On 14/02/2023 20.14, Alexander Bulekov wrote:
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
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?
I assume you should have enough experience with sending PRs now, so if Peter
does not mind, I'd suggest to directly send PRs to Peter now. If that does
not work for some reason, feel free to send a “not for master” pull request
to me instead, then I'll take it along with my next qtest-related PR.
Thomas
