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eld

eld is a minimal dynamic loader for ELF shared objects.

Building

mkdir eld
cd eld
git clone ... src
mkdir build
cd build
cmake ../src/ -G "Unix Makefiles" \
    -DCMAKE_BUILD_TYPE="Debug" \
    -DCMAKE_C_COMPILER="$INSTALL_PATH/bin/clang" \
    -DCMAKE_C_FLAGS="-target or1k-elf" \
    -DOR1K_SIM_PATH="$INSTALL_PATH/bin/or32-elf-sim" \
    -DCMAKE_INSTALL_PREFIX="$INSTALL_PATH/or1k-elf"
make

Components

libeld.a

This is the core dynamic loading library. It exposes the classical dlopen, dlclose and dlsym functions. They work pretty much as expected, except for the fact that the file name provided to dlopen must be a pointer to the buffer of memory where the ELF to load is located.

To try it out, create a C file, name it libyour.c and make it a shared library:

int your() {
  return 99;
}

$ clang -target or1k-elf libyour.c -shared -o libyour.so

Convert the ELF shell object (.so file) in a C array with xxd:

$ xxd -i libyour.so > libyour.so.h
$ cat libyour.so.h
unsigned char libmy_so[] = {
  0x7f, 0x45, 0x4c, 0x46, 0x01, 0x02, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
...

Then include it in your main C file (main.c) and call dlopen on the generated array:

#include <stdlib.h>
#include <stdio.h>
#include "dl.h"
#include "libyour.so.h"

typedef int (*myfunc_ptr)();

int main() {
  void *handle = NULL;

  handle = dlopen((char *) libyour_so, 0);

  if (!handle) {
    printf("Couldn't load the dynamic library\n");
    return EXIT_FAILURE;
  }

  myfunc_ptr your = dlsym(handle, "your");

  if (!your) {
    printf("Couldn't find the symbol \"your\"\n");
    return EXIT_FAILURE;
  }

  printf("your() == %d\n", your());

  return EXIT_SUCCESS;
}

Supposing you are working the eld build directory:

$ clang -target or1k-elf -I. main.c -L. -leld -o main -Wl,--export-dynamic
$ or32-elf-sim -f sim.cfg main
...
your() == 99
...

libyour.so, libmy.so and loader

The project provides some example libraries and a simple application to test the loader.

  • loader (test.c): reads from standard input a series of libraries, loads them and performs some simple operations using eld; libraries are fed into the loader once at a time, first sending an integer (4-byte, little endian) representing the size of the library, and then the library itself; repeat until a 0-sized library is fed in input. This process is facilitated by feed-so.py script.
  • libyour.so (libyour.c): contains just a single function and a global variable;
  • libmy.so (libmy.c): depends on libyour.so and uses its function, has a constructor and a destructor, and expects to have some functions from the libc.

There is a target to build and test with or1ksim the loader:

make loader_sim

Or if you want to launch or1ksim in debug mode:

make loader_sim_debug

Supported platforms

eld primary aim is currently supporting bare-metal OpenRISC targets. eld has been tested with both clang and GCC and works fine with both or1k-elf and or1kle-elf targets.

Current limitations

When applying resolving symbols in shared object or calling dlsym, the symbols are not searched in the proper order. Currently we first search in the shared object currently being relocated (or the handle passed to dlsym) and then all the previously loaded shared objects are searched in load order.

TODO

  • Isolate platform specific parts
  • Move examples to a dedicated directory
  • Add CMake target to generate documentation
  • Import sys/queue.h
  • Add support for PID objects
  • Add support for TLS relocations