AFL in Ubuntu 18.04 is broken

At is has been reported on the discussion list for American Fuzzy Lop lately, unfortunately the fuzzer is broken in Ubuntu 18.04 “Bionic Beaver”. Ubuntu Bionic ships AFL 2.52b, which is the current version at the moment of writing this blog post. But the particular problem comes from the accompanying gcc-7 package, which is pulled by afl via the build-essential package. It was noticed in the development branch for the next Debian release from continuous integration (#895618) that introducing a triplet-prefixed as in gcc-7 7.3.0-16 (like same was changed for gcc-8, see #895251) affected the -B option in way that afl-gcc (the gcc wrapper) can’t use the shipped assembler (/usr/lib/afl-as) anymore to install the instrumentation into the target binary (#896057, thanks to Jakub Wilk for spotting the problem). As a result, the instrumented fuzzying and other things in afl doesn’t work:

$ afl-gcc --version
 afl-cc 2.52b by <lcamtuf@google.com>
 gcc (Ubuntu 7.3.0-16ubuntu3) 7.3.0
$ afl-gcc -o test-instr test-instr.c 
 afl-cc 2.52b by <lcamtuf@google.com>
$ afl-fuzz -i in -o out -- ./test-instr
 afl-fuzz 2.52b by <lcamtuf@google.com>
 [+] You have 2 CPU cores and 1 runnable tasks (utilization: 50%).
 [+] Try parallel jobs - see /usr/share/doc/afl-doc/docs/parallel_fuzzing.txt.
 [*] Creating hard links for all input files...
 [*] Validating target binary...
 
 [-] Looks like the target binary is not instrumented! The fuzzer depends on
     compile-time instrumentation to isolate interesting test cases while
     mutating the input data. For more information, and for tips on how to
     instrument binaries, please see /usr/share/doc/afl-doc/docs/README.
	 
     When source code is not available, you may be able to leverage QEMU
     mode support. Consult the README for tips on how to enable this.
     (It is also possible to use afl-fuzz as a traditional, "dumb" fuzzer.
     For that, you can use the -n option - but expect much worse results.)
	 
 [-] PROGRAM ABORT : No instrumentation detected
          Location : check_binary(), afl-fuzz.c:6920

The same error message is put out e.g. by afl-showmap. gcc-7 7.3.0-18 fixes this. As an alternative before this becomes available, afl-clang which uses the clang compiler might be used instead to prepare the target binary properly:

$ afl-clang --version
 afl-cc 2.52b by <lcamtuf@google.com>
 clang version 4.0.1-10 (tags/RELEASE_401/final)
$ afl-clang -o test-instr test-instr.c 
 afl-cc 2.52b by <lcamtuf@google.com>
 afl-as 2.52b by <lcamtuf@google.com>
 [+] Instrumented 6 locations (64-bit, non-hardened mode, ratio 100%)