instrumentation/README.persistent_mode.md documents in the section about
deferred forkserver initialization:
> With the location selected, add this code in the appropriate spot:
>
> ```c
> #ifdef __AFL_HAVE_MANUAL_CONTROL
> __AFL_INIT();
> #endif
> ```
>
> You don't need the #ifdef guards, but including them ensures that the program
> will keep working normally when compiled with a tool other than afl-clang-fast/
> afl-clang-lto/afl-gcc-fast.
>
> Finally, recompile the program with afl-clang-fast/afl-clang-lto/afl-gcc-fast
> (afl-gcc or afl-clang will *not* generate a deferred-initialization binary) -
> and you should be all set!
This strongly implies that you can compile a program that uses __AFL_INIT()
under an `#ifdef __AFL_HAVE_MANUAL_CONTROL` guard with afl-gcc/-clang.
However, this currently fails:
$ cat example.c
#include <stdio.h>
int main(void) {
#ifdef __AFL_HAVE_MANUAL_CONTROL
__AFL_INIT();
#endif
puts("Hello");
}
$ afl-gcc example.c -o example
afl-cc++4.06a by Michal Zalewski, Laszlo Szekeres, Marc Heuse - mode: GCC-GCC
[!] WARNING: You are using outdated instrumentation, install LLVM and/or gcc-plugin and use afl-clang-fast/afl-clang-lto/afl-gcc-fast instead!
afl-as++4.06a by Michal Zalewski
[+] Instrumented 1 locations (64-bit, non-hardened mode, ratio 100%).
/usr/bin/ld: /tmp/ccuJHcpt.o: in function `main':
/home/jn/dev/fuzz/AFLplusplus/example.c:5: undefined reference to `__afl_manual_init'
collect2: error: ld returned 1 exit status
The issue here is an inconsistency in afl-gcc (i.e. afl-cc operating in GCC mode):
- afl-cc defines __AFL_HAVE_MANUAL_CONTROL and __AFL_INIT unconditionally
- __AFL_INIT relies on __afl_manual_init, which is defined in afl-compiler-rt.o
- afl-cc doesn't link afl-compiler-rt in GCC or CLANG mode
Since afl-gcc/-clang is documented as not supporting deferred forkserver
initialization, this patch omits the definitions of __AFL_HAVE_MANUAL_CONTROL
and related macros in GCC/CLANG mode.
This restores the ability to compile a deferred-forkserver program under
afl-gcc, if it can also be compiled under gcc.
[ In case someone reads this an feels adventurous enough (as I did) to
think about enabling deferred forkserver under afl-gcc: Whether the
deferred forkserver actually works can be verified by placing a
usleep(100000) or similar at the start of main (before __AFL_INIT()),
and watching the execution speed. It doesn't work. ]
With the right -W options, compilers may complain about the cast of
string literals (for PERSIST_SIG and DEFER_SIG) to (char*), and they're
right to do so, because string literals are constant. Since some
projects enable -Werror, this can lead to a broken build with afl-cc.
Let's simply cast to (const char *), which preserves the constness of
the string literal.
When running, the following gets printed in quick succession on startup:
afl-fuzz++4.00c based on afl by Michal Zalewski and a large online community
[...]
[+] NOTE: This is v3.x which changes defaults and behaviours - see README.md
Don't assert that this is v3, just that v3+ changes defaults and
behaviours.
