[llvm-dev] libFuzzer: issue with weak symbols on Mac

[Max Moroz via llvm-dev]

I'd like to discuss the following change: https://reviews.llvm.org/D37526

For the context, there is a comment
in compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeak.cpp:

// Implementation for Linux. This relies on the linker's support for weak
// symbols. We don't use this approach on Apple platforms because it
requires
// clients of LibFuzzer to pass ``-U _<symbol_name>`` to the linker to allow
// weak symbols to be undefined. That is a complication we don't want to
expose
// to clients right now.

That makes sense, but with current implementation, you cannot use
libFuzzer's interface functions other than LLVMFuzzerTestOneInput. Below is
a small example to verify that LLVMFuzzerInitialize is not being called on
Mac:

#include <stddef.h>
#include <stdint.h>
#include <stdio.h>

extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv) {
  printf("Hello from LLVMFuzzerInitialize, argc: %i\n", *argc);
  return *argc;
}

extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
  printf("Hello from LLVMFuzzerTestOneInput, size: %zu\n", size);
  if (size) {
  	return data[0];
  }
  return size;
}


Assuming that there are libFuzzer customers who don't mind to specify
"-U,_%function_name%" explicitly (e.g.
https://chromium-review.googlesource.com/c/chromium/src/+/653846/1/testing/libfuzzer/BUILD.gn),
we need to have a way to use FuzzerExtFunctionsWeak.cpp instead
of FuzzerExtFunctionsDlsym.cpp on Mac.

The CL I've uploaded feels a bit hacky to me, but I don't see any less
intrusive solution that would still comply with existing implementation and
would also support weak symbols to be explicitly allowed if needed.

Thanks!

--
Max
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