Re: Problem with loading 64-bit ELF image with multiboot2

["Vladimir 'φ-coder/phcoder' Serbinenko" <phcoder@gmail.com>]

On 22.07.2013 13:46, Pawel Wojtalczyk wrote:
> Hello,
>
> I use x86 64-bit GRUB2 under EFI and would like to load 64-bit ELF
> image. The image looks as following:
>
Rather than bringing the issue once more time in a different place, look 
that this was already fixed in trunk.
> $ objdump -h Image
>
> Image:     file format elf64-x86-64
>
> Sections:
> Idx Name          Size      VMA               LMA               File
> off  Algn
>    0 .text         0019d870  ffffffff90008000  0000000010008000
> 00000120  2**5
>                    CONTENTS, ALLOC, LOAD, CODE
>    1 .wrs_build_vars 00000150  ffffffff901a5870  00000000101a5870
> 0019d990  2**0
>                    CONTENTS, ALLOC, LOAD, READONLY, DATA
>    2 .data         0000db50  ffffffff901a59c0  00000000101a59c0
> 0019db00  2**6
>                    CONTENTS, ALLOC, LOAD, DATA
>    3 .bss          01016e40  ffffffff901b3520  00000000101b3520
> 001ab660  2**5
>                    ALLOC
> $ objdump -f Image
>
> Image:     file format elf64-x86-64
> architecture: i386:x86-64, flags 0x00000012:
> EXEC_P, HAS_SYMS
> start address 0xffffffff90008000
>
> I tried to load it via GRUB2, but unfortunately it displays 'invalid
> entry point for ELF64'. The problem is that for 64-bit ELF images the
> multiboot_elfxx.c library still requires 32-bit virtual address:
>
> #ifdef MULTIBOOT_LOAD_ELF64
> # if defined( __mips)
>    /* We still in 32-bit mode.  */
>    if (ehdr->e_entry < 0xffffffff80000000ULL)
>      return grub_error (GRUB_ERR_BAD_OS, "invalid entry point for ELF64");
> # else
>    /* We still in 32-bit mode.  */
>    if (ehdr->e_entry > 0xffffffff)
>      return grub_error (GRUB_ERR_BAD_OS, "invalid entry point for ELF64");
> # endif
> #endif
>
> Could you tell me why this assumption is required? The virtual address
> (e_entry is virtual following ELF specification) is related to the image
> itself, how it setup MMU, not the loader (GRUB2). GRUB2 should check
> only if the physical location of loaded image is in 32-bit physical memory.
>
> Besides if we skip thich e_entry check, the image is properly loaded,
> because grub_multiboot_payload_eip is properly computed few lines below.
>
> grub_multiboot_payload_eip = (ehdr->e_entry - phdr(i)->p_vaddr)
>        + phdr(i)->p_paddr;
>
> Could you tell me why GRUB2 checks for 64-bit images this e_entry
> virtual address (which can be everywhere in the 64-bit address space)
> and requires it to be 32-bit address space only?
>
> Regards
> Pawel Wojtalczyk
>
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel
>