# Simulating failures in the POSIX API

When developing robust sofware, developers often consider the cases when the classic POSIX functions return failure.

Testing that fault-handling code is a problem because under normal conditions it's hard to make the POSIX functions fail, and generating abnormal conditions is usually difficult.

For example, getting malloc() to fail can represent using up all your memory, and at that point your test case might not even work. Or getting I/O operations to fail might involve filling up the disk which is very undesirable, or generating a very special environment which is difficult to reproduce.

libfiu comes with some tools that can be used to perform fault injection in the POSIX API (which includes the C standard library functions) without having to modify the application's source code, that can help to simulate scenarios like the ones described above in an easy and reproducible way.

# fiu-run

The first of those tools is an application called fiu-run.

Suppose you want to run the classic program "fortune" (which some would definitely consider mission critical) and see how it behaves on the presence of read() errors. With fiu-run, you can do it like this:

    $ fiu-run -x -c "enable_random name=posix/io/rw/read,probability=0.05" fortune

That enables the failure point with the name posix/io/rw/read with 5% probability to fail on each call, and then runs fortune. The -x parameter tells fiu-run to enable fault injection in the POSIX API.

Run it several times and you can see that sometimes it works, but sometimes it doesn't, reporting an error reading, which means a read() failed as expected.

When fortune is run, every read() has a 5% chance to fail, selecting an errno at random from the list of the ones that read() is allowed to return. If you want to select an specific errno, you can do it by passing its numerical value using the -i parameter.

The name of the failure points are fixed, and there is at least one for each function that libfiu supports injecting failures to. Not all POSIX functions are included, but most of the important pieces are, and it can be easily extended. See below for details.

To see the list of supported functions and names, see the (automatically generated) preload/posix/function_list file that comes in the libfiu tarball.

# fiu-ctrl

Sometimes it is more interesting to simulate failures at a given point in time instead of from the beginning, as fiu-run does.

To that end, you can combine fiu-run with the second tool, called fiu-ctrl.

Let's suppose we want to see what the "top" program does when it can't open files. First, we run it with fiu-run:

    $ fiu-run -x top

Everything should look normal. Then, in another terminal, we make open() fail unconditionally:

    $ fiu-ctrl -c "enable name=posix/io/oc/open" `pidof top`

After that moment, the top display will probably be empty, because it can't read process information. Now let's disable that failure point, so open() works again:

    $ fiu-ctrl -c "disable name=posix/io/oc/open" `pidof top`

And everything should have gone back to normal.

# How does it work

libfiu comes with two preload libraries: fiu_run_preload and fiu_posix_preload.

The first one is loaded using LD_PRELOAD (see ld.so(8) for more information) by fiu-run, and can enable failure points and start libfiu's remote control capabilities before the program begins to run.

The second one is also loaded using LD_PRELOAD by fiu-run when the -x parameter is given, and provides libfiu-enabled wrappers for the POSIX functions, allowing the user to inject failures in them.

fiu-ctrl communicates with the applications launched by fiu-run via the libfiu remote control capabilities.