Question

This shouldn’t be that hard that one may think, if I got it right. Specifically, I’ll begin with iOS and the ELF executable format. Let’s clarify that I have a jailbroken iPhone and I don’t want to do this in any appstore apps, so pleas avoid “good advices” like “you can’t do it as it’s prohibited by Apple”.

So, what I have seen is that there’s a Flash player implementation, called Frash (by Comex btw, developer of recent jailbreaks). This utility requires, after installation, that Android’s libflashplayer.so is present (copied to) the iPhone file system. I digged into the source code and found out that the tweak actually opens the Android (ELF) shared object file, “parses” it and executes code from it. I already asked a friend of mine wheter it is or is not actually possible and he told me that it is, because ELF on ARM and Mach-O on ARM are binary compatible (because they’re both ARM). But he actually failed to explain it to me in detail, so I’d like to ask how can it be done? I can’t exactly understand the source code fragment that handles, but one thing is sure:

int fd = open("libflashplayer.so", O_RDONLY);
_assert(fd > 0);

fds_init();

sandbox_me();

int symtab_size;
Elf32_Sym *symtab;
void **init_array;
Elf32_Word init_array_size;
char *strtab;
TIME(base_load_elf(fd, &symtab, &symtab_size, &init_array, &init_array_size, &strtab));

// Call the init funcs
_assert(init_array);
while(init_array_size >= 4) {
    void (*x)() = *init_array++;
    notice("Calling %p", x);
    x();
    init_array_size -= 4;
}

(from the original code, as of 02/12/2011 on GitHub)

It seems to me that he uses libelf to perform this, right? And that in an ELF file there are symbols that can be executed on a compatible processor just fine?

I’d also like to know whether it is true for all other processor architectures? So maybe one can execute symbols from Linux binaries on OS X?

Answer 1

The important thing about compatibility is the underlying processor architecture, not Linux vs. OS X vs. Android. If the ELF or .so are compiled for the same processor instruction set, then this can work. If not, then they are not compatible. For example, if both were built for Linux but different processors, they would not be compatible.