To report security bugs, see ‘SECURITY’ in the top source directory.

Fuzzing nbdkit using the American Fuzzy Lop (afl) fuzzer
========================================================

We can fuzz nbdkit using the -s (read from stdin) option, and feeding
in a file of "network" input.  The stdout from nbdkit (ie. data that
would normally be sent back to the client) is ignored.

You will need to recompile nbdkit with afl instrumentation:

  ./configure CC=/usr/bin/afl-gcc CXX=/usr/bin/afl-g++
  make clean
  make

The fuzzing/testcase_dir directory contains some initial testcases
that afl can use.

Run multiple copies of afl-fuzz.  Usually you should run 1 master (-M)
and as many slaves (-S) as you can.

Master:

  mkdir -p fuzzing/sync_dir
  afl-fuzz -i fuzzing/testcase_dir -o fuzzing/sync_dir -m 256 -M fuzz01 \
      ./server/nbdkit -s ./plugins/memory/.libs/nbdkit-memory-plugin.so 1M

Slaves:

  # replace fuzzNN with fuzz02, fuzz03, etc.
  afl-fuzz -i fuzzing/testcase_dir -o fuzzing/sync_dir -m 256 -S fuzzNN \
      ./server/nbdkit -s ./plugins/memory/.libs/nbdkit-memory-plugin.so 1M

Test Coverage
-------------

To find out if the fuzzing is covering all of the code, I used afl-cov
(https://github.com/mrash/afl-cov).  Usage is rather complex, so
consult the README of that project, but in brief:

(1) Create a second copy of the nbdkit source, and compile it with
profiling:

  ./configure CFLAGS="-O2 -g -pg -fprofile-arcs -ftest-coverage"
  make clean
  make

(2) Assuming ../nbdkit-afl is the nbdkit source compiled with afl, and
the current directory is nbdkit compiled with profiling, then run the
command below.  You can run this even while afl-fuzz is running.

  afl-cov -d ../nbdkit-afl/fuzzing/sync_dir \
      --code-dir . \
      --coverage-cmd "cat AFL_FILE | ./server/nbdkit -s ./plugins/memory/.libs/nbdkit-memory-plugin.so 1M"

This will create an HTML test coverage report in
../nbdkit-afl/fuzzing/sync_dir/cov/web/

Notes
-----

We only have testcases for the newstyle fixed protocol so far, but if
people report that they are exposing the oldstyle protocol to the
internet / untrusted clients then we could extend the testing for
that.

We have only fuzzed the memory plugin, which is convenient because it
stores everything in memory and throws it away when nbdkit exits.
Since plugins can have bugs as well as the core server, it may be
productive to test other plugins and filters.

Fuzzing nbdkit using Clang + libFuzzer
======================================

Recompile nbdkit with libFuzzer enabled and build the libFuzzer test
binary:

  ./configure \
      CC=clang \
      CFLAGS="-g -O1 -fPIC" \
      --enable-libfuzzer \
      --disable-perl
  make clean
  make CFLAGS="-g -O1 -fPIC -fsanitize=fuzzer,address"

(The awkward CFLAGS on the make command line are necessary because
./configure attempts to test that the compiler works, but this test
fails when -fsanitize=fuzzer is used as that option adds an extra
main() definition.)

",address" enables the Clang Address Sanitizer, and can be omitted for
faster fuzzing.

This builds a specialized version of nbdkit which is the fuzzer test
program (testing only nbdkit-memory-plugin unless you modify the
source).  You can run it like this on the input corpus:

  ./server/nbdkit fuzzing/testcase_dir

New test inputs are written to fuzzing/testcase_dir and will be used
on subsequent runs.  If this is undesirable then delete
fuzzing/testcase_dir/[0-f]* before the run.

There are various extra command line options supported by libFuzzer.
For more details see:

  https://llvm.org/docs/LibFuzzer.html
