CODEWITHSUNDEEP
c++linkerfuzzingobjcopylibfuzzer
matoro
matoro

Reputation: 181

How to call a renamed symbol in an external object file?

I am trying to fuzz a particular piece of code using LLVM libFuzzer that only exposes its main() function externally. I have access to the source of the target code, but cannot change it.

If I try to directly include the object file, it conflicts with the main definition provided by -fsanitize=fuzzer. I thought I might be able to solve this by renaming the main symbol in the object file:

objcopy --redefine-sym main=stub_main main.stub main.o

Then in my harness code, I should just declare:

extern int stub_main(int argc, char **argv)

And call stub_main() instead of main(), while including main.stub on the link line. However, this did not seem to work, and the linker cannot find the reference to stub_main(int, char**).

How can I call this main function from another piece of code that also provides its own main()?

Upvotes: 1

Views: 607

Answers (1)

Kostas
Kostas

Reputation: 4176

You are not accounting for C++'s name mangling. The symbol for stub_main is likely a string containing main as well as some obfuscated info about arguments, type of function, and return type. In my platform it's __Z9stub_mainiPPc. The symbol for main would likely just be main or main_.

You can try looking how main and stub_main definitions mangle in your platform with objdump -d *.o, and then you can replace these strings with objcopy --redefine-sym.

Alternatively, as matoro said, you can declare the function as extern "C" so that no name mangling takes place.

Upvotes: 2

Related Questions