# 9 Debugging NIFs and Port Drivers

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9.1 With great power comes great responsibilty

NIFs and port driver code run inside the Erlang VM OS process (the "Beam"). To maximize performance the code is called directly by the same threads executing Erlang beam code and has full access to all the memory of the OS process. A buggy NIF/driver can thus make severe damage by corrupting memory.

In a best case scenario such memory corruption is detected immediately causing the Beam to crash generating a core dump file which can be analyzed to find the bug. However, it is very common for memory corruption bugs to not be immediately detected when the faulty write happens, but instead much later, for example when the calling Erlang process is garbage collected. When that happens it can be very hard to find the root cause of the memory corruption by analysing the core dump. All traces that could have indicated which specific buggy NIF/driver that caused the corruption may be long gone.

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9.2 The debug emulator

One way to make debugging easier is to run an emulator built with target debug. It will

- Increase probability of detecting bugs earlier. It contains a lot more runtime checks to ensure correct use of internal interfaces and data structures.

- Generate a core dump that is easier to analyze. Compiler optimizations are turned off, which stops the compiler from "optimizing away" variables, thus making it easier/possible to inspect their state.

- Detect lock order violations. A runtime lock checker will verify that the locks in the erl_nif and erl_driver APIs are seized in a consistent order that cannot result in deadlock bugs.

In fact, we recommend to use the debug emulator as default during development of NIFs and drivers, regardless if you are troubleshooting bugs or not. Some subtle bugs may not be detected by the normal emulator and just happen to work anyway by chance. However, another version of the emulator, or even different circumstances within the same emulator, may cause the bug to later provoke all kinds of problems.

The main disadvantage of the debug emulator is its reduced performance. The extra runtime checks and lack of compiler optimizations may result in a slowdown with a factor of two or more depending on load. The memory footprint should be about the same.

If the debug emulator is part of the Erlang/OTP installation, it can be started with the -emu_type option.

    > erl -emu_type debug
    Erlang/OTP 25 [erts-13.0.2] ... [type-assertions] [debug-compiled] [lock-checking]

    Eshell V13.0.2  (abort with ^G)
    1>

If the debug emulator is not part of the installation, you need to build it from the Erlang/OTP source code. After building from source either make an Erlang/OTP installation or you can run the debug emulator directly in the source tree with the cerl script:

    > $ERL_TOP/bin/cerl -debug
    Erlang/OTP 25 [erts-13.0.2] ... [type-assertions] [debug-compiled] [lock-checking]

    Eshell V13.0.2  (abort with ^G)
    1>

The cerl script can also be used as a convenient way to start the debugger gdb for core dump analysis:

    > $ERL_TOP/bin/cerl -debug -core core.12345
    or
    > $ERL_TOP/bin/cerl -debug -rcore core.12345

The first variant starts Emacs and runs gdb within, while the other -rcore runs gdb directly in the terminal. Apart from starting gdb with the correct beam.debug.smp executable file it will also read the file $ERL_TOP/erts/etc/unix/etp-commands which contains a lot of gdb command for inspecting a beam core dump. For example, the command etp that will print the content of an Erlang term (Eterm) in plain Erlang syntax.

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9.3 Address Sanitizer

AddressSanitizer (asan) is an open source programming tool that detects memory corruption bugs such as buffer overflows, use-after-free and memory leaks. AddressSanitizer is based on compiler instrumentation and is supported by both gcc and clang.

Similar to the debug emulator, the asan emulator runs slower than normal, about 2-3 times slower. However, it also has a larger memory footprint, about 3 times more memory than normal.

To get full effect you should compile both your own NIF/driver code as well as the Erlang emulator with AddressSanitizer instrumentation. Compile your own code by passing option -fsanitize=address to gcc or clang. Other recommended options that will improve the fault identification are -fno-common and -fno-omit-frame-pointer.

Build and run the emulator with AddressSanitizer support by using the same procedure as for the debug emulator, except use the asan build target instead of debug.

Run in source tree

  If you run the asan emulator directly in the source tree with the cerl script you only need to set environment variable ASAN_LOG_DIR to the directory where the error log files will be generated.

      > export ASAN_LOG_DIR=/my/asan/log/dir
      > $ERL_TOP/bin/cerl -asan
      Erlang/OTP 25 [erts-13.0.2] ... [address-sanitizer]

      Eshell V13.0.2  (abort with ^G)
      1>

  You may however also want to set ASAN_OPTIONS="halt_on_error=true" if you want the emulator to crash when an error is detected.

Run installed Erlang/OTP

  If you run the asan emulator in an installed Erlang/OTP with erl -emu_type asan you need to set the path to the error log file with

      > export ASAN_OPTIONS="log_path=/my/asan/log/file"

  To avoid false positive memory leak reports from the emulator itself set LSAN_OPTIONS (LSAN=LeakSanitizer):

      > export LSAN_OPTIONS="suppressions=$ERL_TOP/erts/emulator/asan/suppress"

  The suppress file is currently not installed but can be copied manually from the source tree to wherever you want it.

Memory corruption errors are reported by AddressSanitizer when they happen, but memory leaks are only checked and reported by default then the emulator terminates.

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9.4 Valgrind

An even more heavy weight debugging tool is Valgrind. It can also find memory corruption bugs and memory leaks similar to asan. Valgrind is not as good at buffer overflow bugs, but it will find use of undefined data, which is a type of error that asan cannot detect.

Valgrind is much slower than asan and it is incapable at exploiting CPU multicore processing. We therefore recommend asan as the first choice before trying valgrind.

Valgrind runs as a virtual machine itself, emulating execution of hardware machine instructions. This means you can run almost any program unchanged on valgrind. However, we have found that the beam executable benefits from being compiled with special adaptions for running on valgrind.

Build the emulator with valgrind target the same as is done for debug and asan. Note that valgrind needs to be installed on the machine before the build starts.

Run the valgrind emulator directly in the source tree with the cerl script. Set environment variable VALGRIND_LOG_DIR to the directory where the error log files will be generated.

    > export VALGRIND_LOG_DIR=/my/valgrind/log/dir
    > $ERL_TOP/bin/cerl -valgrind
    Erlang/OTP 25 [erts-13.0.2] ... [valgrind-compiled]

    Eshell V13.0.2  (abort with ^G)
    1>

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9.5 rr - Record and Replay

Last but not least, the fantastic interactive debugging tool rr, developed by Mozilla as open source. rr stands for Record and Replay. While a core dump represents only a static snapshot of the OS process when it crashed, with rr you instead record the entire session, from start of the OS process to the end (the crash). You can then replay that session from within gdb. Single step, set breakpoints and watchpoints, and even execute backwards.

Considering its powerful utility, rr is remarkably light weight. It runs on Linux with any reasonably modern x86 CPU. You may get a two times slowdown when executing in recording mode. The big weakness is its inability to exploite CPU multicore processing. If the bug is a race condition between concurrently running threads, it may be hard to reproduce with rr.

rr does not require any special instrumented compilation. However, if possible, run it together with the debug emulator, as that will result in a much nicer debugging experience. You run rr in the source tree using the cerl script.

Here is an example of a typical session. First we catch the crash in an rr recording session:

    > $ERL_TOP/bin/cerl -debug -rr
    rr: Saving execution to trace directory /home/foobar/.local/share/rr/beam.debug.smp-1.
    Erlang/OTP 25 [erts-13.0.2]

    Eshell V13.0.2  (abort with ^G)
    1> mymod:buggy_nif().
    Segmentation fault

Now we can replay that session with rr replay:

    > rr replay
    GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
    :
    (rr) continue
    :
    Thread 2 received signal SIGSEGV, Segmentation fault.
    (rr) backtrace

You get the call stack at the moment of the crash. Bad luck, it is somewhere deep down in the garbage collection of the beam. But you manage to figure out that variable hp points to a broken Erlang term.

Set a watch point on that memory position and resume execution backwards. The debugger will then stop at the exact position when that memory position *hp was written.

    (rr) watch -l *hp
    Hardware watchpoint 1: -location *hp
    (rr) reverse-continue
    Continuing.

    Thread 2 received signal SIGSEGV, Segmentation fault.

This is a quirk to be aware about. We started by executing forward until it crashed with SIGSEGV. We are now executing backwards from that point, so we are hitting the same SIGSEGV again but from the other direction. Just continue backwards once more to move past it.

    (rr) reverse-continue
    Continuing.

    Thread 2 hit Hardware watchpoint 1: -location *hp

    Old value = 42
    New value = 0

And here we are at the position when someone wrote a broken term on the process heap. Note that "Old value" and "New value" are reversed when we execute backwards. In this case the value 42 was written on the heap. Let's see who the guilty one is:

    (rr) backtrace