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Posted: **Sat Nov 17, 2012 11:00 pm**

Hello, I was able to get the JamesM kernel tutorial completely, and I've a full Initrd which is loaded to memory.

now I've the executable file at 0x116834, and I parsed the ELF header to find the entry address which is at 0x100000.
I know I should jump to that address and I did use Fork and so on... but I don't know how can I get the real code address in this file as it's different to the memory.

thanks, and sorry if it's a noob question, I'm trying to learn about all these stuff using tutorials and so on.. )))

Posted: **Sat Nov 17, 2012 11:23 pm**

You should parse the program headers as well. Those headers give information about parts of an ELF which should be loaded into the address space (e.g, a segment with type LOAD should be loaded at the virtual address in its corresponding program header).

a simple loader may look like:

Code: Select all


static
void write_elf_segment(elf32_program_hdr_t *phdr, uint_t image_addr) {
    memcpy((void *) (phdr->p_vaddr), 
           (void *) (image_addr + phdr->p_offset), 
           phdr->p_filesz);
}

int elf32_load(uint_t image_addr) {
    elf32_hdr_t *hdr = (elf32_hdr_t *) image_addr;

    if (hdr->e_ident[0] != 0x7F &&
        hdr->e_ident[1] != 'E' &&
        hdr->e_ident[2] != 'L' &&
        hdr->e_ident[3] != 'F')
        return -1;

    elf32_program_hdr_t *phaddr = 
        (elf32_program_hdr_t *) (image_addr + hdr->e_phoff);

    uint_t idx;
    for (idx = 0; idx < hdr->e_phnum; idx++) {
        if (phaddr[idx].p_type == PT_LOAD) {
            write_elf_segment(&phaddr[idx], image_addr);
            if (phaddr[idx].p_filesz < phaddr[idx].p_memsz) {
                memset((void *) (phaddr[idx].p_vaddr + phaddr[idx].p_filesz),
                       0,
                       phaddr[idx].p_memsz - phaddr[idx].p_filesz);
            }
        }
    }

    return 0;
}

provided that we're in the right address space.

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