How to embed data in shared library?

For example, I want to embed dicmap.bin to a shared library libxxx.so. I write a program to verify it.

test_dicmap.cpp

#include <stdio.h>
#include <stdint.h>

extern "C" {
extern const uint8_t _binary_dicmap_bin_start[];
extern const uint8_t _binary_dicmap_bin_end[];
extern const void* _binary_dicmap_bin_size;
}

int main()
{
    size_t size = (size_t)&_binary_dicmap_bin_size;
    printf("start=%p, end=%pnend-start=%zd, size=%zdn", 
        _binary_dicmap_bin_start, 
        _binary_dicmap_bin_end,
        _binary_dicmap_bin_end - _binary_dicmap_bin_start,
        size);
    printf("data[0..8]=%02x %02x %02x %02x %02x %02x %02x %02xn", 
            _binary_dicmap_bin_start[0], _binary_dicmap_bin_start[1], 
            _binary_dicmap_bin_start[2], _binary_dicmap_bin_start[3],
            _binary_dicmap_bin_start[4], _binary_dicmap_bin_start[5], 
            _binary_dicmap_bin_start[6], _binary_dicmap_bin_start[7]);
}

But its _start, _end and _size is invalid.

]$ ls dicmap.bin  -l
-rw-rw-r-- 1 kirbyzhou kirbyzhou 198600798 Feb 26 10:58 dicmap.bin

]# objcopy -B i386 -I binary -O elf64-x86-64 dicmap.bin dicmap.o && g++ -o libxxx.so dicmap.o -shared  &&  g++ -L. -lxxx  test_dicmap.cpp
/opt/rh/devtoolset-8/root/usr/libexec/gcc/x86_64-redhat-linux/8/ld: warning: type and size of dynamic symbol `_binary_dicmap_bin_size' are not defined
/opt/rh/devtoolset-8/root/usr/libexec/gcc/x86_64-redhat-linux/8/ld: warning: type and size of dynamic symbol `_binary_dicmap_bin_start' are not defined
/opt/rh/devtoolset-8/root/usr/libexec/gcc/x86_64-redhat-linux/8/ld: warning: type and size of dynamic symbol `_binary_dicmap_bin_end' are not defined

]# ./a.out
start=0x601034, end=0x601034
end-start=0, size=6295604
data[0..8]=00 00 00 00 00 00 00 00

end-start and size should be sizeof dicmap.bin (198600798).

My objcopy is binutils-2.30-54.el7 of rhel7 with devtoolset-8.

I try to add share flags to the .o file, but a error happens:

objcopy -B i386 -I binary -O elf64-x86-64 dicmap.bin dicmap.o --set-section-flag .data=share
objcopy: BFD version 2.30-54.el7 internal error, aborting at elf.c:8869 in _bfd_elf_set_section_contents

objcopy: Please report this bug.

binutils-2.27-41.base.el7_7.1.x86_64 of rhel7 also have the same problem.

Is there any method to help me?

Answer

There 2 methods works for me now.

Method 1: Use objcopy to convert data to “.o”, then link to “.so”. Then link the “.so” and main code with “-fPIC”.

objcopy -B i386 -I binary -O elf64-x86-64 dicmap.bin dicmap.o
g++ -shared -fPIC dicmap.o -o libdicmap1.so
# -fPIC is very import in the following line, 
# But it is very unusual when you compile and link main code.
g++ -fPIC test_dicmap.cpp libdicmap1.so -o test_dicmap1-PIC

Method 2: Use assmbler to wrap the data, and store the size in a different way of objcopy.

g++ -shared -fPIC dicmap3.s -o libdicmap3.so
g++ test_dicmap3.cpp libdicmap3.so -o test-dicmap3

Codes:

test_dicmap.cpp:

#include <stdio.h>
#include <stdint.h>
#include <assert.h>
extern "C" {
extern const uint8_t _binary_dicmap_bin_start[];
extern const uint8_t _binary_dicmap_bin_end[];
extern const void* _binary_dicmap_bin_size;
}
int main()
{
    size_t data_size = (size_t)&_binary_dicmap_bin_size;
    printf("start=%p, end=%pnend-start=%zd, size=%zdn", 
        _binary_dicmap_bin_start, 
        _binary_dicmap_bin_end,
        _binary_dicmap_bin_end - _binary_dicmap_bin_start,
        data_size);
    printf("data[0..8]=%02x %02x %02x %02x %02x %02x %02x %02xn", 
            _binary_dicmap_bin_start[0], _binary_dicmap_bin_start[1], 
            _binary_dicmap_bin_start[2], _binary_dicmap_bin_start[3],
            _binary_dicmap_bin_start[4], _binary_dicmap_bin_start[5], 
            _binary_dicmap_bin_start[6], _binary_dicmap_bin_start[7]);
    assert(_binary_dicmap_bin_end - _binary_dicmap_bin_start == data_size);
}

test_dicmap3.cpp:

#include <stdio.h>
#include <stdint.h>
#include <assert.h>

extern "C" {
extern const uint8_t _binary_dicmap_bin_start[];
extern const uint8_t _binary_dicmap_bin_end[];
extern const size_t _binary_dicmap_bin_size;
}


int main()
{
    size_t data_size = _binary_dicmap_bin_size;
    printf("start=%p, end=%pnend-start=%zd, size=%zdn", 
        _binary_dicmap_bin_start, 
        _binary_dicmap_bin_end,
        _binary_dicmap_bin_end - _binary_dicmap_bin_start,
    data_size);
    printf("data[0..8]=%02x %02x %02x %02x %02x %02x %02x %02xn",
            _binary_dicmap_bin_start[0], _binary_dicmap_bin_start[1], 
            _binary_dicmap_bin_start[2], _binary_dicmap_bin_start[3],
            _binary_dicmap_bin_start[4], _binary_dicmap_bin_start[5], 
            _binary_dicmap_bin_start[6], _binary_dicmap_bin_start[7]);
    // _binary_dicmap_bin_end is invalid
    //assert(_binary_dicmap_bin_end - _binary_dicmap_bin_start == data_size);
}

dicmap3.s:

    .globl  _binary_dicmap_bin_start
    .globl  _binary_dicmap_bin_end
    .globl  _binary_dicmap_bin_size
    .section  .rodata
    .type   _binary_dicmap_bin_start, @object
    .align  8
_binary_dicmap_bin_start:
    .incbin "dicmap.bin"
    .align  1
    .size   _binary_dicmap_bin_end, 1
_binary_dicmap_bin_end:
    .byte   0
    .size   _binary_dicmap_bin_start, _binary_dicmap_bin_end - _binary_dicmap_bin_start
    .type   _binary_dicmap_bin_size, @object
    .size   _binary_dicmap_bin_size, 8
    .align  8
_binary_dicmap_bin_size:
    .quad   _binary_dicmap_bin_end - _binary_dicmap_bin_start

dicmap.bin:

helloworld