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afl-cc: Don't offer __AFL_INIT() etc. in GCC/CLANG modes

Browse Source 
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 WARNING: You are using outdated instrumentation, install LLVM and/or gcc-plugin and use afl-clang-fast/afl-clang-lto/afl-gcc-fast instead!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. ]
This commit is contained in:
Jonathan Neuschäfer
2023-04-10 12:22:42 +02:00
parent 0911525194
commit 9e3e1a5512
1 changed files with 60 additions and 48 deletions
Download Patch File Download Diff File Expand all files Collapse all files

108
src/afl-cc.c
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@ -1101,37 +1101,45 @@ static void edit_params(u32 argc, char **argv, char **envp) {
if (!have_c) cc_params[cc_par_cnt++] = "-lrt";
#endif
cc_params[cc_par_cnt++] = "-D__AFL_HAVE_MANUAL_CONTROL=1";
cc_params[cc_par_cnt++] = "-D__AFL_COMPILER=1";
cc_params[cc_par_cnt++] = "-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION=1";
/* When the user tries to use persistent or deferred forkserver modes by
appending a single line to the program, we want to reliably inject a
signature into the binary (to be picked up by afl-fuzz) and we want
to call a function from the runtime .o file. This is unnecessarily
painful for three reasons:
/* As documented in instrumentation/README.persistent_mode.md, deferred
forkserver initialization and persistent mode are not available in afl-gcc
and afl-clang. */
if (compiler_mode != GCC && compiler_mode != CLANG) {
1) We need to convince the compiler not to optimize out the signature.
This is done with __attribute__((used)).
cc_params[cc_par_cnt++] = "-D__AFL_HAVE_MANUAL_CONTROL=1";
2) We need to convince the linker, when called with -Wl,--gc-sections,
not to do the same. This is done by forcing an assignment to a
'volatile' pointer.
/* When the user tries to use persistent or deferred forkserver modes by
appending a single line to the program, we want to reliably inject a
signature into the binary (to be picked up by afl-fuzz) and we want
to call a function from the runtime .o file. This is unnecessarily
painful for three reasons:
3) We need to declare __afl_persistent_loop() in the global namespace,
but doing this within a method in a class is hard - :: and extern "C"
are forbidden and __attribute__((alias(...))) doesn't work. Hence the
__asm__ aliasing trick.
1) We need to convince the compiler not to optimize out the signature.
This is done with __attribute__((used)).
*/
2) We need to convince the linker, when called with -Wl,--gc-sections,
not to do the same. This is done by forcing an assignment to a
'volatile' pointer.
cc_params[cc_par_cnt++] =
"-D__AFL_FUZZ_INIT()="
"int __afl_sharedmem_fuzzing = 1;"
"extern unsigned int *__afl_fuzz_len;"
"extern unsigned char *__afl_fuzz_ptr;"
"unsigned char __afl_fuzz_alt[1048576];"
"unsigned char *__afl_fuzz_alt_ptr = __afl_fuzz_alt;";
3) We need to declare __afl_persistent_loop() in the global namespace,
but doing this within a method in a class is hard - :: and extern "C"
are forbidden and __attribute__((alias(...))) doesn't work. Hence the
__asm__ aliasing trick.
*/
cc_params[cc_par_cnt++] =
"-D__AFL_FUZZ_INIT()="
"int __afl_sharedmem_fuzzing = 1;"
"extern unsigned int *__afl_fuzz_len;"
"extern unsigned char *__afl_fuzz_ptr;"
"unsigned char __afl_fuzz_alt[1048576];"
"unsigned char *__afl_fuzz_alt_ptr = __afl_fuzz_alt;";
}
if (plusplus_mode) {
@ -1169,35 +1177,39 @@ static void edit_params(u32 argc, char **argv, char **envp) {
"(*__afl_fuzz_len = read(0, __afl_fuzz_alt_ptr, 1048576)) == 0xffffffff "
"? 0 : *__afl_fuzz_len)";
cc_params[cc_par_cnt++] =
"-D__AFL_LOOP(_A)="
"({ static volatile const char *_B __attribute__((used,unused)); "
" _B = (const char*)\"" PERSIST_SIG
"\"; "
"extern int __afl_connected;"
#ifdef __APPLE__
"__attribute__((visibility(\"default\"))) "
"int _L(unsigned int) __asm__(\"___afl_persistent_loop\"); "
#else
"__attribute__((visibility(\"default\"))) "
"int _L(unsigned int) __asm__(\"__afl_persistent_loop\"); "
#endif /* ^__APPLE__ */
// if afl is connected, we run _A times, else once.
"_L(__afl_connected ? _A : 1); })";
if (compiler_mode != GCC && compiler_mode != CLANG) {
cc_params[cc_par_cnt++] =
"-D__AFL_INIT()="
"do { static volatile const char *_A __attribute__((used,unused)); "
" _A = (const char*)\"" DEFER_SIG
"\"; "
cc_params[cc_par_cnt++] =
"-D__AFL_LOOP(_A)="
"({ static volatile const char *_B __attribute__((used,unused)); "
" _B = (const char*)\"" PERSIST_SIG
"\"; "
"extern int __afl_connected;"
#ifdef __APPLE__
"__attribute__((visibility(\"default\"))) "
"void _I(void) __asm__(\"___afl_manual_init\"); "
"__attribute__((visibility(\"default\"))) "
"int _L(unsigned int) __asm__(\"___afl_persistent_loop\"); "
#else
"__attribute__((visibility(\"default\"))) "
"void _I(void) __asm__(\"__afl_manual_init\"); "
"__attribute__((visibility(\"default\"))) "
"int _L(unsigned int) __asm__(\"__afl_persistent_loop\"); "
#endif /* ^__APPLE__ */
"_I(); } while (0)";
// if afl is connected, we run _A times, else once.
"_L(__afl_connected ? _A : 1); })";
cc_params[cc_par_cnt++] =
"-D__AFL_INIT()="
"do { static volatile const char *_A __attribute__((used,unused)); "
" _A = (const char*)\"" DEFER_SIG
"\"; "
#ifdef __APPLE__
"__attribute__((visibility(\"default\"))) "
"void _I(void) __asm__(\"___afl_manual_init\"); "
#else
"__attribute__((visibility(\"default\"))) "
"void _I(void) __asm__(\"__afl_manual_init\"); "
#endif /* ^__APPLE__ */
"_I(); } while (0)";
}
if (x_set) {