[libFuzzer] Improve documentation

[kcc]

This is very cool, thanks!
I've made several minor comments.
The patch does not apply to ToT (arc patch D19585 says 2 out of 14 hunks FAILED), for some reason, you may need to svn up / merge.

docs/LibFuzzer.rst

2

For no particular reason I prefer libFuzzer.

96

via a specific fuzzing entrypoint (aka "target function");

97

s/generations/generates

107–108

I don't want to be this verbose.
Instead we should clearly state that the only supported variant (at least at this point) is trunk (or something very close to trunk).

307

(unless you disable this by `-reload=0` command line flag)

320

s/blocks/blocks or edges

330

Remove outer ()
Add something like "in this mode the fuzzer binary could be used as a regression test, i.e. on a continuous integration system"

377–380

why not svn?

629–630

While you are at it, remove this part (about function-scope statics).
It's not that bad in fact.

[daviddrysdale]

Markups from kcc initial pass. Merged up to later upstream version.

[daviddrysdale]

In D19585#413936, @kcc wrote:

This is very cool, thanks!
I've made several minor comments.
The patch does not apply to ToT (arc patch D19585 says 2 out of 14 hunks FAILED), for some reason, you may need to svn up / merge.

Sorry, forgot to merge up first. BTW, does Phabricator show whether the patch applies cleanly somewhere? I can't immediately see it in the UI.

docs/LibFuzzer.rst

2

OK, but how do you want to handle sentences that start with the name -- capitalize or not?

(At the moment I've left sentence starts as "LibFuzzer does xyz...")

377–380

When I tried it, the current SVN tip wouldn't build, so I thought it safer to stick with a fixed release -- less vulnerable that way.

[kcc]

Thanks again, let me commit this.

docs/LibFuzzer.rst

-========================================================
-LibFuzzer -- a library for coverage-guided fuzz testing.
-========================================================
+=======================================================
+libFuzzer – a library for coverage-guided fuzz testing.
+=======================================================
 .. contents::
    :local:
    :depth: 1
 
 Introduction
 ============
 
-libFuzzer -- library for in-process evolutionary fuzzing of other libraries.
+LibFuzzer is a library for in-process, coverage-guided, evolutionary fuzzing
+of other libraries.
 
-The typical workflow looks like the following.
-First, implement a fuzzing target function, like this::
+LibFuzzer is similar in concept to American Fuzzy Lop (AFL_), but it performs
+all of its fuzzing inside a single process.  This in-process fuzzing can be more
+restrictive and fragile, but is potentially much faster as there is no overhead
+for process start-up.
+
+The fuzzer is linked with the library under test, and feeds fuzzed inputs to the
+library via a specific fuzzing entrypoint (aka "target function"); the fuzzer
+then tracks which areas of the code are reached, and generates mutations on the
+corpus of input data in order to maximize the code coverage.  The code coverage
+information for libFuzzer is provided by LLVM's SanitizerCoverage_
+instrumentation.
+
+
+Versions
+========
+
+LibFuzzer is under active development so a current (or at least very recent)
+version of Clang is the only supported variant.
+
+(If `building Clang from trunk`_ is too time-consuming or difficult, then
+the Clang binaries that the Chromium developers build are likely to be
+fairly recent:
+
+.. code-block:: console
+
+  mkdir TMP_CLANG
+  cd TMP_CLANG
+  git clone https://chromium.googlesource.com/chromium/src/tools/clang
+  cd ..
+  TMP_CLANG/clang/scripts/update.py
+
+This installs the Clang binary as
+``./third_party/llvm-build/Release+Asserts/bin/clang``)
+
+The libFuzzer code resides in the LLVM repository, and requires a recent Clang
+compiler to build (and is used to `fuzz various parts of LLVM itself`_).
+However the fuzzer itself does not (and should not) depend on any part of LLVM
+infrastructure and can be used for other projects without requiring the rest
+of LLVM.
+
+
+Corpus
+======
+
+Coverage-guided fuzzers like libFuzzer rely on a corpus of sample inputs for the
+code under test.  This corpus should ideally be seeded with a varied collection
+of valid and invalid inputs for the code under test; for example, for a graphics
+library the initial corpus might hold a variety of different small PNG/JPG/GIF
+files.  The fuzzer generates random mutations based around the sample inputs in
+the current corpus.  If a mutation triggers execution of a previously-uncovered
+path in the code under test, then that mutation is saved to the corpus for
+future variations.
+
+LibFuzzer will work fine without any initial seeds, but will be less
+efficient.   In particular, if the library under test accepts complex,
+structured inputs then starting from a varied corpus is very important.
+
+The corpus can also act as a sanity/regression check, to confirm that the
+fuzzing entrypoint still works and that all of the sample inputs run through
+the code under test without problems.
+
+
+Getting Started
+===============
+
+.. contents::
+   :local:
+   :depth: 1
+
+Building
+--------
+
+The first step for using libFuzzer on a library is to implement a fuzzing
+target function that accepts a sequence of bytes, like this:
+
+.. code-block:: c++
 
   // fuzz_target.cc
   extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
     DoSomethingInterestingWithMyAPI(Data, Size);
     return 0;  // Non-zero return values are reserved for future use.
   }
 
-Next, build the Fuzzer library as a static archive. Note that libFuzzer contains the `main()` function::
+Next, build the libFuzzer library as a static archive, without any sanitizer
+options. Note that the libFuzzer library contains the ``main()`` function:
+
+.. code-block:: console
 
   svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
   # Alternative: get libFuzzer from a dedicated git mirror:
   # git clone https://chromium.googlesource.com/chromium/llvm-project/llvm/lib/Fuzzer
   clang++ -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
   ar ruv libFuzzer.a Fuzzer*.o
 
-Then build the target function and the library you are going to test.
-You should use SanitizerCoverage_ and one of ASan, MSan, or UBSan.
-Link it with `libFuzzer.a`::
+Then build the fuzzing target function and the library under test using
+the SanitizerCoverage_ option, which instruments the code so that the fuzzer
+can retrieve code coverage information (to guide the fuzzing).  Linking with
+the libFuzzer code then gives an fuzzer executable.
+
+You should also enable one or more of the *sanitizers*, which help to expose
+latent bugs by making incorrect behavior generate errors at runtime:
+
+ - AddressSanitizer_ detects memory access errors.
+ - MemorySanitizer_ detects uninitialized reads: code whose behavior relies on memory
+   contents that have not been initialized to a specific value.
+ - UndefinedBehaviorSanitizer_ detects the use of various features of C/C++ that are explicitly
+   listed as resulting in undefined behavior.
+
+Finally, link with ``libFuzzer.a``::
 
   clang -fsanitize-coverage=edge -fsanitize=address your_lib.cc fuzz_target.cc libFuzzer.a -o my_fuzzer
 
-Create a directory with the initial "seed" samlpes.
-For some input types libFuzzer will work just fine w/o any seeds,
-but for complex inputs this step is very important::
+Running
+-------
+
+To run the fuzzer, first create a Corpus_ directory that holds the
+initial "seed" sample inputs:
+
+.. code-block:: console
 
   mkdir CORPUS_DIR
   cp /some/input/samples/* CORPUS_DIR
 
-Finally, run the fuzzer on the `CORPUS_DIR`::
+Then run the fuzzer on the corpus directory:
 
-  ./my_fuzzer CORPUS_DIR  # -max_len=1000 -jobs=20 -more_lags=...
+.. code-block:: console
 
+  ./my_fuzzer CORPUS_DIR  # -max_len=1000 -jobs=20 ...
 
-As new interesting test cases are discovered they will be added to the corpus.
-If a bug is discovered by the sanitizer (ASan, etc) it will be reported as usual and the reproducer
-will be written to disk.
-Each Fuzzer process is single-threaded (unless the library starts its own
-threads). You can run the libFuzzer on the same corpus in multiple processes
-in parallel (use the flags `-jobs=N` and `-workers=N`).
-
-libFuzzer is similar in concept to AFL_,
-but uses in-process Fuzzing, which is more fragile and restrictive, but
-potentially much faster as it has no overhead for process start-up.
-It uses LLVM's SanitizerCoverage_ instrumentation to get in-process
-coverage-feedback
-
-The code resides in the LLVM repository,
-requires the fresh Clang compiler to build
-and is used to fuzz various parts of LLVM,
-but the Fuzzer itself does not (and should not) depend on any
-part of LLVM and can be used for other projects w/o requiring the rest of LLVM.
+As the fuzzer discovers new interesting test cases (i.e. test cases that
+trigger coverage of new paths through the code under test), those test cases
+will be added to the corpus directory.
 
-Fresh Clang
------------
+By default, the fuzzing process will continue indefinitely – at least until
+a bug is found.  Any crashes or sanitizer failures will be reported as usual,
+stopping the fuzzing process, and the particular input that triggered the bug
+will be written to disk (typically as ``crash-<sha1>`` or ``timeout-<sha1>``).
 
-If you don't know where to get the fresh Clang binaries and don't want to build
-it from trunk (why wouldn't you?) you may grab the fresh Clang binaries
-maintained by the Chromium developers::
 
-  mkdir TMP_CLANG
-  cd TMP_CLANG
-  git clone https://chromium.googlesource.com/chromium/src/tools/clang
-  cd ..
-  TMP_CLANG/clang/scripts/update.py
+Parallel Fuzzing
+----------------
 
-This will install a reasonably fresh and well tested clang binaries as
-`third_party/llvm-build/Release+Asserts/bin/clang`
+Each libFuzzer process is single-threaded, unless the library under test starts
+its own threads.  However, it is possible to run multiple libFuzzer processes in
+parallel with a shared corpus directory; this has the advantage that any new
+inputs found by one fuzzer process will be available to the other fuzzer
+processes (unless the ``-reload=0`` option is set).
 
-Usage
-=====
-To run fuzzing pass 0 or more directories. New samples will be written into `dir1`, other directories will be read once during startup.::
+This is primarily controlled by the ``-jobs=N`` option, which indicates that
+that `N` fuzzing jobs should be run to completion (i.e. until a bug is found or
+time/iteration limits are reached).  These jobs will be run across a set of
+worker processes, by default using half of the available CPU cores; the count of
+worker processes can be overridden by the ``-workers=N`` option.  For example,
+running with ``-jobs=30`` on a 12-core machine would run 6 workers by default,
+with each worker averaging 5 bugs by completion of the entire process.
 
-./fuzzer [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]
 
-To run individual tests without fuzzing pass 1 or more files::
+Options
+=======
 
-./fuzzer [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...]
+To run the fuzzer, pass zero or more corpus directories as command line
+arguments.  The fuzzer will read test inputs from each of these corpus
+directories, and any new test inputs that are generated will be written
+back to the first corpus directory:
 
-The most important flags are::
+.. code-block:: console
 
-  seed                                0 Random seed. If 0, seed is generated.
-  runs                                -1  Number of individual test runs (-1 for infinite runs).
-  max_len                               0       Maximum length of the test input. If 0, libFuzzer tries to guess a good value based on the corpus and reports it.
-  timeout                             1200  Timeout in seconds (if positive). If one unit runs more than this number of seconds the process will abort.
-  timeout_exitcode                     77       Unless abort_on_timeout is set, use this exitcode on timeout.
-  max_total_time                        0       If positive, indicates the maximal total time in seconds to run the fuzzer.
-  help                                0 Print help.
-  merge                                 0       If 1, the 2-nd, 3-rd, etc corpora will be merged into the 1-st corpus. Only interesting units will be taken.
-  jobs                                 0  Number of jobs to run. If jobs >= 1 we spawn this number of jobs in separate worker processes with stdout/stderr redirected to fuzz-JOB.log.
-  workers                             0 Number of simultaneous worker processes to run the jobs. If zero, "min(jobs,NumberOfCpuCores()/2)" is used.
-  use_traces                            0       Experimental: use instruction traces
-  only_ascii                            0       If 1, generate only ASCII (isprint+isspace) inputs.
-  artifact_prefix                       ""      Write fuzzing artifacts (crash, timeout, or slow inputs) as $(artifact_prefix)file
-  exact_artifact_path                   ""      Write the single artifact on failure (crash, timeout) as $(exact_artifact_path). This overrides -artifact_prefix and will not use checksum in the file name. Do not use the same path for several parallel processes.
-  print_final_stats                     0       If 1, print statistics at exit.
-  close_fd_mask                         0       If 1, close stdout at startup; if 2, close stderr; if 3, close both. Be careful, this will also close e.g. asan's stderr/stdout.
+  ./fuzzer [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]
+
+If a list of files (rather than directories) are passed to the fuzzer program,
+then it will re-run those files as test inputs but will not perform any fuzzing.
+In this mode the fuzzer binary can be used as a regression test (e.g. on a
+continuous integration system) to check the target function and saved inputs
+still work.
+
+The most important command line options are:
+
+``-help``
+  Print help message.
+``-seed``
+  Random seed. If 0 (the default), the seed is generated.
+``-runs``
+  Number of individual test runs, -1 (the default) to run indefinitely.
+``-max_len``
+  Maximum length of a test input. If 0 (the default), libFuzzer tries to guess
+  a good value based on the corpus (and reports it).
+``-timeout``
+  Timeout in seconds, default 1200. If an input takes longer than this timeout,
+  the process is treated as a failure case.
+``-timeout_exitcode``
+  Exit code (default 77) to emit when terminating due to timeout, when
+  ``-abort_on_timeout`` is not set.
+``-max_total_time``
+  If positive, indicates the maximum total time in seconds to run the fuzzer.
+  If 0 (the default), run indefinitely.
+``-merge``
+  If set to 1, any corpus inputs from the 2nd, 3rd etc. corpus directories
+  that trigger new code coverage will be merged into the first corpus
+  directory.  Defaults to 0.
+``-reload``
+  If set to 1 (the default), the corpus directory is re-read periodically to
+  check for new inputs; this allows detection of new inputs that were discovered
+  by other fuzzing processes.
+``-jobs``
+  Number of fuzzing jobs to run to completion. Default value is 0, which runs a
+  single fuzzing process until completion.  If the value is >= 1, then this
+  number of jobs performing fuzzing are run, in a collection of parallel
+  separate worker processes; each such worker process has its
+  ``stdout``/``stderr`` redirected to ``fuzz-<JOB>.log``.
+``-workers``
+  Number of simultaneous worker processes to run the fuzzing jobs to completion
+  in. If 0 (the default), ``min(jobs, NumberOfCpuCores()/2)`` is used.
+``-dict``
+  Provide a dictionary of input keywords; see Dictionaries_.
+``-use_counters``
+  Use `coverage counters`_ to generate approximate counts of how often code
+  blocks are hit; defaults to 1.
+``-use_traces``
+  Use instruction traces (experimental, defaults to 0); see `Data-flow-guided fuzzing`_.
+``-only_ascii``
+  If 1, generate only ASCII (``isprint``+``isspace``) inputs. Defaults to 0.
+``-artifact_prefix``
+  Provide a prefix to use when saving fuzzing artifacts (crash, timeout, or
+  slow inputs) as ``$(artifact_prefix)file``.  Defaults to empty.
+``-exact_artifact_path``
+  Ignored if empty (the default).  If non-empty, write the single artifact on
+  failure (crash, timeout) as ``$(exact_artifact_path)``. This overrides
+  ``-artifact_prefix`` and will not use checksum in the file name. Do not use
+  the same path for several parallel processes.
+``-print_final_stats``
+  If 1, print statistics at exit.  Defaults to 0.
+``-close_fd_mask``
+  Indicate output streams to close at startup. Be careful, this will also
+  remove diagnostic output from the tools in use; for example the messages
+  AddressSanitizer_ sends to ``stderr``/``stdout`` will also be lost.
+
+   - 0 (default): close neither ``stdout`` nor ``stderr``
+   - 1 : close ``stdout``
+   - 2 : close ``stderr``
+   - 3 : close both ``stdout`` and ``stderr``.
 
 For the full list of flags run the fuzzer binary with ``-help=1``.
 
-Usage examples
-==============
+Output
+======
+
+During operation the fuzzer prints information to ``stderr``, for example::
+
+  INFO: Seed: 3338750330
+  Loaded 1024/1211 files from corpus/
+  INFO: -max_len is not provided, using 64
+  #0  READ   units: 1211 exec/s: 0
+  #1211 INITED cov: 2575 bits: 8855 indir: 5 units: 830 exec/s: 1211
+  #1422 NEW    cov: 2580 bits: 8860 indir: 5 units: 831 exec/s: 1422 L: 21 MS: 1 ShuffleBytes-
+  #1688 NEW    cov: 2581 bits: 8865 indir: 5 units: 832 exec/s: 1688 L: 19 MS: 2 EraseByte-CrossOver-
+  #1734 NEW    cov: 2583 bits: 8879 indir: 5 units: 833 exec/s: 1734 L: 27 MS: 3 ChangeBit-EraseByte-ShuffleBytes-
+  ...
+
+The early parts of the output include information about the fuzzer options and
+configuration, including the current random seed (in the ``Seed:`` line; this
+can be overridden with the ``-seed=N`` flag).
+
+Further output lines have the form of an event code and statistics.  The
+possible event codes are:
+
+``READ``
+  The fuzzer has read in all of the provided input samples from the corpus
+  directories.
+``INITED``
+  The fuzzer has completed initialization, which includes running each of
+  the initial input samples through the code under test.
+``NEW``
+  The fuzzer has created a test input that covers new areas of the code
+  under test.  This input will be saved to the primary corpus directory.
+``pulse``
+  The fuzzer has generated 2\ :sup:`n` inputs (generated periodically to reassure
+  the user that the fuzzer is still working).
+``DONE``
+  The fuzzer has completed operation because it has reached the specified
+  iteration limit (``-runs``) or time limit (``-max_total_time``).
+``MIN<n>``
+  The fuzzer is minimizing the combination of input corpus directories into
+  a single unified corpus (due to the ``-merge`` command line option).
+``RELOAD``
+  The fuzzer is performing a periodic reload of inputs from the corpus
+  directory; this allows it to discover any inputs discovered by other
+  fuzzer processes (see `Parallel Fuzzing`_).
+
+Each output line also reports the following statistics (when non-zero):
+
+``cov:``
+  Total number of code blocks or edges covered by the executing the current
+  corpus.
+``bits:``
+  Rough measure of the number of code blocks or edges covered, and how often;
+  only valid if the fuzzer is run with ``-use_counters=1``.
+``indir:``
+  Number of distinct function `caller-callee pairs`_ executed with the
+  current corpus; only valid if the code under test was built with
+  ``-fsanitize-coverage=indirect-calls``.
+``units:``
+  Number of entries in the current input corpus.
+``exec/s:``
+  Number of fuzzer iterations per second.
+
+For ``NEW`` events, the output line also includes information about the mutation
+operation that produced the new input:
+
+``L:``
+  Size of the new input in bytes.
+``MS: <n> <operations>``
+  Count and list of the mutation operations used to generate the input.
+
+
+Examples
+========
 .. contents::
    :local:
    :depth: 1
 
 Toy example
 -----------
 
-A simple function that does something interesting if it receives the input "HI!"::
+A simple function that does something interesting if it receives the input
+"HI!"::
 
   cat << EOF >> test_fuzzer.cc
   #include <stdint.h>
   #include <stddef.h>
   extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     if (size > 0 && data[0] == 'H')
       if (size > 1 && data[1] == 'I')
          if (size > 2 && data[2] == '!')
          __builtin_trap();
     return 0;
   }
   EOF
   # Build test_fuzzer.cc with asan and link against libFuzzer.a
   clang++ -fsanitize=address -fsanitize-coverage=edge test_fuzzer.cc libFuzzer.a
   # Run the fuzzer with no corpus.
   ./a.out
 
 You should get an error pretty quickly::
 
   #0  READ   units: 1 exec/s: 0
   #1  INITED cov: 3 units: 1 exec/s: 0
-  #2  NEW    cov: 5 units: 2 exec/s: 0 L: 64 MS: 0 
-  #19237  NEW    cov: 9 units: 3 exec/s: 0 L: 64 MS: 0 
+  #2  NEW    cov: 5 units: 2 exec/s: 0 L: 64 MS: 0
+  #19237  NEW    cov: 9 units: 3 exec/s: 0 L: 64 MS: 0
   #20595  NEW    cov: 10 units: 4 exec/s: 0 L: 1 MS: 4 ChangeASCIIInt-ShuffleBytes-ChangeByte-CrossOver-
   #34574  NEW    cov: 13 units: 5 exec/s: 0 L: 2 MS: 3 ShuffleBytes-CrossOver-ChangeBit-
   #34807  NEW    cov: 15 units: 6 exec/s: 0 L: 3 MS: 1 CrossOver-
   ==31511== ERROR: libFuzzer: deadly signal
   ...
   artifact_prefix='./'; Test unit written to ./crash-b13e8756b13a00cf168300179061fb4b91fefbed
 
 
 PCRE2
 -----
 
 Here we show how to use libFuzzer on something real, yet simple: pcre2_::
 
   COV_FLAGS=" -fsanitize-coverage=edge,indirect-calls,8bit-counters"
   # Get PCRE2
-  svn co svn://vcs.exim.org/pcre2/code/trunk pcre
-  # Build PCRE2 with AddressSanitizer and coverage.
-  (cd pcre; ./autogen.sh; CC="clang -fsanitize=address $COV_FLAGS" ./configure --prefix=`pwd`/../inst && make -j && make install)
+  wget ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/pcre2-10.20.tar.gz
+  tar xf pcre2-10.20.tar.gz
+  # Build PCRE2 with AddressSanitizer and coverage; requires autotools.
+  (cd pcre2-10.20; ./autogen.sh; CC="clang -fsanitize=address $COV_FLAGS" ./configure --prefix=`pwd`/../inst && make -j && make install)
   # Build the fuzzing target function that does something interesting with PCRE2.
   cat << EOF > pcre_fuzzer.cc
   #include <string.h>
   #include <stdint.h>
   #include "pcre2posix.h"
   extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     if (size < 1) return 0;
     char *str = new char[size+1];
     memcpy(str, data, size);
     str[size] = 0;
     regex_t preg;
     if (0 == regcomp(&preg, str, 0)) {
       regexec(&preg, str, 0, 0, 0);
       regfree(&preg);
     }
     delete [] str;
     return 0;
   }
   EOF
   clang++ -g -fsanitize=address $COV_FLAGS -c -std=c++11  -I inst/include/ pcre_fuzzer.cc
   # Link.
   clang++ -g -fsanitize=address -Wl,--whole-archive inst/lib/*.a -Wl,-no-whole-archive libFuzzer.a pcre_fuzzer.o -o pcre_fuzzer
 
 This will give you a binary of the fuzzer, called ``pcre_fuzzer``.
-Now, create a directory that will hold the test corpus::
+Now, create a directory that will hold the test corpus:
+
+.. code-block:: console
 
   mkdir -p CORPUS
 
 For simple input languages like regular expressions this is all you need.
-For more complicated inputs populate the directory with some input samples.
-Now run the fuzzer with the corpus dir as the only parameter::
+For more complicated/structured inputs, the fuzzer works much more efficiently
+if you can populate the corpus directory with a variety of valid and invalid
+inputs for the code under test.
+Now run the fuzzer with the corpus directory as the only parameter:
+
+.. code-block:: console
 
   ./pcre_fuzzer ./CORPUS
 
-You will see output like this::
+Initially, you will see Output_ like this::
 
-  Seed: 1876794929
-  #0      READ   cov 0 bits 0 units 1 exec/s 0
-  #1      pulse  cov 3 bits 0 units 1 exec/s 0
-  #1      INITED cov 3 bits 0 units 1 exec/s 0
-  #2      pulse  cov 208 bits 0 units 1 exec/s 0
-  #2      NEW    cov 208 bits 0 units 2 exec/s 0 L: 64
-  #3      NEW    cov 217 bits 0 units 3 exec/s 0 L: 63
-  #4      pulse  cov 217 bits 0 units 3 exec/s 0
-
-* The ``Seed:`` line shows you the current random seed (you can change it with ``-seed=N`` flag).
-* The ``READ``  line shows you how many input files were read (since you passed an empty dir there were inputs, but one dummy input was synthesised).
-* The ``INITED`` line shows you that how many inputs will be fuzzed.
-* The ``NEW`` lines appear with the fuzzer finds a new interesting input, which is saved to the CORPUS dir. If multiple corpus dirs are given, the first one is used.
-* The ``pulse`` lines appear periodically to show the current status.
+  INFO: Seed: 2938818941
+  INFO: -max_len is not provided, using 64
+  INFO: A corpus is not provided, starting from an empty corpus
+  #0  READ   units: 1 exec/s: 0
+  #1  INITED cov: 3 bits: 3 units: 1 exec/s: 0
+  #2  NEW    cov: 176 bits: 176 indir: 3 units: 2 exec/s: 0 L: 64 MS: 0
+  #8  NEW    cov: 176 bits: 179 indir: 3 units: 3 exec/s: 0 L: 63 MS: 2 ChangeByte-EraseByte-
+  ...
+  #14004  NEW    cov: 1500 bits: 4536 indir: 5 units: 406 exec/s: 0 L: 54 MS: 3 ChangeBit-ChangeBit-CrossOver-
 
 Now, interrupt the fuzzer and run it again the same way. You will see::
 
-  Seed: 1879995378
-  #0      READ   cov 0 bits 0 units 564 exec/s 0
-  #1      pulse  cov 502 bits 0 units 564 exec/s 0
+  INFO: Seed: 3398349082
+  INFO: -max_len is not provided, using 64
+  #0  READ   units: 405 exec/s: 0
+  #405  INITED cov: 1499 bits: 4535 indir: 5 units: 286 exec/s: 0
+  #587  NEW    cov: 1499 bits: 4540 indir: 5 units: 287 exec/s: 0 L: 52 MS: 2 InsertByte-EraseByte-
+  #667  NEW    cov: 1501 bits: 4542 indir: 5 units: 288 exec/s: 0 L: 39 MS: 2 ChangeBit-InsertByte-
+  #672  NEW    cov: 1501 bits: 4543 indir: 5 units: 289 exec/s: 0 L: 15 MS: 2 ChangeASCIIInt-ChangeBit-
+  #739  NEW    cov: 1501 bits: 4544 indir: 5 units: 290 exec/s: 0 L: 64 MS: 4 ShuffleBytes-ChangeASCIIInt-InsertByte-ChangeBit-
   ...
-  #512    pulse  cov 2933 bits 0 units 564 exec/s 512
-  #564    INITED cov 2991 bits 0 units 344 exec/s 564
-  #1024   pulse  cov 2991 bits 0 units 344 exec/s 1024
-  #1455   NEW    cov 2995 bits 0 units 345 exec/s 1455 L: 49
 
-This time you were running the fuzzer with a non-empty input corpus (564 items).
-As the first step, the fuzzer minimized the set to produce 344 interesting items (the ``INITED`` line)
+On the second execution the fuzzer has a non-empty input corpus (405 items).  As
+the first step, the fuzzer minimized this corpus (the ``INITED`` line) to
+produce 286 interesting items, omitting inputs that do not hit any additional
+code.
+
+(Aside: although the fuzzer only saves new inputs that hit additional code, this
+does not mean that the corpus as a whole is kept minimized.  For example, if
+an input hitting A-B-C then an input that hits A-B-C-D are generated,
+they will both be saved, even though the latter subsumes the former.)
+
 
-You may run ``N`` independent fuzzer jobs in parallel on ``M`` CPUs::
+You may run ``N`` independent fuzzer jobs in parallel on ``M`` CPUs:
+
+.. code-block:: console
 
   N=100; M=4; ./pcre_fuzzer ./CORPUS -jobs=$N -workers=$M
 
-By default (``-reload=1``) the fuzzer processes will periodically scan the CORPUS directory
+By default (``-reload=1``) the fuzzer processes will periodically scan the corpus directory
 and reload any new tests. This way the test inputs found by one process will be picked up
 by all others.
 
 If ``-workers=$M`` is not supplied, ``min($N,NumberOfCpuCore/2)`` will be used.
 
 Heartbleed
 ----------
 Remember Heartbleed_?
 As it was recently `shown <https://blog.hboeck.de/archives/868-How-Heartbleed-couldve-been-found.html>`_,
-fuzzing with AddressSanitizer can find Heartbleed. Indeed, here are the step-by-step instructions
-to find Heartbleed with LibFuzzer::
+fuzzing with AddressSanitizer_ can find Heartbleed. Indeed, here are the step-by-step instructions
+to find Heartbleed with libFuzzer::
 
   wget https://www.openssl.org/source/openssl-1.0.1f.tar.gz
   tar xf openssl-1.0.1f.tar.gz
   COV_FLAGS="-fsanitize-coverage=edge,indirect-calls" # -fsanitize-coverage=8bit-counters
   (cd openssl-1.0.1f/ && ./config &&
     make -j 32 CC="clang -g -fsanitize=address $COV_FLAGS")
-  # Get and build LibFuzzer
+  # Get and build libFuzzer
   svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
   clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
   # Get examples of key/pem files.
   git clone   https://github.com/hannob/selftls
   cp selftls/server* . -v
   cat << EOF > handshake-fuzz.cc
   #include <openssl/ssl.h>
   #include <openssl/err.h>
Voila::
   =================================================================
   ==17488==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x629000004748 at pc 0x00000048c979 bp 0x7fffe3e864f0 sp 0x7fffe3e85ca8
   READ of size 60731 at 0x629000004748 thread T0
       #0 0x48c978 in __asan_memcpy
       #1 0x4db504 in tls1_process_heartbeat openssl-1.0.1f/ssl/t1_lib.c:2586:3
       #2 0x580be3 in ssl3_read_bytes openssl-1.0.1f/ssl/s3_pkt.c:1092:4
 
 Note: a `similar fuzzer <https://boringssl.googlesource.com/boringssl/+/HEAD/FUZZING.md>`_
-is now a part of the boringssl source tree.
+is now a part of the BoringSSL_ source tree.
 
 Advanced features
 =================
 .. contents::
    :local:
    :depth: 1
 
 Dictionaries

 It will later use those recorded inputs during mutations.
 
 This mode can be combined with DataFlowSanitizer_ to achieve better sensitivity.
 
 AFL compatibility
 -----------------
 LibFuzzer can be used together with AFL_ on the same test corpus.
 Both fuzzers expect the test corpus to reside in a directory, one file per input.
-You can run both fuzzers on the same corpus, one after another::
+You can run both fuzzers on the same corpus, one after another:
+
+.. code-block:: console
 
   ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program @@
   ./llvm-fuzz testcase_dir findings_dir  # Will write new tests to testcase_dir
 
 Periodically restart both fuzzers so that they can use each other's findings.
 Currently, there is no simple way to run both fuzzing engines in parallel while sharing the same corpus dir.
 
 How good is my fuzzer?
 ----------------------
 
 Once you implement your target function ``LLVMFuzzerTestOneInput`` and fuzz it to death,
 you will want to know whether the function or the corpus can be improved further.
 One easy to use metric is, of course, code coverage.
-You can get the coverage for your corpus like this::
+You can get the coverage for your corpus like this:
+
+.. code-block:: console
 
   ASAN_OPTIONS=coverage=1 ./fuzzer CORPUS_DIR -runs=0
 
 This will run all the tests in the CORPUS_DIR but will not generate any new tests
 and dump covered PCs to disk before exiting.
 Then you can subtract the set of covered PCs from the set of all instrumented PCs in the binary,
 see SanitizerCoverage_ for details.
 
 User-supplied mutators
 ----------------------
 
 LibFuzzer allows to use custom (user-supplied) mutators,
 see FuzzerInterface.h_
 
 Startup initialization
 ----------------------
 If the library being tested needs to be initialized, there are several options.
 
-The simplest way is to have a statically initialized global object::
+The simplest way is to have a statically initialized global object:
+
+.. code-block:: c++
 
    static bool Initialized = DoInitialization();
 
 Alternatively, you may define an optional init function and it will receive
-the program arguments that you can read and modify::
+the program arguments that you can read and modify:
+
+.. code-block:: c++
 
    extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv) {
     ReadAndMaybeModify(argc, argv);
     return 0;
    }
 
-Try to avoid initialization inside the target function itself as
-it will skew the coverage data. Don't do this::
-
-    extern "C" int LLVMFuzzerTestOneInput(...) {
-      static bool initialized = false;
-      if (!initialized) { 
-         ...
-      }
-    }
 
 Leaks
 -----
 
-When running libFuzzer with AddressSanitizer_ the latter will be able to report
-memory leaks, but only when the process exits, so if you suspect memory leaks
-in your target you should run libFuzzer with `-runs=N` or `-max_total_time=N`.
-If a leak is reported at the end, you will not get the reproducer from libFuzzer.
-You will need to re-run the target on every file in the corpus separately to
-find which one causes the leak.
+Code that has been built with AddressSanitizer_ will report memory leaks,
+but only when the process exits.  If you suspect memory leaks in the code
+under test, you will therefore need to use the ``-runs=N`` or
+``-max_total_time=N`` command line options to ensure that the fuzzing
+process completes and gives AddressSanitizer_ a chance to report leaks.
+Because the leak is only reported at the end of the process, this also means
+that it is not clear which input triggered the leak.  To narrow this down,
+re-run each input file in the corpus separately through the target function.
 
 If your target has massive leaks you will eventually run out of RAM.
 To protect your machine from OOM death you may use
-e.g. `ASAN_OPTIONS=hard_rss_limit_mb=2000` (with AddressSanitizer_).
+e.g. ``ASAN_OPTIONS=hard_rss_limit_mb=2000`` (with AddressSanitizer_).
 
-In future libFuzzer may support finding/reporting leaks better than this, stay tuned.
 
 Fuzzing components of LLVM
 ==========================
 .. contents::
    :local:
    :depth: 1
 
 clang-format-fuzzer
 -------------------
 The inputs are random pieces of C++-like text.
 
-Build (make sure to use fresh clang as the host compiler)::
+Build (make sure to use fresh clang as the host compiler):
+
+.. code-block:: console
 
     cmake -GNinja  -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=YES -DCMAKE_BUILD_TYPE=Release /path/to/llvm
     ninja clang-format-fuzzer
     mkdir CORPUS_DIR
     ./bin/clang-format-fuzzer CORPUS_DIR
 
-Optionally build other kinds of binaries (asan+Debug, msan, ubsan, etc).
+Optionally build other kinds of binaries (ASan+Debug, MSan, UBSan, etc).
 
 Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23052
 
 clang-fuzzer
 ------------
 
 The behavior is very similar to ``clang-format-fuzzer``.
 

 
 When run in dissassembly mode, the inputs are opcodes to be disassembled. The
 fuzzer will consume as many instructions as possible and will stop when it
 finds an invalid instruction or runs out of data.
 
 Please note that the command line interface differs slightly from that of other
 fuzzers. The fuzzer arguments should follow ``--fuzzer-args`` and should have
 a single dash, while other arguments control the operation mode and target in a
-similar manner to ``llvm-mc`` and should have two dashes. For example::
+similar manner to ``llvm-mc`` and should have two dashes. For example:
+
+.. code-block:: console
 
   llvm-mc-fuzzer --triple=aarch64-linux-gnu --disassemble --fuzzer-args -max_len=4 -jobs=10
 
 Buildbot
 --------
 
-We have a buildbot that runs the above fuzzers for LLVM components
-24/7/365 at http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer .
+A buildbot continuously runs the above fuzzers for LLVM components, with results
+shown at http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer .
 
 FAQ
 =========================
 
-Q. Why libFuzzer does not use any of the LLVM support?
-------------------------------------------------------
+Q. Why doesn't libFuzzer use any of the LLVM support?
+-----------------------------------------------------
 
 There are two reasons.
 
-First, we want this library to be used outside of the LLVM w/o users having to
+First, we want this library to be used outside of the LLVM without users having to
 build the rest of LLVM. This may sound unconvincing for many LLVM folks,
 but in practice the need for building the whole LLVM frightens many potential
 users -- and we want more users to use this code.
 
 Second, there is a subtle technical reason not to rely on the rest of LLVM, or
 any other large body of code (maybe not even STL). When coverage instrumentation
 is enabled, it will also instrument the LLVM support code which will blow up the
 coverage set of the process (since the fuzzer is in-process). In other words, by
 using more external dependencies we will slow down the fuzzer while the main
 reason for it to exist is extreme speed.
 
-Q. What about Windows then? The Fuzzer contains code that does not build on Windows.
+Q. What about Windows then? The fuzzer contains code that does not build on Windows.
 ------------------------------------------------------------------------------------
 
 Volunteers are welcome.
 
 Q. When this Fuzzer is not a good solution for a problem?
 ---------------------------------------------------------
 
 * If the test inputs are validated by the target library and the validator
   asserts/crashes on invalid inputs, in-process fuzzing is not applicable.
-* Bugs in the target library may accumulate w/o being detected. E.g. a memory
+* Bugs in the target library may accumulate without being detected. E.g. a memory
   corruption that goes undetected at first and then leads to a crash while
   testing another input. This is why it is highly recommended to run this
   in-process fuzzer with all sanitizers to detect most bugs on the spot.
 * It is harder to protect the in-process fuzzer from excessive memory
   consumption and infinite loops in the target library (still possible).
 * The target library should not have significant global state that is not
   reset between the runs.
-* Many interesting target libs are not designed in a way that supports
+* Many interesting target libraries are not designed in a way that supports
   the in-process fuzzer interface (e.g. require a file path instead of a
   byte array).
 * If a single test run takes a considerable fraction of a second (or
   more) the speed benefit from the in-process fuzzer is negligible.
 * If the target library runs persistent threads (that outlive
   execution of one test) the fuzzing results will be unreliable.
 
 Q. So, what exactly this Fuzzer is good for?

 * `Linux Kernel's BPF verifier <https://github.com/iovisor/bpf-fuzzer>`_
 
 * Capstone: `[1] <https://github.com/aquynh/capstone/issues/600>`__ `[2] <https://github.com/aquynh/capstone/commit/6b88d1d51eadf7175a8f8a11b690684443b11359>`__
 
 * Radare2: `[1] <https://github.com/revskills?tab=contributions&from=2016-04-09>`__
 
 * gRPC: `[1] <https://github.com/grpc/grpc/pull/6071/commits/df04c1f7f6aec6e95722ec0b023a6b29b6ea871c>`__ `[2] <https://github.com/grpc/grpc/pull/6071/commits/22a3dfd95468daa0db7245a4e8e6679a52847579>`__ `[3] <https://github.com/grpc/grpc/pull/6071/commits/9cac2a12d9e181d130841092e9d40fa3309d7aa7>`__ `[4] <https://github.com/grpc/grpc/pull/6012/commits/82a91c91d01ce9b999c8821ed13515883468e203>`__ `[5] <https://github.com/grpc/grpc/pull/6202/commits/2e3e0039b30edaf89fb93bfb2c1d0909098519fa>`__ `[6] <https://github.com/grpc/grpc/pull/6106/files>`__
 
-
 * LLVM: `Clang <https://llvm.org/bugs/show_bug.cgi?id=23057>`_, `Clang-format <https://llvm.org/bugs/show_bug.cgi?id=23052>`_, `libc++ <https://llvm.org/bugs/show_bug.cgi?id=24411>`_, `llvm-as <https://llvm.org/bugs/show_bug.cgi?id=24639>`_, Disassembler: http://reviews.llvm.org/rL247405, http://reviews.llvm.org/rL247414, http://reviews.llvm.org/rL247416, http://reviews.llvm.org/rL247417, http://reviews.llvm.org/rL247420, http://reviews.llvm.org/rL247422.
 
 .. _pcre2: http://www.pcre.org/
-
 .. _AFL: http://lcamtuf.coredump.cx/afl/
-
 .. _SanitizerCoverage: http://clang.llvm.org/docs/SanitizerCoverage.html
 .. _SanitizerCoverageTraceDataFlow: http://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow
 .. _DataFlowSanitizer: http://clang.llvm.org/docs/DataFlowSanitizer.html
 .. _AddressSanitizer: http://clang.llvm.org/docs/AddressSanitizer.html
-
 .. _Heartbleed: http://en.wikipedia.org/wiki/Heartbleed
-
 .. _FuzzerInterface.h: https://github.com/llvm-mirror/llvm/blob/master/lib/Fuzzer/FuzzerInterface.h
+.. _3.7.0: http://llvm.org/releases/3.7.0/docs/LibFuzzer.html
+.. _building Clang from trunk: http://clang.llvm.org/get_started.html
+.. _MemorySanitizer: http://clang.llvm.org/docs/MemorySanitizer.html
+.. _UndefinedBehaviorSanitizer: http://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
+.. _`coverage counters`: http://clang.llvm.org/docs/SanitizerCoverage.html#coverage-counters
+.. _`caller-callee pairs`: http://clang.llvm.org/docs/SanitizerCoverage.html#caller-callee-coverage
+.. _BoringSSL: https://boringssl.googlesource.com/boringssl/
+.. _`fuzz various parts of LLVM itself`: `Fuzzing components of LLVM`_