Re: [Xen-devel] [PATCH] fuzz/x86_emulate: fix bounds for input size

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Feb 23, 2018 at 12:57:26AM +0100, Paul Semel wrote:
> The minimum size for the input size was set to DATA_OFFSET + 1 which was 
> meaning
> that we were requesting at least one character of the data array to be filled.
> This is not needed for the fuzzer to get working correctly.

Sorry, I don't follow -- what do you expect the emulator to do if there
is no instruction to emulate?

> 
> The maximum size for the input size was set to INPUT_SIZE, which is actually
> the size of the data array inside the fuzz_corpus structure and so was not
> abling user (or AFL) to fill in the whole structure. Changing to
> sizeof(struct fuzz_corpus) correct this problem.
> 
> Signed-off-by: Paul Semel <semelpaul@xxxxxxxxx>
> ---
>  tools/fuzz/x86_instruction_emulator/fuzz-emul.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c 
> b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
> index 964682aa1a..f3ce2e7e27 100644
> --- a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
> +++ b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
> @@ -33,6 +33,7 @@ struct fuzz_corpus
>      unsigned char data[INPUT_SIZE];
>  } input;
>  #define DATA_OFFSET offsetof(struct fuzz_corpus, data)
> +#define FUZZ_CORPUS_SIZE (sizeof(struct fuzz_corpus))
>  
>  /*
>   * Internal state of the fuzzing harness.  Calculated initially from the 
> input
> @@ -822,13 +823,13 @@ int LLVMFuzzerTestOneInput(const uint8_t *data_p, 
> size_t size)
>      /* Reset all global state variables */
>      memset(&input, 0, sizeof(input));
>  
> -    if ( size <= DATA_OFFSET )
> +    if ( size < DATA_OFFSET )
>      {
>          printf("Input too small\n");
>          return 1;
>      }
>  
> -    if ( size > INPUT_SIZE )
> +    if ( size > FUZZ_CORPUS_SIZE )
>      {
>          printf("Input too large\n");
>          return 1;
> @@ -859,9 +860,9 @@ int LLVMFuzzerTestOneInput(const uint8_t *data_p, size_t 
> size)
>  
>  unsigned int fuzz_minimal_input_size(void)
>  {
> -    BUILD_BUG_ON(DATA_OFFSET > INPUT_SIZE);
> +    BUILD_BUG_ON(DATA_OFFSET > FUZZ_CORPUS_SIZE);

Thinking more about it, this BUILD_BUG_ON is probably irrelevant
nowadays because we've opted to use struct fuzz_corpus instead of a
bunch of data structures (when the fuzzer was first implemented). I
don't think we will go back to the old model in the future so deleting
this BUILD_BUG_ON should be fine.

Wei.

>  
> -    return DATA_OFFSET + 1;
> +    return DATA_OFFSET;
>  }
>  
>  /*
> -- 
> 2.16.1
> 

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