objcopy -O binary

[432]

Hello,
I'm trying to convert ELF file to binary file. How do I do so if I want some symbol to be at the beginning of the file? I want entry to be at offset 0x0 of the file and then load the file at 0x80000.

[alexfru]

For 32-bit ELF object files you may try to use Smaller C's linker, smlrl.

Code: Select all

int add(int a, int b) { return a + b; }

int one = 1;
int two = 2;

void _start(void) { add(one, two); }

Compiling:

Code: Select all

$ smlrcc -c -o bin.o bin.c
$ smlrl -flat32 -origin 0x80000 -o bin.bin bin.o

Disassembly:

Code: Select all

$ ndisasm -b32 -o0x80000 bin.bin
00080000  E916000000        jmp dword 0x8001b
00080005  CC                int3
00080006  CC                int3
00080007  CC                int3
00080008  CC                int3
00080009  CC                int3
0008000A  CC                int3
0008000B  CC                int3
0008000C  CC                int3
0008000D  CC                int3
0008000E  CC                int3
0008000F  CC                int3
00080010  55                push ebp
00080011  89E5              mov ebp,esp
00080013  8B4508            mov eax,[ebp+0x8]
00080016  03450C            add eax,[ebp+0xc]
00080019  C9                leave
0008001A  C3                ret
0008001B  55                push ebp
0008001C  89E5              mov ebp,esp
0008001E  FF3538000800      push dword [dword 0x80038]
00080024  FF3534000800      push dword [dword 0x80034]
0008002A  E8E1FFFFFF        call dword 0x80010
0008002F  83ECF8            sub esp,byte -0x8
00080032  C9                leave
00080033  C3                ret
00080034  0100              add [eax],eax
00080036  0000              add [eax],al
00080038  0200              add al,[eax]
0008003A  0000              add [eax],al

You don't have to use the entire compiler for this, just the linker. However, note that ELF object files produced by other compilers may not be fully supported.

[nullplan]

My tip would be to put the entry symbol into a special section, and use a linker script to link that section to address 0x80000 and all other sections after it.

[bzt]

Hello,
I'm trying to convert ELF file to binary file. How do I do so if I want some symbol to be at the beginning of the file? I want entry to be at offset 0x0 of the file and then load the file at 0x80000.

See a working example here

Code: Select all

objcopy -O binary kernel8.elf kernel8.img

You use a linker script to place the text segment and the entry point at the beginning of the raw image:

Code: Select all

    . = 0x80000;
    .text : { *(.text.boot) }

and in start.S:

Code: Select all

.section ".text.boot"
_start:

(If you don't define otherwise in the linker script, the standard entry point is at label _start).

Cheers,
bzt

[432]

It looks like I have nothing to do then. I already do it this way. Thank you.