Re: [PATCH 2/2] fuzz: add instructions for building reproducers

[Darren Kenny <darren.kenny@oracle.com>]

On Saturday, 2021-03-13 at 23:23:57 -05, Alexander Bulekov wrote:
> We have several scripts that help build reproducers, but no
> documentation for how they should be used. Add some documentation
>
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

> ---
>  docs/devel/fuzzing.rst | 45 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/docs/devel/fuzzing.rst b/docs/devel/fuzzing.rst
> index 97797c4f8c..025fb0c19b 100644
> --- a/docs/devel/fuzzing.rst
> +++ b/docs/devel/fuzzing.rst
> @@ -210,6 +210,51 @@ Build details:
>  - The script responsible for building the fuzzers can be found in the
>    QEMU source tree at ``scripts/oss-fuzz/build.sh``
>  
> +Building Crash Reproducers
> +-----------------------------------------
> +When we find a crash, we should try to create an independent reproducer, that
> +can be used on a non-fuzzer build of QEMU. This filters out any potential
> +false-positives, and improves the debugging experience for developers.
> +Here are the steps for building a reproducer for a crash found by the
> +generic-fuzz target.
> + - Ensure the crash reproduces::
> +   qemu-fuzz-i386 --fuzz-target... ./crash-...
> +
> + - Gather the QTest output for the crash::
> +   QEMU_FUZZ_TIMEOUT=0 QTEST_LOG=1 FUZZ_SERIALIZE_QTEST=1 \
> +   qemu-fuzz-i386 --fuzz-target... ./crash-... &> /tmp/trace
> +
> + - Reorder and clean-up the resulting trace::
> +   scripts/oss-fuzz/reorder_fuzzer_qtest_trace.py /tmp/trace > /tmp/reproducer
> +
> + - Get the arguments needed to start qemu, and provide a path to qemu::
> +   less /tmp/trace # The args should be logged at the top of this file
> +   export QEMU_ARGS="-machine ..."
> +   export QEMU_PATH="path/to/qemu-system"
> +
> + - Ensure the crash reproduces in qemu-system::
> +   $QEMU_PATH $QEMU_ARGS -qtest stdio < /tmp/reproducer
> +
> + - From the crash output, obtain some string that identifies the crash. This
> +   can be a line in the stack-trace, for example::
> +   export CRASH_TOKEN="hw/usb/hcd-xhci.c:1865"
> +
> + - Minimize the reproducer::
> +   scripts/oss-fuzz/minimize_qtest_trace.py -M1 -M2 \
> +   /tmp/reproducer /tmp/reproducer-minimized
> +
> + - Confirm that the minimized reproducer still crashes::
> +   $QEMU_PATH $QEMU_ARGS -qtest stdio < /tmp/reproducer-minimized
> +
> + - Create a one-liner reproducer that can be sent over email::
> +   ./scripts/oss-fuzz/output_reproducer.py -bash /tmp/reproducer-minimized
> +
> + - Output the C source code for a test case that will reproduce the bug ::
> +   ./scripts/oss-fuzz/output_reproducer.py -owner "John Smith <john@smith.com>"\
> +    -name "test_function_name" /tmp/reproducer-minimized
> +
> + - Report the bug and send a patch with the C reproducer upstream
> +
>  Implementation Details / Fuzzer Lifecycle
>  -----------------------------------------
>  
> -- 
> 2.28.0