Compile a binary file for linking OSX

Question

I'm trying to compile a binary file into a MACH_O object file so that it can be linked it into a dylib. The dylib is written in c/c++.

On linux the following command is used: ld -r -b binary -o foo.o foo.bin

I have tried various option on OSX but to no avail:

ld -r foo.bin -o foo.o
gives:
ld: warning: -arch not specified
ld: warning: ignoring file foo.bin, file was built for unsupported file format which is not the architecture being linked (x86_64)

An empty .o file is created

ld -arch x86_64 -r foo.bin -o foo.o 
ld: warning: ignoring file foo.bin, file was built for unsupported file format which is not the architecture being linked (x86_64)

Again and empty .o file is created. Checking the files with nm gives: nm foo.o nm: no name list

The binary file is actually, firmware that will be downloaded to an external device.

Thanks for looking

Answer 1

Here's the closest translation to the Linux linker command to perform binary embedding with the OSX linker:

touch stub.c
gcc -o stub.o -c stub.c
ld -r -o foo.o -sectcreate binary foo_bin foo.bin stub.o

foo.bin will be stored in segment binary, section foo_bin (both names are arbitrary but chosen to mimic GNU ld for ELF on Linux) of the foo.o object.

stub is necessary because ld refuses to create just a custom segment/section. You don't need it if you link directly with a real code object.

To get data back from the section, use getsectbyname (struct is defined in mach-o/loader.h):

#include <mach-o/getsect.h>
const struct section_64 *sect = getsectbyname("binary", "foo_bin");
char *buffer = calloc(1, sect->size+1);
memcpy(buffer, sect->addr, sect->size); // whatever

or getsectdata:

#include <mach-o/getsect.h>
size_t size;
char *data = getsectdata("binary", "foo_bin", &size);
char *buffer = calloc(1, size+1);
memcpy(buffer, data, size); // whatever

(I used it to store text data, hence the stringification via calloc zeroing of size+1 plus blob copying)

Warning: Since 10.7, ASLR got stronger and messes badly with getsect* functions, resulting in segfaults. set disable-aslr off in GDB before running to reproduce EXC_BAD_ACCESS (SIGSEGV) in debug conditions. People had to jump through inordinate hoops to find the real address and get this working again.

A simple workaround is to get the offset and size, open the binary and read the data straight from disk. Here is a working example:

// main.c, build with gcc -o main main.c foo.o
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <mach-o/getsect.h>

int main() {
    // finding the filename of the running binary is left as an exercise to the reader
    char *filename = "main";

    const struct section_64 *sect = getsectbyname("binary", "foo_bin");
    if (sect == NULL) {
        exit(1);
    }

    char *buffer = calloc(1, sect->size+1);
    int fd = open(filename, O_RDONLY);
    if (fd < 0) {
        exit(1);
    }
    lseek(fd, sect->offset, SEEK_SET);
    if (read(fd, buffer, sect->size) != sect->size) {
        close(fd);
        exit(1);
    }

    printf("%s", buffer);
}