foreign-dlopen

Intro

Calling dlopen() function from statically-linked binaries is a well-known (in narrow circles) problem *. A common approach is not supporting it at all. A usual explanation goes along the lines of:

1. You usually link statically to exclude unneeded functions from the binary.
2. But if you want to dynamically load a shared library, it likely itself is linked dynamically against libc (doing otherwise requires extra legwork, and if you have a few such shared libraries, themselves statically linked, you duplicate code in each).
3. But that means you would need to carry around (big) libc.so, which undermines the original idea of static linking (you could link your app against dynamic libc and save space).
4. Alternatively, you could link entire libc into your static executable, and export dynamic symbols (ld --export-dynamic). That avoids carriying around extra libc.so file, but again requires extra legwork. And it still undermines the original benefit of static linking, as your app will be the size of libc.so (+ your app's code).

The summary is that if you want to use dlopen() from static binary, you would need to do extra legwork, and would lose benefits of static linking. Ergo, don't use dlopen() with static linking, use dynamic linking instead! And as no reasonable person would use dlopen() with static linking, let's remove dlopen() (and that's entire dynamic loader, quite a bloaty piece of code!) support from statically linked libc, to let people who really need static linking, reap the maximum benefits of it.

Those are all very valid arguments, but they are all based on a "plugin" model: you have a common system, sharing a common libc.

That's not the only usage model though. There's another model, which we'll call "FFI (Foreign Function Interface) model". It goes along the lines of:

1. Suppose you have a perfect, closed world application. Statically linked of course.
2. But you want to go out to dirty bustling outside world (in other words, let your application, or users of your appplication, to dlopen() outside shared libraries).
3. There're absolutely no expectations or stipulations about which libc is used by those shared libraries. In particular, there's no expectations that libc of your application and external shared lib are the same. Or that you know which libc is used by external lib at all. For example, your static binary may be linked against musl libc, but you may want to load (allow to load) glibc bloat lying in abundance on a typical Linux system.

Again, the only thing you want is to maintain your static perfect world, independent from outside hustle. But, at user discretion, you want to allow this hustling outside world into your address space, by means of dlopen().

This cute project is a proof-of-concept solution for this usecase.

References

• https://www.openwall.com/lists/musl/2012/12/08/4

Details

Implementation idea #1: a (custom) ELF loader. Problem: trying to implement "full" ELF loader (recursively load dependent .so, etc.) is prolematic, e.g. because glibc (libc.so) is tightly coupled with dynamic loader aka interpreter (ld.so). If you just load libc.so, and its dependency ld.so, a lot of stuff in ld.so will remain uninitialized, then libc.so will call into ld.so, which will crash. To properly initialize ld.so, it must be "run", i.e. execution should go into its entry point (and not just ELF INIT func). But when started that way, ld.so loads an executable, which then terminates.

Idea #2: Make ld.so load an executable which will jump back into our custom loader. This way, both ld.so will be initialized, and we get back control. Coupled with setjmp/longjmp, this can be turned into a reusable library. Its structure is:

1. Custom ELF loader, built against any libc or lack thereof. This should be simplified loader, without support for loading shared libs, etc. The only thing it needs to load is a "helper" target executable and its INTERP.
2. It calls into INTERP, the call wrapped with setjmp. The corresponding longjmp is wrapped into a global func, whose address (in ascii) we pass as a command-line argument to the "helper" target binary.
3. The "helper" binary should be linked against native libc of the target environment whose shared libs we want to load (e.g., glibc).
4. Target binary is also linked agains target libc's libdl. The binary captures addresses of dlopen()/dlsym()/etc. funcs into an array, and performs function call into the address passed as a command-line arg, passing array as a function arg.
5. That address is, as mentioned above, is a function which performs longjmp, after storing the passed dlopen(), etc. function addresses for future use.
6. After longjmp, we're back to our application, which now has access to dlopen(), etc. of the target system.

Building and running

1. cd src
2. Build target helper executable: make -f Makefile.fdlhelper. As explained above, it should be built against target system from which you want to load shared libs dynamically using dlopen(). (If you build this on a typical Linux system, it will be built against glibc.)
3. Build static, stdlib-less sample application: make STDLIB=0. (You must pass STDLIB=0 for full effect. By default, the sample is built against stdlib, which is helpful during development/debugging.)
4. Run the sample: ./foreign_dlopen_demo. While it is static, it will dynamically load libc.so.6 and call printf() from it. (All this using fdlhelper executable built in a previous step.)

Credits

"Foreign dlopen" idea and implementation is by Paul Sokolovsky. The implementation is based on the ELF loader by Mikhail Ilyin: https://github.com/MikhailProg/elf , the original README of that project follows.

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ELF loader

A small elf loader. It can load static and dynamically linked ELF EXEC and DYN (pie) binaries. The loader is PIE program that doesn't depend on libc and calls kernel services directly (z_syscall.c).

If the loader needs to load a dynamically linked ELF it places an interpreter (usually ld.so) and a requested binary into a memory and then calls the interpreter entry point.

Build

Default build is for amd64:

$ make

Build for i386:

$ make ARCH=i386

Small build (exclude all messages and printf):

$ make SMALL=1

Load binaries

Load ls:

$ ./loader /bin/ls

Load galculator:

$ ./loader /usr/bin/galculator