Update fuzz documentation

Reviewed-by: NRich Salz <rsalz@openssl.org>
GH: #2182

fuzz/README.md

@@ -43,7 +43,8 @@ Configure for fuzzing:
             -fsanitize-coverage=edge,indirect-calls,8bit-counters \
             enable-ec_nistp_64_gcc_128 -fno-sanitize=alignment enable-tls1_3 \
             enable-weak-ssl-ciphers enable-rc5 enable-md2 \
-            enable-ssl3 enable-ssl3-method enable-nextprotoneg
+            enable-ssl3 enable-ssl3-method enable-nextprotoneg \
+            --debug
     $ sudo apt-get install make
     $ LDCMD=clang++ make -j
     $ fuzz/helper.py $FUZZER
@@ -51,9 +52,7 @@ Configure for fuzzing:
 Where $FUZZER is one of the executables in `fuzz/`.
 
 If you get a crash, you should find a corresponding input file in
-`fuzz/corpora/$FUZZER-crash/`. You can reproduce the crash with
-
-    $ fuzz/$FUZZER <crashfile>
+`fuzz/corpora/$FUZZER-crash/`.
 
 AFL
 ===
@@ -64,7 +63,8 @@ Configure for fuzzing:
     $ CC=afl-clang-fast ./config enable-fuzz-afl no-shared -DPEDANTIC \
         enable-tls1_3 enable-weak-ssl-ciphers enable-rc5 enable-md2 \
         enable-ssl3 enable-ssl3-method enable-nextprotoneg \
-        enable-ec_nistp_64_gcc_128
+        enable-ec_nistp_64_gcc_128 -fno-sanitize=alignment \
+        --debug
     $ make
 
 The following options can also be enabled: enable-asan, enable-ubsan, enable-msan
@@ -74,3 +74,46 @@ Run one of the fuzzers:
     $ afl-fuzz -i fuzz/corpora/$FUZZER -o fuzz/corpora/$FUZZER/out fuzz/$FUZZER
 
 Where $FUZZER is one of the executables in `fuzz/`.
+
+Reproducing issues
+==================
+
+If a fuzzer generates a reproducible error, you can reproduce the problem using
+the fuzz/*-test binaries and the file generated by the fuzzer. They binaries
+don't need to be build for fuzzing, there is no need to set CC or the call
+config with enable-fuzz-* or -fsanitize-coverage, but some of the other options
+above might be needed. For instance the enable-asan or enable-ubsan option might
+be useful to show you when the problem happens. For the client and server fuzzer
+it might be needed to use -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION to
+reproduce the generated random numbers.
+
+To reproduce the crash you can run:
+
+    $ fuzz/$FUZZER-test $file
+
+Random numbers
+==============
+
+The client and server fuzzer normally generate random numbers as part of the TLS
+connection setup. This results in the coverage of the fuzzing corpus changing
+depending on the random numbers. This also has an effect for coverage of the
+rest of the test suite and you see the coverage change for each commit even when
+no code has been modified.
+
+Since we want to maximize the coverage of the fuzzing corpus, the client and
+server fuzzer will use predictable numbers instead of the random numbers. This
+is controlled by the FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION define.
+
+The coverage depends on the way the numbers are generated. We don't disable any
+check of hashes, but the corpus has the correct hash in it for the random
+numbers that were generated. For instance the client fuzzer will always generate
+the same client hello with the same random number in it, and so the server, as
+emulated by the file, can be generated for that client hello.
+
+Coverage changes
+================
+
+Since the corpus depends on the default behaviour of the client and the server,
+changes in what they send by default will have an impact on the coverage. The
+corpus will need to be updated in that case.
+