# FRIDA mode

The purpose of FRIDA mode is to provide an alternative binary only fuzzer for
AFL++ just like that provided by QEMU mode. The intention is to provide a very
similar user experience, right down to the options provided through environment
variables.

In FRIDA mode, binary programs are instrumented, similarly to QEMU mode.

## Current progress

As FRIDA mode is new, it is missing a lot of features. The design is such that
it should be possible to add these features in a similar manner to QEMU mode and
perhaps leverage some of its design and implementation.

| Feature/Instrumentation  | FRIDA mode | Notes                                         |
| -------------------------|:----------:|:---------------------------------------------:|
| NeverZero                |     x      |                                               |
| Persistent Mode          |     x      | (x86/x64/aarch64 only)                        |
| LAF-Intel / CompCov      |     -      | (CMPLOG is better 90% of the time)            |
| CMPLOG                   |     x      | (x86/x64/aarch64 only)                        |
| Selective Instrumentation|     x      |                                               |
| Non-Colliding Coverage   |     -      | (not possible in binary-only instrumentation) |
| Ngram prev_loc Coverage  |     -      |                                               |
| Context Coverage         |     -      |                                               |
| Auto Dictionary          |     -      |                                               |
| Snapshot LKM Support     |     -      |                                               |
| In-Memory Test Cases     |     x      | (x86/x64/aarch64 only)                        |

## Compatibility

Currently FRIDA mode supports Linux and macOS targets on both x86/x64
architecture and aarch64. Later releases may add support for aarch32 and Windows
targets as well as embedded linux environments.

FRIDA has been used on various embedded targets using both uClibc and musl C
runtime libraries, so porting should be possible. However, the current build
system does not support cross compilation.

## Getting started

To build everything, run `make`. To build for x86, run `make 32`. Note that in
x86 bit mode, it is not necessary for afl-fuzz to be built for 32-bit. However,
the shared library for FRIDA mode must be since it is injected into the target
process.

Various tests can be found in subfolders within the `test/` directory. To use
these, first run `make` to build any dependencies. Then run `make qemu` or `make
frida` to run on either QEMU of FRIDA mode respectively. To run frida tests in
32-bit mode, run `make ARCH=x86 frida`. When switching between architectures, it
may be necessary to run `make clean` first for a given build target to remove
previously generated binaries for a different architecture.

### Android

In order to build, you need to download the Android SDK:

[https://developer.android.com/ndk/downloads](https://developer.android.com/ndk/downloads)

Then creating locally a standalone chain as follows:

[https://developer.android.com/ndk/guides/standalone_toolchain](https://developer.android.com/ndk/guides/standalone_toolchain)

## Usage

FRIDA mode added some small modifications to `afl-fuzz` and similar tools in
AFL++. The intention was that it behaves identically to QEMU, but it uses the
'O' switch rather than 'Q'. Whilst the options 'f', 'F', 's' or 'S' may have
made more sense for a mode powered by FRIDA Stalker, they were all taken, so
instead we use 'O' in homage to the [author](https://github.com/oleavr) of
FRIDA.

Similarly, the intention is to mimic the use of environment variables used by
QEMU where possible (by replacing `s/QEMU/FRIDA/g`). Accordingly, the following
options are currently supported:

* `AFL_FRIDA_DEBUG_MAPS` - See `AFL_QEMU_DEBUG_MAPS`.
* `AFL_FRIDA_EXCLUDE_RANGES` - See `AFL_QEMU_EXCLUDE_RANGES`.
* `AFL_FRIDA_INST_RANGES` - See `AFL_QEMU_INST_RANGES`.
* `AFL_FRIDA_PERSISTENT_ADDR` - See `AFL_QEMU_PERSISTENT_ADDR`.
* `AFL_FRIDA_PERSISTENT_CNT` - See `AFL_QEMU_PERSISTENT_CNT`.
* `AFL_FRIDA_PERSISTENT_HOOK` - See `AFL_QEMU_PERSISTENT_HOOK`.
* `AFL_FRIDA_PERSISTENT_RET` - See `AFL_QEMU_PERSISTENT_RET`.

To enable the powerful CMPLOG mechanism, set `-c 0` for `afl-fuzz`.

## Scripting

One of the more powerful features of FRIDA mode is its support for
configuration by JavaScript, rather than using environment variables. For
details of how this works, see [Scripting.md](Scripting.md).

## Performance

Additionally, the intention is to be able to make a direct performance
comparison between the two approaches. Accordingly, FRIDA mode includes various
test targets based on the [libpng](https://libpng.sourceforge.io/) benchmark
used by [fuzzbench](https://google.github.io/fuzzbench/) and integrated with the
[StandaloneFuzzTargetMain](https://raw.githubusercontent.com/llvm/llvm-project/main/compiler-rt/lib/fuzzer/standalone/StandaloneFuzzTargetMain.c)
from the llvm project. These tests include basic fork-server support, persistent
mode and persistent mode with in-memory test-cases. These are built and linked
without any special modifications to suit FRIDA or QEMU. The test data provided
with libpng is used as the corpus.

The intention is to add support for FRIDA mode to the FuzzBench project and
perform a like-for-like comparison with QEMU mode to get an accurate
appreciation of its performance.

## Design

FRIDA mode is supported by using `LD_PRELOAD` (`DYLD_INSERT_LIBRARIES` on macOS)
to inject a shared library (`afl-frida-trace.so`) into the target. This shared
library is built using the [frida-gum](https://github.com/frida/frida-gum)
devkit from the [FRIDA](https://github.com/frida/frida) project. One of the
components of frida-gum is
[Stalker](https://medium.com/@oleavr/anatomy-of-a-code-tracer-b081aadb0df8),
this allows the dynamic instrumentation of running code for AARCH32, AARCH64,
x86 and x64 architectures. Implementation details can be found
[here](https://frida.re/docs/stalker/).

Dynamic instrumentation is used to augment the target application with similar
coverage information to that inserted by `afl-gcc` or `afl-clang`. The shared
library is also linked to the `compiler-rt` component of AFL++ to feedback this
coverage information to AFL++ and also provide a fork server. It also makes use
of the FRIDA
[prefetch](https://github.com/frida/frida-gum/blob/56dd9ba3ee9a5511b4b0c629394bf122775f1ab7/gum/gumstalker.h#L115)
support to feedback instrumented blocks from the child to the parent using a
shared memory region to avoid the need to regenerate instrumented blocks on each
fork.

Whilst FRIDA allows for a normal C function to be used to augment instrumented
code, FRIDA mode instead makes use of optimized assembly instead on AARCH64 and
x86/64 targets. By injecting these small snippets of assembly, we avoid having
to push and pop the full register context. Note that since this instrumentation
is used on every basic block to generate coverage, it has a large impact on
performance.

CMPLOG support also adds code to the assembly, however, at present this code
makes use of a basic C function and is yet to be optimized. Since not all
instances run CMPLOG mode and instrumentation of the binary is less frequent
(only on CMP, SUB and CALL instructions) performance is not quite so critical.

## Advanced configuration options

* `AFL_FRIDA_DRIVER_NO_HOOK` - See `AFL_QEMU_DRIVER_NO_HOOK`. When using the
  QEMU driver to provide a `main` loop for a user provided
  `LLVMFuzzerTestOneInput`, this option configures the driver to read input from
  `stdin` rather than using in-memory test cases.
* `AFL_FRIDA_INST_COVERAGE_ABSOLUTE` - Generate coverage files using absolute
  virtual addresses rather than relative virtual addresses.
* `AFL_FRIDA_INST_COVERAGE_FILE` - File to write DynamoRIO format coverage
  information (e.g., to be loaded within IDA lighthouse).
* `AFL_FRIDA_INST_DEBUG_FILE` - File to write raw assembly of original blocks
  and their instrumented counterparts during block compilation.

```
Creating block for 0x7ffff7953313:
        0x7ffff7953313  mov qword ptr [rax], 0
        0x7ffff795331a  add rsp, 8
        0x7ffff795331e  ret

Generated block 0x7ffff75e98e2
        0x7ffff75e98e2  mov qword ptr [rax], 0
        0x7ffff75e98e9  add rsp, 8
        0x7ffff75e98ed  lea rsp, [rsp - 0x80]
        0x7ffff75e98f5  push rcx
        0x7ffff75e98f6  movabs rcx, 0x7ffff795331e
        0x7ffff75e9900  jmp 0x7ffff75e9384


  ***
```
* `AFL_FRIDA_INST_CACHE_SIZE` - Set the size of the instrumentation cache used
as a look-up table to cache real to instrumented address block translations.
Default is 256Mb.
* `AFL_FRIDA_INST_INSN` - Generate instrumentation for conditional
  instructions (e.g. `CMOV` instructions on x64).
* `AFL_FRIDA_INST_JIT` - Enable the instrumentation of Just-In-Time compiled
  code. Code is considered to be JIT if the executable segment is not backed by
  a file.
* `AFL_FRIDA_INST_NO_OPTIMIZE` - Don't use optimized inline assembly coverage
  instrumentation (the default where available). Required to use
* `AFL_FRIDA_INST_REGS_FILE` - File to write raw register contents at the start
  of each block.
  `AFL_FRIDA_INST_TRACE`.
* `AFL_FRIDA_INST_NO_CACHE` - Don't use a look-up table to cache real to
instrumented address block translations.
* `AFL_FRIDA_INST_NO_PREFETCH` - Disable prefetching. By default, the child will
  report instrumented blocks back to the parent so that it can also instrument
  them and they be inherited by the next child on fork, implies
  `AFL_FRIDA_INST_NO_PREFETCH_BACKPATCH`.
* `AFL_FRIDA_INST_NO_PREFETCH_BACKPATCH` - Disable prefetching of stalker
  backpatching information. By default, the child will report applied
  backpatches to the parent so that they can be applied and then be inherited by
  the next child on fork.
* `AFL_FRIDA_INST_SEED` - Sets the initial seed for the hash function used to
  generate block (and hence edge) IDs. Setting this to a constant value may be
  useful for debugging purposes, e.g., investigating unstable edges.
* `AFL_FRIDA_INST_TRACE` - Log to stdout the address of executed blocks, implies
  `AFL_FRIDA_INST_NO_OPTIMIZE`.
* `AFL_FRIDA_INST_TRACE_UNIQUE` - As per `AFL_FRIDA_INST_TRACE`, but each edge
  is logged only once, requires `AFL_FRIDA_INST_NO_OPTIMIZE`.
* `AFL_FRIDA_INST_UNSTABLE_COVERAGE_FILE` - File to write DynamoRIO format
  coverage information for unstable edges (e.g., to be loaded within IDA
  lighthouse).
* `AFL_FRIDA_JS_SCRIPT` - Set the script to be loaded by the FRIDA scripting
  engine. See [Scipting.md](Scripting.md) for details.
* `AFL_FRIDA_OUTPUT_STDOUT` - Redirect the standard output of the target
  application to the named file (supersedes the setting of `AFL_DEBUG_CHILD`).
* `AFL_FRIDA_OUTPUT_STDERR` - Redirect the standard error of the target
  application to the named file (supersedes the setting of `AFL_DEBUG_CHILD`).
* `AFL_FRIDA_PERSISTENT_DEBUG` - Insert a Breakpoint into the instrumented code
  at `AFL_FRIDA_PERSISTENT_HOOK` and `AFL_FRIDA_PERSISTENT_RET` to allow the
  user to detect issues in the persistent loop using a debugger.

  ```
  gdb \
      --ex 'set environment AFL_FRIDA_PERSISTENT_ADDR=XXXXXXXXXX' \
      --ex 'set environment AFL_FRIDA_PERSISTENT_RET=XXXXXXXXXX' \
      --ex 'set environment AFL_FRIDA_PERSISTENT_DEBUG=1' \
      --ex 'set environment AFL_DEBUG_CHILD=1' \
      --ex 'set environment LD_PRELOAD=afl-frida-trace.so' \
      --args <my-executable> [my arguments]
  ```

* `AFL_FRIDA_SECCOMP_FILE` - Write a log of any syscalls made by the target to
  the specified file.
* `AFL_FRIDA_STALKER_ADJACENT_BLOCKS` - Configure the number of adjacent blocks
  to fetch when generating instrumented code. By fetching blocks in the same
  order they appear in the original program, rather than the order of execution
  should help reduce locality and adjacency. This includes allowing us to vector
  between adjacent blocks using a NOP slide rather than an immediate branch.
* `AFL_FRIDA_STALKER_IC_ENTRIES` - Configure the number of inline cache entries
  stored along-side branch instructions which provide a cache to avoid having to
  call back into FRIDA to find the next block. Default is 32.
* `AFL_FRIDA_STALKER_NO_BACKPATCH` - Disable backpatching. At the end of executing
  each block, control will return to FRIDA to identify the next block to
  execute.
* `AFL_FRIDA_STATS_FILE` - Write statistics information about the code being
  instrumented to the given file name. The statistics are written only for the
  child process when new block is instrumented (when the
  `AFL_FRIDA_STATS_INTERVAL` has expired). Note that just because a new path is
  found does not mean a new block needs to be compiled. It could be that the
  existing blocks instrumented have been executed in a different order.

  ```
  stats
  -----
  Time                  2021-07-21 11:45:49
  Elapsed                                 1 seconds


  Transitions                    cumulative               delta
  -----------                    ----------               -----
  total                              753619               17645
  call_imm                             9193 ( 1.22%)        344 ( 1.95%) [       344/s]
  call_reg                                0 ( 0.00%)          0 ( 0.00%) [         0/s]
  call_mem                                0 ( 0.00%)          0 ( 0.00%) [         0/s]
  ret_slow_path                       67974 ( 9.02%)       2988 (16.93%) [      2988/s]
  post_call_invoke                     7996 ( 1.06%)        299 ( 1.69%) [       299/s]
  excluded_call_imm                    3804 ( 0.50%)        200 ( 1.13%) [       200/s]
  jmp_imm                              5445 ( 0.72%)        255 ( 1.45%) [       255/s]
  jmp_reg                             42081 ( 5.58%)       1021 ( 5.79%) [      1021/s]
  jmp_mem                            578092 (76.71%)      10956 (62.09%) [     10956/s]
  jmp_cond_imm                        38951 ( 5.17%)       1579 ( 8.95%) [      1579/s]
  jmp_cond_mem                            0 ( 0.00%)          0 ( 0.00%) [         0/s]
  jmp_cond_reg                            0 ( 0.00%)          0 ( 0.00%) [         0/s]
  jmp_cond_jcxz                           0 ( 0.00%)          0 ( 0.00%) [         0/s]
  jmp_continuation                       84 ( 0.01%)          3 ( 0.02%) [         3/s]


  Instrumentation
  ---------------
  Instructions                         7907
  Blocks                               1764
  Avg Instructions / Block                4


  EOB Instructions
  ----------------
  Total                                1763 (22.30%)
  Call Immediates                       358 ( 4.53%)
  Call Immediates Excluded               74 ( 0.94%)
  Call Register                           0 ( 0.00%)
  Call Memory                             0 ( 0.00%)
  Jump Immediates                       176 ( 2.23%)
  Jump Register                           8 ( 0.10%)
  Jump Memory                            10 ( 0.13%)
  Conditional Jump Immediates          1051 (13.29%)
  Conditional Jump CX Immediate           0 ( 0.00%)
  Conditional Jump Register               0 ( 0.00%)
  Conditional Jump Memory                 0 ( 0.00%)
  Returns                               160 ( 2.02%)


  Relocated Instructions
  ----------------------
  Total                                 232 ( 2.93%)
  addsd                                   2 ( 0.86%)
  cmp                                    46 (19.83%)
  comisd                                  2 ( 0.86%)
  divsd                                   2 ( 0.86%)
  divss                                   2 ( 0.86%)
  lea                                   142 (61.21%)
  mov                                    32 (13.79%)
  movsd                                   2 ( 0.86%)
  ucomisd                                 2 ( 0.86%)
  ```

* `AFL_FRIDA_STATS_INTERVAL` - The maximum frequency to output statistics
  information. Stats will be written whenever they are updated if the given
  interval has elapsed since last time they were written.
* `AFL_FRIDA_TRACEABLE` - Set the child process to be traceable by any process
  to aid debugging and overcome the restrictions imposed by YAMA. Supported on
  Linux only. Permits a non-root user to use `gcore` or similar to collect a
  core dump of the instrumented target. Note that in order to capture the core
  dump you must set a sufficient timeout (using `-t`) to avoid `afl-fuzz`
  killing the process whilst it is being dumped.
* `AFL_FRIDA_VERBOSE` - Enable verbose output from FRIDA mode.

## FASAN - FRIDA Address Sanitizer mode

FRIDA mode also supports FASAN. The design of this is actually quite simple and
very similar to that used when instrumenting applications compiled from source.

### Address Sanitizer basics

When Address Sanitizer is used to instrument programs built from source, the
compiler first adds a dependency (`DT_NEEDED` entry) for the Address Sanitizer
dynamic shared object (DSO). This shared object contains the main logic for
Address Sanitizer, including setting and managing up the shadow memory. It also
provides replacement implementations for a number of functions in standard
libraries.

These replacements include things like `malloc` and `free` which allows for
those allocations to be marked in the shadow memory, but also a number of other
functions. Consider `memcpy`, for example. This is instrumented to validate the
parameters (test the source and destination buffers against the shadow memory).
This is much easier than instrumenting those standard libraries, since first, it
would require you to re-compile them and secondly it would mean that the
instrumentation would be applied at a more expensive granular level. Lastly,
load-widening (typically found in highly optimized code) can also make this
instrumentation more difficult.

Since the DSO is loaded before all of the standard libraries (in fact it insists
on being first), the dynamic loader will use it to resolve imports from other
modules which depend on it.

### FASAN implementation

FASAN takes a similar approach. It requires the user to add the Address
Sanitizer DSO to the `AFL_PRELOAD` environment variable such that it is loaded
into the target. Again, it must be first in the list. This means that it is not
necessary to instrument the standard libraries to detect when an application has
provided an incorrect argument to `memcpy`, for example. This avoids issues with
load-widening and should also mean a huge improvement in performance.

FASAN then adds instrumentation for any instructions which use memory operands
and then calls into the `__asan_loadN` and `__asan_storeN` functions provided by
the DSO to validate memory accesses against the shadow memory.

## Collisions

FRIDA mode has also introduced some improvements to reduce collisions in the
map. For details, see [MapDensity.md](MapDensity.md).

## OSX library fuzzing

An example of how to fuzz a dynamic library on OSX is included, see
[test/osx-lib](test/osx-lib). This requires the use of a simple test harness
executable which will load the library and call a target function within it. The
dependent library can either be loaded in using `dlopen` and `dlsym` in a
function marked `__attribute__((constructor()))` or the test harness can be
linked against it. It is important that the target library is loaded before
execution of `main`, since this is the point where FRIDA mode is initialized.
Otherwise, it will not be possible to configure coverage for the test library
using `AFL_FRIDA_INST_RANGES` or similar.

## Debugging

Should you encounter problems with FRIDA mode, refer to
[DEBUGGING.md](DEBUGGING.md) for assistance.

## To do

The next features to be added are Aarch32 support as well as looking at
potential performance improvements. The intention is to achieve feature parity
with QEMU mode in due course. Contributions are welcome, but please get in touch
to ensure that efforts are deconflicted.
