Re: [PATCH v2 07/15] fuzz: Add support for custom crossover functions

[Darren Kenny]

On Wednesday, 2020-08-19 at 02:11:02 -04, Alexander Bulekov wrote:
> libfuzzer supports a "custom crossover function". Libfuzzer often tries
> to blend two inputs to create a new interesting input. Sometimes, we
> have a better idea about how to blend inputs together. This change
> allows fuzzers to specify a custom function for blending two inputs
> together.
>
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>

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

> ---
>  tests/qtest/fuzz/fuzz.c | 13 +++++++++++++
>  tests/qtest/fuzz/fuzz.h | 26 ++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+)
>
> diff --git a/tests/qtest/fuzz/fuzz.c b/tests/qtest/fuzz/fuzz.c
> index 8234b68754..248fab5f37 100644
> --- a/tests/qtest/fuzz/fuzz.c
> +++ b/tests/qtest/fuzz/fuzz.c
> @@ -118,6 +118,19 @@ static FuzzTarget *fuzz_get_target(char* name)
>  }
>  
>  
> +/* Sometimes called by libfuzzer to mutate two inputs into one */
> +size_t LLVMFuzzerCustomCrossOver(const uint8_t *data1, size_t size1,
> +                                 const uint8_t *data2, size_t size2,
> +                                 uint8_t *out, size_t max_out_size,
> +                                 unsigned int seed)
> +{
> +    if(fuzz_target->crossover) {
> +        return fuzz_target->crossover(data1, size1, data2, size2, out,
> +                                      max_out_size, seed);
> +    }
> +    return 0;
> +}
> +
>  /* Executed for each fuzzing-input */
>  int LLVMFuzzerTestOneInput(const unsigned char *Data, size_t Size)
>  {
> diff --git a/tests/qtest/fuzz/fuzz.h b/tests/qtest/fuzz/fuzz.h
> index 9ca3d107c5..d36642b5ec 100644
> --- a/tests/qtest/fuzz/fuzz.h
> +++ b/tests/qtest/fuzz/fuzz.h
> @@ -77,6 +77,28 @@ typedef struct FuzzTarget {
>       */
>      void(*fuzz)(QTestState *, const unsigned char *, size_t);
>  
> +    /*
> +     * The fuzzer can specify a "Custom Crossover" function for combining two
> +     * inputs from the corpus. This function is sometimes called by libfuzzer
> +     * when mutating inputs.
> +     *
> +     * data1: location of first input
> +     * size1: length of first input
> +     * data1: location of second input
> +     * size1: length of second input
> +     * out: where to place the resulting, mutated input
> +     * max_out_size: the maximum length of the input that can be placed in 
> out
> +     * seed: the seed that should be used to make mutations deterministic, 
> when needed
> +     *
> +     * See libfuzzer's LLVMFuzzerCustomCrossOver API for more info.
> +     *
> +     * Can be NULL
> +     */
> +    size_t(*crossover)(const uint8_t *data1, size_t size1,
> +                       const uint8_t *data2, size_t size2,
> +                       uint8_t *out, size_t max_out_size,
> +                       unsigned int seed);
> +
>  } FuzzTarget;
>  
>  void flush_events(QTestState *);
> @@ -91,6 +113,10 @@ void fuzz_qtest_set_serialize(bool option);
>   */
>  void fuzz_add_target(const FuzzTarget *target);
>  
> +size_t LLVMFuzzerCustomCrossOver(const uint8_t *data1, size_t size1,
> +                                 const uint8_t *data2, size_t size2,
> +                                 uint8_t *out, size_t max_out_size,
> +                                 unsigned int seed);
>  int LLVMFuzzerTestOneInput(const unsigned char *Data, size_t Size);
>  int LLVMFuzzerInitialize(int *argc, char ***argv, char ***envp);
>  
> -- 
> 2.27.0