How to remove strings from a compiled binary (.so)

Question

How do I remove strings from / obfuscate a compiled binary? The goal is to avoid having people read the names of the functions/methods inside.

It is a dynamic library (.so) compiled from C++ code for Android with the NDK tools (includes GCC)

I compile with -O3 and already use arm-eabi-strip -g mylib.so to remove debugging symbols, but when I do strings mylib.so all the names of the functions/methods are still readable.

Answer 1

These strings are in the dynamic symbol table, which is used when the library is loaded at runtime. readelf -p .dynstr mylib.so will show these entries.

strip -g will remove debugging symbols, but it can't remove entries from the dynamic symbol table, as these may be needed at runtime. Your problem is that you have entries in the dynamic symbol table for functions which are never going to be called from outside your library. Unless you tell it, the compiler/linker has no way of knowing which functions form part of the external API (and therefore need entries in the dynamic symbol table) and which functions are private to your library (and so don't need entries in the dynamic symbol table), so it just creates dynamic symbol table entries for all non-static functions.

There are two main ways you can inform the compiler which functions are private.

1. Mark the private functions static. Obviously, this only works for functions only needed within a single compilation unit, though for some libraries this technique might be sufficient.

2. Use the gcc "visibility" attribute to mark the functions as visible or hidden. You have two options: either mark all the private functions as hidden, or change the default visibility to hidden using the -fvisibility=hidden compiler option and mark all the public functions as visible. The latter is probably the best option for you, as it means that you don't have to worry about accidentally adding a function and forgetting to mark it as hidden.

If you have a function:

int foo(int a, int b);

then the syntax for marking it hidden is:

int foo(int a, int b) __attribute__((visibility("hidden")));

and the syntax for marking it visible is:

int foo(int a, int b) __attribute__((visibility("default")));

For further details, see this document, which is an excellent source of information on this subject.