InProcessForkExecutor about libafl

[andreafioraldi]

The immediate goal of ForkserverExecutor is btw to execute AFL++ binaries compiled with afl-clang-fast

[domenukk]

Basically this:
https://github.com/AFLplusplus/LibAFL-legacy/blob/53e339a38e27bcf5decf892928df459904442498/src/aflpp.c#L98

Or the "real" afl++ version, that also add sharedmem input, here:
https://github.com/AFLplusplus/AFLplusplus/blob/stable/src/afl-forkserver.c

(it doesn't have to fork itself but can use a rust std::process::Command to execute the target)

/edit:
The Angora forkserver is here (but won't work 1:1 as it uses Unix sockets instead of pipes):
https://github.com/AngoraFuzzer/Angora/blob/3cedcac8e65595cd2cdd950b60f654c93cf8cc2e/fuzzer/src/executor/forksrv.rs#L29

[tokatoka]

I've pushed a piece of very WIP code to the forkserver branch.
We'll also need to take care of the shared memory (and make a observer (ShmemObserver) to monitor the shared map), right?

[domenukk]

Yes, I assume we can use a normal map observer, with the map pointing to an ShMem.
Sharedmap input would eventually also be nice to have, as it's a lot quicker than (ab)using the file system

/edit: looking at your code, it might be a good idea to implement Drop for the Pipe struct so that they out close when going out of scope :)

[tokatoka]

Are we gonna go with injecting fork-server assembly code into instrumented PUT?
(https://github.com/google/AFL/blob/master/afl-as.h#L252)
, or do we prefer to use #[ctor] to spin up a forkserver before main() (it seems Angora is doing in this way).
The latter way seems much simpler, though.

[andreafioraldi]

For ForkserverExecutor this is an implementation detail of the instrumentation backend, has nothing to do with the lib.
For InProcessForkExecutor, it is simply an InProcessExecutor-like executor that instead of calling the harness function forks, call the harness in the child and waitpid in the parent

[tokatoka]

I'll use the Pipe struct you've pushed to the launcher branch

[domenukk]

@tokatoka want to create a PR for the current state, so we can finish it up together?

[tokatoka]

Yes, I was kind of waiting for the launcher's Pipe class to be merged into the main branch.
The one last thing left to do for ForkserverExecutor is to check the execution result. (check if the fuzzed program has crashed or not)

[andreafioraldi]

Good job

[tokatoka]

BTW, for what kind of target programs do we prefer to use InProcessForkExecutor over InProcessExecutor?

[domenukk]

InProcessForkExecutor would help when the target is instable, as state gets reset every once in a while.
However, a trivial solution already exists: it's possible to

• Using the restarting mngr
• running fuzz_loop_for with a low number of iters
• call manager.on_restart(state)?;, then exiting the child

This still needs some serialization that will cost time, maybe there is a better way (for example, by having large parts of the fuzzer on a shared map, directly?)

[tokatoka]

ok, makes sense.

[tokatoka]

I think we can call
fd = open("/dev/zero", O_RDWR);
mmap(EDGES_MAP.as_mut_ptr() as *mut c_void, 65536, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, 0);
to make the EDGES_MAP shared between the child and the parent

but in this case, we need to make EDGES_MAP aligned to 65536 bytes, so can we make EDGES_MAP into a struct then we can attach #[repr(C, align(65536))]?

and I think we can wrap that mmap in a new function ShMemProvider->new_fixed()?
(not sure what to do for ashmem and windows 🤔 )

[domenukk]

What's the benefit over using a normal ShMem? I'd just use that if possible, then it works on every os.
Instrumentation will need a way to replace the map pointer (and,potentially, a size field?)

[tokatoka]

My idea was to remap EDGES_MAP as MAP_SHARED, so that the bitmap is shared between the child and the parent, but normal ShMem does not have a way to remap a memory region as MAP_SHARED.

[domenukk]

How do you mean? The contents of ShMem are shared just fine, else the llmp wouldn't work

[tokatoka]

Nevermind, I've misunderstood,
yes I can just directly use ShMem. I'll make a wip pr when it's done

[tokatoka]

this can be closed now?

[domenukk]

Yes, great work