Add RUZZY_ARGV0 to support libFuzzer re-execution

Author: mschwager

entrypoint.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
 
+SCRIPT_PATH="$(cd "$(dirname "$0")" && pwd)/$(basename "$0")"
+
 LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
-    ruby -e 'require "ruzzy"; Ruzzy.dummy' -- "$@"
+    RUZZY_ARGV0="$SCRIPT_PATH" ruby -e 'require "ruzzy"; Ruzzy.dummy' -- "$@"

lib/ruzzy.rb

@@ -7,7 +7,12 @@
 module Ruzzy
   require 'cruzzy/cruzzy'
 
-  DEFAULT_ARGS = [$PROGRAM_NAME] + ARGV
+  # libFuzzer features like -merge and -fork re-execute argv[0] via system().
+  # RUZZY_ARGV0 allows a wrapper script (e.g. entrypoint.sh) to advertise its
+  # own path as argv[0], so re-execution goes through the wrapper that sets
+  # LD_PRELOAD and other required environment.
+  ARGV0 = ENV.fetch('RUZZY_ARGV0', $PROGRAM_NAME)
+  DEFAULT_ARGS = [ARGV0] + ARGV
   EXT_PATH = Pathname.new(__FILE__).parent.parent / 'ext' / 'cruzzy'
   ASAN_PATH = (EXT_PATH / 'asan_with_fuzzer.so').to_s
   UBSAN_PATH = (EXT_PATH / 'ubsan_with_fuzzer.so').to_s