I wish to inject an object file into an existing binary. The method I am attempting is:
- Convert a compiled binary into a relocatable object file.
- Use
gcc/ldto link the relocatable object file with the object file to be embedded.
Given the source:
#include <stdlib.h>
#include <stdio.h>
int main(void)
{
puts("main");
return EXIT_SUCCESS;
}
I compile this to host with the following:
gcc -Wall host.c -o host
I do the conversion to relocatable object file with:
objcopy -B i386 -I binary -O elf64-x86-64 host host.o
I then attempt a link with:
gcc host.o -o host
Ideally, this would relink the relocatable object file back to a binary. This would also give a chance to link in any extra object files. Unfortunately the command gives the following error:
/usr/lib/gcc/x86_64-linux-gnu/4.6.1/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status
My question is why is this error appearing and how would I go about properly relinking?
Something I tried was to link in another object file at this point which contained a dummy main (because I figured I could manually patch up the entry point later anyway), but what happened was that the new binary seemed to relocate the old code in a weird way with the symbol table completely messed up.
Extra Information
readelf on the binary yields the following:
mike@mike-ubuntu:~/Desktop/inject-obj$ readelf -h host
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x400410
Start of program headers: 64 (bytes into file)
Start of section headers: 4424 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 9
Size of section headers: 64 (bytes)
Number of section headers: 30
Section header string table index: 27
And on the relocatable object file:
mike@mike-ubuntu:~/Desktop/inject-obj$ readelf -h host.o
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 8480 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 5
Section header string table index: 2
Rationale
For those interested, the rationale can be found here.
