logbt

Short for "Log Backtrace", this is a bash wrapper for displaying a backtrace when a program crashes.

The goal of logbt is to provide immediate feedback on why and how a C/C++ program crashed.

Normally when a C/C++ program crashes the kernel will only print something minimal like Segmentation fault: 11. However, with logbt you will see a detailed output that includes:

• the exit code of the program printed to stdout (normally the exit code is not visible in logs)
• the backtrace that shows the lines of code involved in the crash aka. the callstack for the crash

The logbt command can also:

• Respond to the USR1 signal and generate a backtrace of a healthy program (which will continue to run)
• Act as init process (aka "PID1") for docker containers (receives signals and reaps child processes)
• Automatically clean up coredumps on the system (to avoid your disk filling up)

Supported signals

Logbt notices all signals associated with a crash and translates this to more detailed output. For example, without logbt, for a program that segfaults (hits the SIGSEGV signal) you would see Segmentation fault: 11. With logbt you would see:

[logbt] saw '<program name' exit with code:139 (SEGV)
[logbt] Found corefile at /cores/core.<pid>
<backtrace>

Where:

• <program name> is the name of the program that you launched with logbt -- <program name>
• <pid> is unique program ID that the system assigned that process
• <backtrace> will be the unique backtrace from gdb (on linux) or lldb (on os x) that shows what lines of code were executing that lead to the segfault.

These are the signals that logbt will report detailed output for:

• signal SIGSEGV, exit code 139, common name Segmentation fault: 11
• signal SIGABRT, exit code 134, common name Abort trap: 6
• signal SIGFPE, exit code 136, common name Floating-point exception: 8
• signal SIGTERM, exit code 143, common name terminated
• signal SIGILL, exit code 132 common name Illegal instruction: 4
• signal SIGHUP, exit code 129, common name Hangup
• signal SIGKILL, exit code 137, common name Killed
• signal SIGINT, exit code 130, common name Interrupt
• signal SIGBUS, exit code 138 (os x) / 135 (linux), common name Bus error: 10
• signal SIGUSR1, exit code 158 (os x) / 128 (linux), common name User-defined signal 1

For more info on these signals see http://man7.org/linux/man-pages/man7/signal.7.html

Upgrading

If upgrading from a previous logbt version see Upgrading.md for details on how to adapt your code.

Supports

• Linux and OS X
• Docker (see Docker below)

Depends

The logbt run command requires gdb on linux and lldb on OS X.

Recommended install of gdb on linux:

mkdir mason && curl -sSfL https://github.com/mapbox/mason/archive/v0.6.0.tar.gz | tar --gunzip --extract --strip-components=1 --directory=./mason
./mason/mason install gdb 7.12
export PATH=$(./mason/mason prefix gdb 7.12)/bin:${PATH}
which gdb

Recommended install of lldb on OS X is to get latest XCode.

Installing

To install logbt to /usr/local/bin:

curl -sSfL https://github.com/mapbox/logbt/archive/v3.0.0.tar.gz | tar --gunzip --extract --strip-components=1 --exclude="*md" --exclude="test*" --directory=/usr/local
which logbt
/usr/local/bin/logbt
logbt --version

Locally (perhaps if your user cannot write to /usr/local):

curl -sSfL https://github.com/mapbox/logbt/archive/v3.0.0.tar.gz | tar --gunzip --extract --strip-components=2 --exclude="*md" --exclude="test*" --directory=.
./logbt --version

Usage

There are two main modes to using logbt. First you run logbt --setup and second you run logbt -- <your program> to launch your program with it.

Setup logbt

sudo logbt --setup

This command sets the system core_pattern to ensure it is ready for logbt to use.

This is required on Linux (modifies /proc/sys/kernel/core_pattern).

Running logbt --setup is optional on OS X if these conditions are met:

• The system default for kern.corefile is intact (This means on OS X that $(sysctl -n kern.corefile) == '/cores/core.%P')
• The /cores directory exists and is writeable by normal users. This can be accomplished by doing sudo mkdir -p /cores && sudo chmod a+w /cores/

Note, to restore the default on OS X you can run sudo sysctl kern.corefile=/cores/core.%P.

Common default values for core_pattern on linux (which do not work with logbt) are:

• |/usr/libexec/abrt-hook-ccpp %s %c %p %u %g %t e Seen on Centos 6 (won't work because data is piped to abrt-hook-ccpp)
• |/usr/share/apport/apport %p %s %c Seen on Ubuntu Precise (won't work because data is piped to apport)
• |/usr/share/apport/apport %p %s %c %P Seen on Ubuntu Trusty (won't work because data is piped to apport)
• core Seen on various systems (won't work because logbt needs the pid and program name in the core_pattern on linux)

Run logbt

All commands passed to logbt after -- are interpreted as the program to run and any arguments to pass to that program.

This is known as the run command.

Therefore, to launch your program with logbt run:

logbt -- <your program> <your program args>

Then logbt will run as long as your program runs. If logbt your program will be killed with SIGTERM. If your program exits then logbt will exit with the same exit code. If your program crashes then logbt will display a backtrace and exit with the crashing exit code.

Additional options

• logbt --test: tests that logbt is functioning correctly. Should be run after logbt --setup
• logbt --current-pattern: displays the current core_pattern value on the system (/proc/sys/kernel/core_pattern on linux and sysctl -n kern.corefile on OS X)
• logbt --target-pattern: displays the target core_pattern value that logbt --setup will apply to the system which is /tmp/logbt-coredumps/core.%p.%E on linux and /tmp/logbt-coredumps/core.%P on OS X)
• logbt --version: Prints the logbt version
• logbt --help: Prints the logbt usage help
• logbt --keep-core: The default behavior of logbt is to clear all corefiles found in the core directory listed by the core_pattern. Passing this option modifies this behavior such that the corefiles are kept and not deleted
• logbt --debug-command "<command>": The default command sent to gdb (on linux) is thread apply all bt and lldb (on OS X) is thread backtrace all, respectively. If you pass this argument, which should be quoted, then you can customize what is sent. For example if you want full backtraces from gdb you could pass logbt --debug-command "thread apply all bt full"

Snapshotting

A experimental feature of logbt >= 2.x is the ability to send a USR1 signal and to generate backtrace of the healthy child program.

kill -USR1 <pid of logbt>

Or, if running logbt in a docker container, you can send this via the host like:

# see ./test/docker-snapshotting.sh for a full example
docker kill --signal="SIGUSR1" <container id>

When USR1 is received by logbt the child program is paused (SIGSTOP), a backtrace is generated, and then the child program is resumed (SIGCONT).

There are several limitations to consider before using this feature. Future logbt versions will likely change this interface to use bcc tools for snapshotting due to limitations 1/2 below.

1. Not recommended for production

This is useful for checking on what the child program is doing for debugging, but should not be used in production systems because the child program is stopped for potentially >= several seconds.

2. ptrace support

ptrace support is needed to allow gdb on linux to attach to the child process. To enable ptrace in a docker container you must run with --cap-add SYS_PTRACE or --privileged. And ptrace_scope scope in the kernel likely will need to be set to zero like: sudo bash -c "echo 0 > /proc/sys/kernel/yama/ptrace_scope". Another final limitation of ptrace is that only one tool may be attached at one time.

3. child limitations

The child must be the program you want snapshotted. While logbt supports tracking crashes of any children or grandchildren of the program run by logbt the snapshotting will only be done on the direct child.

Running unit tests

The unit tests additionally depend on:

• nodejs 4.x
• timeout command

On OS X these can be installed and enabled like:

brew install node coreutils || true
export PATH=$(brew --prefix)/opt/coreutils/libexec/gnubin:${PATH}

They can be run like:

./test/unit.sh

Docker considerations

Docker linux containers inherit their kernel settings from the linux host. Because the core_pattern modified by logbt --setup is kernel-level the --setup command must be either be run as root on the linux host (recommended) or within a container run with the --privileged flag.

If you try to run logbt --setup in a container without the --privileged flag you will see an error like: /proc/sys/kernel/core_pattern: Read-only file system

⚠️ Running logbt --setup in a privileged container will change the core_pattern value for the host. On OS X (with docker for mac) the core_pattern value will also be changed in the underlying linux host run by the hypervisor. You can see this by logging into the linux vm with screen by doing screen ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/tty and then cat /proc/sys/kernel/core_pattern. (ctrl a \ to exit). By default it will be cores and after running a docker container that runs logbt --setup with docker run --privileged it will be equal to the logbt internal value for linux of /tmp/logbt-coredumps/core.%p.%E. This will be inherited for all other docker containers you run on OS X.

The --privileged only applies to docker run and not docker build (refs moby/moby#1916)

With AWS, the ECS container definition is how you ask for privileged runs.

The logbt --setup command may not work on some CI systems unless you have permissions to modify the kernal pattern or if the kernal pattern is already set up to match logbt expectations.

One other alternative to running --privileged is mounting a writable /proc directory like:

docker run --volume /proc:/writable-proc <image name> bash

Then within that container you can write to writable-proc and it will be reflected in /proc:

cat $(./bin/logbt --target-pattern) > /writable-proc/sys/kernel/core_pattern

After that command both /writable-proc/sys/kernel/core_pattern and /proc/sys/kernel/core_pattern will be equivalent and logbt --test should work.

But surprisingly this still modifies the host core_pattern so there is no major advantage to this method over running logbt --setup directly on the host.

FAQ

Q: core file may not match

I'm seeing a warning in the backtrace that says "core file may not match specified executable file". Why is that happening and is it a problem?

Answer:

This is normal and harmless if the program you launched with logbt has customized its process "title". For example, with node you can do:

process.title = 'custom-name';

When gdb prints core file may not match specified executable file it is saying that it noticed your modification of the process title.

Q: non-tracked corefiles

I'm seeing a message from logbt like [logbt] No corefile found at /tmp/logbt-coredumps/core.641.*. Is that a problem or indication that backtraces are not working?

Answer:

If you also see a message following it like:

[logbt] Found corefile (non-tracked) at /tmp/logbt-coredumps/core.642.!root!.nvm!versions!node!v4.7.2!bin!node
[logbt] Processing cores...

Then everything is okay. What is happening is that logbt uses the <pid> (process id) of the program it launches to look for a corefile when that program crashes. Let's call that program the parent process. In this case the parent did not crash (pid of 641) but a program it launched (a child) crashed (pid of 642). Because the parent correctly reported the crash to logbt (via returning an exit code indicating a crash) then logbt knows to look harder for corefiles that may have been created from a crashing child. In this case logbt prints a message that it found a (non-tracked) corefile to indicate a child crashed rather than a parent.