getting it work on Android.

I found about userfaultfd recently so in order to get to know how it works I started by writing a standalone test app for desktop. When I got it to work as expected i ndk-build the c source for Android (12) and tried it out on an adb shell. When I saw that it was working there too I started applying the mechanism in the library.

The library is compiled in a .so and is used by other applications (executables) on desktop and (android apps) on Android. After I finished I tried it on desktop and it works. Unfortunately this is not the case for the Android apps. It seems that the thread that is causing the fault never get halted and the thread that is listening (polling/ALooping) on the userfaultfd file descriptor never gets any notification (only time outs)

Any ideas? What am I missing? 

[enh]

i've never used userfautfd personally so i don't know how it deals with multiple callers, but i think ART uses it for GC on devices where it's available, so if the kernel only reports events to _one_ fd rather than all fds, it might be that in an app, all the events are going to ART instead of you?

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[Panagiotis Apostolou]

Right. This didn't occur to me and I guess it kinda makes sense. I wonder how I can verify it.

[enh]

write a test program that opens two userfaultfds and checks whether they both see everything, or it just goes to the first?

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