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fdtrack

[TOC]

fdtrack is a file descriptor leak checker added to Android in API level 30.

fdtrack consists of two parts: a set of hooks in bionic to register a callback that's invoked on file descriptor operations, and a library that implements a hook to perform and store backtraces for file descriptor creation.

bionic hooks

bionic provides a header in the bionic_libc_platform_headers header_lib at <bionic/fdtrack.h>. Register a callback with android_fdtrack_compare_exchange_hook to receive callbacks upon file descriptor creation and destruction. This function can be called at any point in order to start capturing events, but be sure to properly handle unbalanced closes. This callback may be called from an async signal safe context, but not vfork (bionic tracks whether a thread is vforked, and chooses not to call callbacks when this is the case).

libfdtrack

libfdtrack implements a library that uses libunwindstack to unwind and store fd creation backtraces.

Using libfdtrack

libfdtrack registers its hook upon being loaded, so to start capturing backtraces, dlopen("libfdtrack.so", RTLD_GLOBAL) is all that's needed. To dump its output to logcat, either use fdtrack_dump, or send the signal BIONIC_SIGNAL_FDTRACK (available from <bionic/reserved_signals.h>) to the process. If you wish to iterate through the results programmatically, fdtrack_iterate can be used (warning: this interface is currently unstable, don't use it in code that can be used on multiple platform versions.)

libfdtrack adds a significant amount of overhead, so for processes that are latency-critical like system_server, it's not feasible to always capture backtraces. Instead, if you can detect that an fd leak is ongoing, turning on backtraces for a while and then triggering a dump can be sufficient. system_server implements this approach, spawning a thread that regularly checks the count of fds in the process, turns on fdtrack when it hits a threshold, and then aborts after another threshold. This dumps the output to logcat, which will be available in both the tombstone and logcat from bugreports.

Implementation details

There are multiple methods to unwind in Android:

  • libunwindstack
    • Pros
      • Primary method on the platform
      • Able to unwind through ART
    • Cons
      • Uses malloc internally: unsafe unless a separate allocator is statically-linked and steps are taken to prevent the unwind from being interrupted by a signal
      • Slow - infeasible to be used always in latency-sensitive processes
  • android_unsafe_frame_pointer_chase
    • Pros
      • Definitely async signal safe
      • Fast
    • Cons
      • Unable to unwind through ART because it doesn't maintain the frame pointer
      • Requires -fno-omit-frame-pointer to be used on all code being unwound through, which currently isn't the case on Android
      • Frame layout is a mess on 32-bit ARM: the ARM standard, clang, and GCC all disagree
      • Chasing the frame pointer will often result in multiple frames inside the same function

libfdtrack chooses to use libunwindstack for now, since unwinding through ART is critical to being useful for the initial user, system_server.