Loading .so Files From Memory

Question

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dlopen from memory?

I've seen this for Windows' DLL files, being loaded from a memory buffer, but I cant find it anywhere for Linux, and "ld" source code is the most complex code I've ever seen. So:

Is there any example of loading .so files from memory? Even a simple one that I can finish? I just don't know where to start, even though I've read most of the ELF specifications it's still mysterious to me.

Answer 1

You're looking at the source code of a wrong thing: ld doesn't do program and library loading. Instead, you should look at the source code of dlopen and dlsym functions found in libc. Also, you should look at the source of the dynamic linker: ld-linux.so (the true name varies with the platform; execute ldd /bin/ls to find out where the dynamic linker resides).

ELF parsing isn't difficult, but it requires attention to detail and understanding of assembly code for the particular CPU; you need also ABI specification for your platform (and it's different for 32- and 64-bit linux, and is also different between CPUs.)

If you just need to load object files from memory at run-time (i.e., it doesn't have to be a SO), you can look at X11 project: they have implemented a module system which, basically, loads object code at some address and relocates it.

Answer 2

You need dlopen() family of functions (on GNU/Linux, they are defined in /usr/include/dlfcn.h).

For an example, take a look at how PHP does modules.

Answer 3

What does "loading .so files from memory" means to you?

If you have any *.so file, then it is in some file system, and has a path. Then just use dlopen on it.

If it is not a file, what is it? How did you get in memory? What exactly have you in memory? (Do you have an ELF header and ELF layout in memory?)

If you have enough information to make an ELF *.so file, dump (i.e. write) such file into some file system (use a temporary filesystem like tmpfs if you are concerned with disk performance). Then dlopen that.

If you don't have enough information to make an ELF .so file, then probably you are dynamically building code in memory. Look at what existing machine code generating infrastructure (like LLVM, GCCJIT, libjit, GNU lightning, LuaJit ....) are doing.

If you have a full functional code in memory, ensure that the memory is executable with mmap & mprotect and jump into it (e.g. using function pointer tricks).