# AFL Dyninst

AFL Dyninst uses Dyninst to insert AFL instrumentations into binaries
for binary fuzzing.

The tool has two parts. The instrumentation tool and the instrumentation 
library. Instrumentation library has an initialization callback and basic 
block callback functions which are designed to emulate what AFL is doing
with afl-gcc/afl-g++/afl-as. 

Instrumentation tool (afl-dyninst) instruments the supplied binary by
inserting callbacks for each basic block and an initialization
callback either at `_init` or at specified entry point.

## Installation

As its name suggest, AFL Dyninst requires [AFL++] 4.22 or above
and [Dyninst] 10 or above at running time.  To build afl-dyninst,
you also need make(1p), m4(1p) and a C++11 compiler.

Since `afl-dyninst` does not relink the output binary,
path to Dyninst library needs to be passed to the build tools
to be injected into wrapper scripts:

    make DYNINST_LIB=$dyninst_prefix/lib PREFIX=$afl_dyninst_prefix install

## Usage

```shell
$ afl-dyninst --help
Usage: afl-dyninst [OPTIONS]... INFILE OUTFILE

Instrument binary to be fuzzed by AFL.

Options:
  -h, --help               show this help message and exit
      --version            show program's version number and exit
  -e ADDR, --entry=ADDR    entry point address to patch (required for stripped binaries)
  -E ADDR, --exit=ADDR     force exit(0) at this address (multiple use)
  -D                       instrument only a simple fork server and also forced exit functions
  -r PATH, --library=PATH  runtime library to instrument (multiple use)
  -I NAME, --include=NAME  instrument only this function and nothing else (multiple use)
  -S NAME, --exclude=NAME  do not instrument this function (multiple use)
  -m N, --min-size=N       minimum size of a basic bock to instrument (default to 10)
  -s N, --skip=N           number of initial basic blocks to skip in binary
  -v, --verbose            enable verbose output (up to 3 levels)
  -x                       experimental performance mode (multiple use, ~25-50% speed improvement)
```

Switch -e is used to manually specify the entry point where initialization
callback is to be inserted. For unstripped binaries, afl-dyninst defaults
to using `_init` of the binary as an entry point. In case of stripped binaries
this option is required and is best set to the address of main which 
can easily be determined by disassembling the binary and looking for an 
argument to `__libc_start_main`.

Switch -E is used to specify addresses that should force a clean exit
when reached. This can speed up the fuzzing tremendously.

Switch -s instructs afl-dyninst to skip the first NUMBER of basic blocks. 
Currently, it is used to work around a bug in Dyninst but doubles as an
optimization option, as skipping the basic blocks of the initialization
routines makes things run faster.  If the instrumented binary is crashing by
itself, try skiping a number of blocks.

Switch -r allows you to specify a path to a library that is loaded
via dlopen() at runtime. Instrumented runtime libraries will be 
written to the same location with a `.ins` suffix as not to overwrite
the original ones. Make sure to backup the originals and then rename the
instrumented ones to original name. 

Switch -m allows you to only instrument basic blocks of a minimum size - the
default minimum size is 10.

Switch -S allows you to not instrument specific functions.
This options is mainly to hunt down bugs in dyninst.
Can be specified multiple times.

Switch -I specified to only instrument specific functions.
This option is amazing with large and threaded targets.
Can be specified multiple times.

Switch -D installs the afl fork server and forced exit functions but no
basic block instrumentation. That would serve no purpose - unless there are
other tools that need that: 
* [afl-dynamorio]
* [afl-pin]

Switch -x enables an experimental performance mode (+25-50% speed). Just try it
and if the target crashes too often, instrument again without this. Should not
crash though.

After instrumenting the target binary always check if it works.
Dyninst makes big changes to the code, and hence more often than not
things are not working anymore.

The instrumentation libraries `libafldyninst.so` and `libdyninstAPI_RT.so`
must be available in `LD_LIBRARY_PATH` as the instrumented binary
will be looking for them.  The helper script afl-dyninst-env sets up
the environment accordingly.

## Examples

### Instrumentation

    afl-dyninst -e 0x4034c0 -x unrar unrar-ins

Here we are instrumenting the `unrar` binary with entry point at 0x4034c0
(manually found address of `main`), skipping the first 10 basic blocks
and outputting to `unrar-ins`.  This instrumented binary can be tested via

    afl-dyninst-env ./unrar-ins ...

### Fuzzing instrumented binary

Since AFL checks if the binary has been instrumented by afl-gcc,
the `AFL_SKIP_BIN_CHECK` environment variable needs to be set.
No modifications to AFL itself is needed. 

    export AFL_SKIP_BIN_CHECK=1

You can use the afl-dyninst-env helper script
to sets the required environment variables:

    afl-dyninst-env afl-fuzz -i testcases/archives/common/gzip/ -o test_gzip\
      -- ./gzip_ins -d -c

## Problems

### Instrumented binary not working

If the instrumented binary crashes or hangs, try to increase
the `--min-size` parameter.  8 is the minimum recommended,
on some targets e.g. 16 is required.

You can also try to remove the performance enhancer flag `-x`.

### AFL reporting executions as crash

If the target is using throw/catch, Dyninst fails to handle
the caught exception, hence `abort` is triggered.
AFL will report most fuzzing test case is reported as a crash
although it does not when running it from the command line.

No solution to this issue is known yet.
Binary editing the target binary to perform `_exit(0)` would help though.

## Copying

AFL Dyninst is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

AFL Dyninst was originally developed as part of a project
with the Cisco Talos VULNDEV Team and released under the Apache License,
version 2.0.

[AFL++]: https://github.com/AFLplusplus/AFLplusplus
[Dyninst]: https://github.com/dyninst/dyninst
[afl-dynamorio]: https://github.com/vanhauser-thc/afl-dynamorio
[afl-pin]: https://github.com/vanhauser-thc/afl-pin