Make the random number generator predictable when fuzzing.

Reviewed-by: Rich Salz <rsalz@openssl.org>
GH: #2023

fuzz/README.md

diff --git a/fuzz/README.md b/fuzz/README.md
index c5a1ba9..d0c30f4 100644
--- a/fuzz/README.md
+++ b/fuzz/README.md
@@ -38,7 +38,8 @@
     $ CC=clang ./config enable-fuzz-libfuzzer \
             --with-fuzzer-include=../../svn-work/Fuzzer \
             --with-fuzzer-lib=../../svn-work/Fuzzer/libFuzzer \
-            -DPEDANTIC enable-asan enable-ubsan no-shared
+            -DPEDANTIC enable-asan enable-ubsan no-shared \
+            -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
     $ sudo apt-get install make
     $ LDCMD=clang++ make -j
     $ fuzz/helper.py $FUZZER

fuzz/server.c

diff --git a/fuzz/server.c b/fuzz/server.c
index b8a3ac4..4f2c794 100644
--- a/fuzz/server.c
+++ b/fuzz/server.c
@@ -191,6 +191,10 @@
 
 static SSL_CTX *ctx;
 
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+extern int rand_predictable;
+#endif
+
 int FuzzerInitialize(int *argc, char ***argv)
 {
     const uint8_t *bufp = kRSAPrivateKeyDER;
@@ -214,6 +218,10 @@
     OPENSSL_assert(ret == 1);
     X509_free(cert);
 
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+    rand_predictable = 1;
+#endif
+
     return 1;
 }