tiny elf loader

see thesis/*

cd thesis && make thesis

usage

make tinyld.a
# or
make ARCH=i386 tinyld.a

take the generated tinyld.a and the header file tinyld.h

include the header in source.c:

#include "tinyld.h"

void main (){
    void *blob = malloc(BLOB_SZ);
    read(blob, somewhere, size);
    void *handle = t_dlmopen(blob, size);
    void *symbol = t_dlsym(handle, "symbol");
}

and compile it with

gcc source.c tinyld.a
./a.out

try it out!

make test

status

test1(pure asm) and test2(syscall) passed, test3(use libc functions) fails

see dlopen.c#L221-L239

the idea

https://github.com/m1m1x/memdlopen replaces the functions used for dlopening, but addresses are fixed for specific ld versions. the code is not reusable. also, it relies on system ld to work.
the idea is to re-implement the needed code for loading a dylib, and get rid of system linker.

pitfalls

libc is now firmly bounded to the loader. the loader needs to find the link_map to load a library, which is hidden in the libc, so in order to build a loader, we must re-implement the libc entirely, which is quite impractical.

an idea suggests that if we write a loader that is capable of loading the libc.so, we might build the link_map on our own. but the libc.so for glibc also has tons of GNU specified relocations, thread stuff, etc. it's like hell implementing a loader for glibc.

glibc is a total disaster.

workarounds

a workaround is to implement only the functions that may be used in the loader, and manually write them to the GOT of the loaded library. see libc.h, libc/*.c. this is currently implemented.

another one is quite dirty, but might be somewhat usable. since the loader itself is compiled non-statically, the loader itself has the link_map and dl_runtime_resolve() stored in it's GOT already. as long as we have an elegant way of finding our own GOT, we can put them into the GOT of the loaded library. it's just... we don't have an elegant way of doing this. is there any?