[PATCH] fuzz: add information about useful libFuzzer flags

[PATCH] fuzz: add information about useful libFuzzer flags

[Alexander Bulekov]

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 docs/devel/fuzzing.txt | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/docs/devel/fuzzing.txt b/docs/devel/fuzzing.txt
index 324d2cd92b..3458763e53 100644
--- a/docs/devel/fuzzing.txt
+++ b/docs/devel/fuzzing.txt
@@ -45,6 +45,39 @@ Information about these is available by passing -help=1
 Now the only thing left to do is wait for the fuzzer to trigger potential
 crashes.
 
+== Useful libFuzzer flags ==
+
+As mentioned above, libFuzzer accepts some arguments. Passing -help=1 will list
+the available arguments. In particular, these arguments might be helpful:
+
+$CORPUS_DIR/ : Specify a directory as the last argument to libFuzzer. libFuzzer
+stores each "interesting" input in this corpus directory. The next time you run
+libFuzzer, it will read all of the inputs from the corpus, and continue fuzzing
+from there.
+
+-max_len=4096 : specify the maximum byte-length of the inputs libFuzzer will
+generate.
+
+-close_fd_mask={1,2,3} : close, stderr, or both. Useful for targets that
+trigger many debug/error messages, or create output on the serial console.
+
+-jobs=4 -workers=4 : These arguments configure libFuzzer to run 4 fuzzers in
+parallel. Replace 4 with a number appropriate for your machine. Make sure to
+specify a $CORPUS_DIR, which will allow the parallel fuzzers to share
+information about the interesting inputs they find.
+
+-use_value_profile=1 : For each comparison operation, libFuzzer computes 
+(caller_pc&4095) | (popcnt(Arg1 ^ Arg2) << 12) and places this in the coverage
+table. Useful for targets with "magic" constants. If Arg1 came from the fuzzer's
+input and Arg2 is a magic constant, then each time the hamming distance
+between Arg1 and Arg2 decreases, libFuzzer adds the input to the corpus.
+
+-shrink=1 : Tries to make elements of the corpus "smaller". Might lead to
+better coverage performance, depending on the target.
+
+Note that libFuzzer's exact behavior will depend on the version of
+clang and libFuzzer used to build the device fuzzers.
+
 == Adding a new fuzzer ==
 Coverage over virtual devices can be improved by adding additional fuzzers.
 Fuzzers are kept in tests/qtest/fuzz/ and should be added to
-- 
2.26.2

Re: [PATCH] fuzz: add information about useful libFuzzer flags

[Philippe Mathieu-Daudé]

On 5/24/20 4:37 PM, Alexander Bulekov wrote:
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  docs/devel/fuzzing.txt | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/docs/devel/fuzzing.txt b/docs/devel/fuzzing.txt
> index 324d2cd92b..3458763e53 100644
> --- a/docs/devel/fuzzing.txt
> +++ b/docs/devel/fuzzing.txt
> @@ -45,6 +45,39 @@ Information about these is available by passing -help=1
>  Now the only thing left to do is wait for the fuzzer to trigger potential
>  crashes.
>  
> +== Useful libFuzzer flags ==
> +
> +As mentioned above, libFuzzer accepts some arguments. Passing -help=1 will list
> +the available arguments. In particular, these arguments might be helpful:
> +
> +$CORPUS_DIR/ : Specify a directory as the last argument to libFuzzer. libFuzzer
> +stores each "interesting" input in this corpus directory. The next time you run
> +libFuzzer, it will read all of the inputs from the corpus, and continue fuzzing
> +from there.

Maybe describe we can use multiple directories, the first one is where
new inputs are written.

> +
> +-max_len=4096 : specify the maximum byte-length of the inputs libFuzzer will
> +generate.
> +
> +-close_fd_mask={1,2,3} : close, stderr, or both. Useful for targets that
> +trigger many debug/error messages, or create output on the serial console.
> +
> +-jobs=4 -workers=4 : These arguments configure libFuzzer to run 4 fuzzers in
> +parallel.

The difference between job/worker is not clear...

> Replace 4 with a number appropriate for your machine. Make sure to
> +specify a $CORPUS_DIR, which will allow the parallel fuzzers to share
> +information about the interesting inputs they find.
> +
> +-use_value_profile=1 : For each comparison operation, libFuzzer computes 
> +(caller_pc&4095) | (popcnt(Arg1 ^ Arg2) << 12) and places this in the coverage
> +table. Useful for targets with "magic" constants. If Arg1 came from the fuzzer's
> +input and Arg2 is a magic constant, then each time the hamming distance

"Hamming" (lastname).

Thanks!

> +between Arg1 and Arg2 decreases, libFuzzer adds the input to the corpus.
> +
> +-shrink=1 : Tries to make elements of the corpus "smaller". Might lead to
> +better coverage performance, depending on the target.
> +
> +Note that libFuzzer's exact behavior will depend on the version of
> +clang and libFuzzer used to build the device fuzzers.
> +
>  == Adding a new fuzzer ==
>  Coverage over virtual devices can be improved by adding additional fuzzers.
>  Fuzzers are kept in tests/qtest/fuzz/ and should be added to
>