Re: [PATCH 00/10] Retire Fork-Based Fuzzing

[Alexander Bulekov]

On 230214 2009, Thomas Huth wrote:
> On 14/02/2023 17.08, Philippe Mathieu-Daudé wrote:
> > On 14/2/23 16:38, Stefan Hajnoczi wrote:
> > > On Sat, Feb 04, 2023 at 11:29:41PM -0500, Alexander Bulekov wrote:
> > > > Hello,
> > > > This series removes fork-based fuzzing.
> > > > How does fork-based fuzzing work?
> > > >   * A single parent process initializes QEMU
> > > >   * We identify the devices we wish to fuzz (fuzzer-dependent)
> > > >   * Use QTest to PCI enumerate the devices
> > > >   * After that we start a fork-server which forks the process and 
> > > > executes
> > > >     fuzzer inputs inside the disposable children.
> > > > 
> > > > In a normal fuzzing process, everything happens in a single process.
> > > > 
> > > > Pros of fork-based fuzzing:
> > > >   * We only need to do common configuration once (e.g. PCI enumeration).
> > > >   * Fork provides a strong guarantee that fuzzer inputs will not
> > > > interfere with
> > > >     each-other
> > > >   * The fuzzing process can continue even after a child-process crashes
> > > >   * We can apply our-own timers to child-processes to exit slow
> > > > inputs, early
> > > > 
> > > > Cons of fork-based fuzzing:
> > > >   * Fork-based fuzzing is not supported by libfuzzer. We had to
> > > > build our own
> > > >     fork-server and rely on tricks using linker-scripts and 
> > > > shared-memory to
> > > >     support fuzzing. (
> > > > https://physics.bu.edu/~alxndr/libfuzzer-forkserver/ )
> > > >   * Fork-based fuzzing is currently the main blocker preventing
> > > > us from enabling
> > > >     other fuzzers such as AFL++ on OSS-Fuzz
> > > >   * Fork-based fuzzing may be a reason why coverage-builds are failing 
> > > > on
> > > >     OSS-Fuzz. Coverage is an important fuzzing metric which
> > > > would allow us to
> > > >     find parts of the code that are not well-covered.
> > > >   * Fork-based fuzzing has high overhead. fork() is an expensive
> > > > system-call,
> > > >     especially for processes running ASAN (with large/complex) VMA 
> > > > layouts.
> > > >   * Fork prevents us from effectively fuzzing devices that rely on
> > > >     threads (e.g. qxl).
> > > > 
> > > > These patches remove fork-based fuzzing and replace it with reboot-based
> > > > fuzzing for most cases. Misc notes about this change:
> > > >   * libfuzzer appears to be no longer in active development. As such, 
> > > > the
> > > >     current implementation of fork-based fuzzing (while having some nice
> > > >     advantages) is likely to hold us back in the future. If these 
> > > > changes
> > > >     are approved and appear to run successfully on OSS-Fuzz, we should 
> > > > be
> > > >     able to easily experiment with other fuzzing engines (AFL++).
> > > >   * Some device do not completely reset their state. This can lead to
> > > >     non-reproducible crashes. However, in my local tests, most crashes
> > > >     were reproducible. OSS-Fuzz shouldn't send us reports unless it can
> > > >     consistently reproduce a crash.
> > > >   * In theory, the corpus-format should not change, so the existing
> > > >     corpus-inputs on OSS-Fuzz will transfer to the new reset()-able
> > > >     fuzzers.
> > > >   * Each fuzzing process will now exit after a single crash is found. To
> > > >     continue the fuzzing process, use libfuzzer flags such as -jobs=-1
> > > >   * We no long control input-timeouts (those are handled by libfuzzer).
> > > >     Since timeouts on oss-fuzz can be many seconds long, I added a limit
> > > >     on the number of DMA bytes written.
> > > > 
> > > > Alexander Bulekov (10):
> > > >    hw/sparse-mem: clear memory on reset
> > > >    fuzz: add fuzz_reboot API
> > > >    fuzz/generic-fuzz: use reboots instead of forks to reset state
> > > >    fuzz/generic-fuzz: add a limit on DMA bytes written
> > > >    fuzz/virtio-scsi: remove fork-based fuzzer
> > > >    fuzz/virtio-net: remove fork-based fuzzer
> > > >    fuzz/virtio-blk: remove fork-based fuzzer
> > > >    fuzz/i440fx: remove fork-based fuzzer
> > > >    fuzz: remove fork-fuzzing scaffolding
> > > >    docs/fuzz: remove mentions of fork-based fuzzing
> > > > 
> > > >   docs/devel/fuzzing.rst              |  22 +-----
> > > >   hw/mem/sparse-mem.c                 |  13 +++-
> > > >   meson.build                         |   4 -
> > > >   tests/qtest/fuzz/fork_fuzz.c        |  41 ----------
> > > >   tests/qtest/fuzz/fork_fuzz.h        |  23 ------
> > > >   tests/qtest/fuzz/fork_fuzz.ld       |  56 --------------
> > > >   tests/qtest/fuzz/fuzz.c             |   6 ++
> > > >   tests/qtest/fuzz/fuzz.h             |   2 +-
> > > >   tests/qtest/fuzz/generic_fuzz.c     | 111 +++++++---------------------
> > > >   tests/qtest/fuzz/i440fx_fuzz.c      |  27 +------
> > > >   tests/qtest/fuzz/meson.build        |   6 +-
> > > >   tests/qtest/fuzz/virtio_blk_fuzz.c  |  51 ++-----------
> > > >   tests/qtest/fuzz/virtio_net_fuzz.c  |  54 ++------------
> > > >   tests/qtest/fuzz/virtio_scsi_fuzz.c |  51 ++-----------
> > > >   14 files changed, 72 insertions(+), 395 deletions(-)
> > > >   delete mode 100644 tests/qtest/fuzz/fork_fuzz.c
> > > >   delete mode 100644 tests/qtest/fuzz/fork_fuzz.h
> > > >   delete mode 100644 tests/qtest/fuzz/fork_fuzz.ld
> > > > 
> > > > -- 
> > > > 2.39.0
> > > > 
> > > 
> > > Whose tree should this go through? Laurent's qtest tree?
> > 
> > Do you mean Thomas?
> 
> I thought Alexander would be doing pull requests for fuzzing-related patches
> nowadays (since he's the listed maintainer for these files)? Or did I get
> that wrong?

I have, though in the past I've been asked to send the PR to different
people. Who should I send this PR to?
-Alex