Error 13: Invalid or unsupported executable while booting simple kernel in grub with string literal

Question

I've written a simple kernel that tries to write two characters to the frame buffer.

If I define a string literal in the kernel, I get the following output when it boots:

  Booting 'os'                                                                  

kernel /boot/kernel.elf                                                         

Error 13: Invalid or unsupported executable format                              

Press any key to continue... 

Otherwise, if I define two characters I get the following (note 'ab' at the start of the output):

abBooting 'os'                                                                  

kernel /boot/kernel.elf                                                      
   [Multiboot-elf, <0x100000:0x201:0x0>, <0x101000:0x0:0x1000>,     shtab=0x102168, 
   entry=0x1001f0]

loader

I wrote the loader in assembly:

global loader                   ; the entry symbol for ELF

MAGIC_NUMBER equ 0x1BADB002     ; define the magic number constant
FLAGS        equ 0x0            ; multiboot flags
CHECKSUM     equ -MAGIC_NUMBER  ; calculate the checksum
                                ; (magic number + checksum + flags should equal 0)
KERNEL_STACK_SIZE equ 4096      ; size of stack in bytes

section .text:                  ; start of the text (code) section
align 4                         ; the code must be 4 byte aligned
    dd MAGIC_NUMBER             ; write the magic number to the machine code,
    dd FLAGS                    ; the flags,
    dd CHECKSUM                 ; and the checksum

loader:                         ; the loader label (defined as entry point in linker script)
    mov eax, 0xCAFEBABE         ; place the number 0xCAFEBABE in the register eax

    mov esp, kernel_stack + KERNEL_STACK_SIZE   ; point esp to the start of the
                                                ; stack (end of memory area)
    extern run  
    call run

.loop:
    jmp .loop                   ; loop forever

section .bss
align 4                         ; align at 4 bytes
kernel_stack:                   ; label points to beginning of memory
    resb KERNEL_STACK_SIZE          ; reserve stack for the kernel

The kernel is written in c

#include "io.h"
#include "fb.h"

void run()
{   
    // try writing message to port
    char* c = (char *) 10000;
    c[0] = 'a';
    c[1] = 'b';

    fb_write(c, 2);  // this does not cause the error

    // fb_write("ab",2); // this line would cause the error
}

External headers

There are two external headers. One for IO ports called io.h and one for writing to the frame buffer called fb.h

Here is io.h and the implementation io.s

io.h:

#ifndef INCLUDE_IO_H
#define INCLUDE_IO_H

/** outb:
 *  Sends the given data to the given I/O port. Defined in io.s
 *
 *  @param port The I/O port to send the data to
 *  @param data The data to send to the I/O port
 */
void outb(unsigned short port, unsigned char data);

#endif /* INCLUDE_IO_H */

io.s:

global outb     ; make the label outb visible outside this file

; outb - send a byte to an I/O port
; stack: [esp + 8] the data byte
;        [esp + 4] the I/O port
;        [esp    ] return address
outb:
    mov al, [esp + 8]
    mov dx, [esp + 4]
    out dx, al
    ret

fb.h

#include "io.h"

// FRAME BUFFER ================================

// Text colors
#define FB_BLACK        0
#define FB_BLUE         1
#define FB_GREEN        2
#define FB_CYAN         3
#define FB_RED          4
#define FB_MAGENTA      5
#define FB_BROWN        6
#define FB_LT_GREY      7
#define FB_DARK_GREY    8
#define FB_LT_BLUE      9
#define FB_LT_GREEN    10
#define FB_LT_CYAN     11
#define FB_LT_RED      12
#define FB_LT_MAGENTA  13
#define FB_LT_BROWN    14
#define FB_WHITE       15

// IO PORTS
#define FB_COMMAND_PORT 0x3D4
#define FB_DATA_PORT    0x3D5

// IO PORT COMMANDS
#define FB_HIGH_BYTE_COMMAND    14 // move cursor command low
#define FB_LOW_BYTE_COMMAND     15 // move cursor command high


/** fb_write_cell:
 *  used to write a character to a cell in the framebuffer
 *
 * param i which cell to write to
 * param c the ascii char to write
 * param fg foreground color
 * param bf background color
 */
void fb_write_cell(unsigned int i, char c, unsigned char fg, unsigned char bg);


/** fb_move_cursor:
 *  used to move the cursor within the frame buffer
 *
 *  param pos position within frame buffer to move cursor to
 */
void fb_move_cursor(unsigned short pos);


/** fb_write:
 *  write some text to the cursor
 *
 *  param buf pointer to character string
 *  param len length of string to write
 */
int fb_write(char *buf, unsigned int len);

fb.c

#include "fb.h"

void fb_write_cell(unsigned int i, char c, unsigned char fg, unsigned char bg)
{
    char *fb = (char *) 0x000B8000;
    fb[i*2] = c;
    fb[i*2 + 1] = ((fg & 0x0F) << 4) | (bg & 0x0F);
}

void fb_move_cursor(unsigned short pos) {
    outb(FB_COMMAND_PORT, FB_HIGH_BYTE_COMMAND);
    outb(FB_DATA_PORT, ((pos>>8) & 0x00FF));
    outb(FB_COMMAND_PORT, FB_LOW_BYTE_COMMAND);
    outb(FB_DATA_PORT, pos & 0x00FF);
}

int fb_write(char *buf, unsigned int len) {

    unsigned int i = 0;
    for(i = 0; i < len; i++) {
        fb_write_cell(i, buf[i], FB_BLACK, FB_WHITE);
    }

    return 0;

}

Building it

I have a linker script called link.ld and a Makefile. I'm using gcc cross compiler for i386-elf That I compiled using this guide (http://wiki.osdev.org/GCC_Cross-Compiler).

ENTRY(loader)                /* the name of the entry label */

SECTIONS {
    . = 0x00100000;          /* the code should be loaded at 1 MB */

    .text ALIGN (0x1000) :   /* align at 4 KB */
    {
        *(.text)             /* all text sections from all files */
    }

    .rodata ALIGN (0x1000) : /* align at 4 KB */
    {
        *(.rodata*)          /* all read-only data sections from all files */
    }

    .data ALIGN (0x1000) :   /* align at 4 KB */
    {
        *(.data)             /* all data sections from all files */
    }

    .bss ALIGN (0x1000) :    /* align at 4 KB */
    {
        sbss = .;
        *(COMMON)            /* all COMMON sections from all files */
        *(.bss)              /* all bss sections from all files */
        ebss = .;


    }
}

And here is my makefile

OBJECTS = io.o fb.o loader.o kmain.o
#CC = gcc
CC = /home/albertlockett/opt/cross/bin/i386-elf-gcc
CFLAGS = -m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector \
         -nostartfiles -nodefaultlibs -Wall -Wextra -Werror -c
LDFLAGS = -T link.ld -melf_i386
AS = nasm
ASFLAGS = -f elf

all: kernel.elf

kernel.elf: $(OBJECTS)
    ld $(LDFLAGS) $(OBJECTS) -o kernel.elf

os.iso: kernel.elf
    cp kernel.elf iso/boot/kernel.elf
    genisoimage -R                              \
                -b boot/grub/stage2_eltorito    \
                -no-emul-boot                   \
                -boot-load-size 4               \
                -A os                           \
                -input-charset utf8             \
                -quiet                          \
                -boot-info-table                \
                -o os.iso                       \
                iso

run: os.iso
    bochs -f bochsrc.txt -q

%.o: %.c
    $(CC) $(CFLAGS)  $< -o $@

%.o: %.s
    $(AS) $(ASFLAGS) $< -o $@

clean:
    rm -rf *.o kernel.elf os.iso

Run it

The makefile builds an iso from the contents of a directory called iso. That folder contains a preconfigured version of grub that I got here (https://github.com/littleosbook/littleosbook/blob/master/files/stage2_eltorito) and a menu.lst file for grub

menu.lst:

default=0
timeout=0

title os
kernel /boot/kernel.elf

contents of iso directory:

iso
`-- boot
    |-- grub
    |   |-- menu.lst
    |   `-- stage2_eltorito
    `-- kernel.elf

The iso image boots in bochs. Here is my bochsrc.txt file

megs:            32
display_library: term
romimage:        file=/usr/share/bochs/BIOS-bochs-latest
vgaromimage:     file=/usr/share/bochs/VGABIOS-lgpl-latest
ata0-master:     type=cdrom, path=os.iso, status=inserted
boot:            cdrom
log:             bochslog.txt
clock:           sync=realtime, time0=local
cpu:             count=1, ips=1000000
com1:            enabled=1, mode=file, dev=com1.out

Does anyone know why the string literal in the kernel file produces the error when I try to boot the iso?

Answer 1

You have an extra colon at the end of section .text: so that creates a new section named .text:. For some obscure reason that I couldn't find out from a quick glance at the documentation, this section is emitted to the output even though it is not listed in your linker script. When you have no literal data in the C code, you are lucky that it still falls within the first 8kiB of the image, so that the multiboot header is in the required portion. If you do have a string literal, you will get a new section .rodata and that, for yet another obscure reason, gets sorted before your .text: but after the standard .text. Example:

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000001  00100000  00100000  00001000  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .rodata       00000005  00101000  00101000  00002000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  2 .text:        00000018  00101008  00101008  00002008  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  3 .bss          0000100a  00102000  00102000  00003000  2**2
                  ALLOC

As you can see it's no longer within the first 8kiB of the image, so grub will be very sad.

TL;DR: remove the extra colon after section .text:.