Re: 8bit and 16bit relocations in gold

["H. Peter Anvin" <hpa at zytor dot com>]

Andi Kleen wrote:
> Ian,
>
> With the help of H. Peter Anvin I poked a bit more at the gold linux kernel 
> boot problems.
>
> First the crash happen really in the real mode code which has 8 and 16bit
> relocations (arch/x86/boot/* in a modern linux kernel tree). This is
> generated from a custom linker script (arch/x86/boot/setup.ld) 
>
> Comparing to a good kernel we already see some differences just in the header:
>
> e.g. in the setup.bin which is just the binary dump of that code
>
> -0000200 46eb 6448 5372 0207 0000 0000 1000 2900      (bfd)
> +0000200 46eb 6449 5372 0207 0000 0000 1000 2900      (gold) 
>
> which is generated from this assembler code in header.S
>
>         .section ".header", "a"
>         .globl  hdr
> hdr:
> setup_sects:    .byte SETUPSECTS
> root_flags:     .word ROOT_RDONLY
> syssize:        .long SYSSIZE
> ram_size:       .word RAMDISK
> vid_mode:       .word SVGA_MODE
> root_dev:       .word ROOT_DEV
> boot_flag:      .word 0xAA55

Actually, that snippet *ends* at 0x200, the code that generates the 
snipped above is:

        # offset 512, entry point

        .globl  _start
_start:
                # Explicitly enter this as bytes, or the assembler
                # tries to generate a 3-byte jump here, which causes
                # everything else to push off to the wrong offset.
                .byte   0xeb            # short (2-byte) jump
                .byte   start_of_setup-1f
1:

        # Part 2 of the header, from the old setup.S

                .ascii  "HdrS"          # header signature
                .word   0x0208          # header version number (>= 0x0105)
                                        # or else old loadlin-1.5 will 
fail)
                .globl realmode_swtch
realmode_swtch: .word   0, 0            # default_switch, SETUPSEG
start_sys_seg:  .word   SYSSEG
                .word   kernel_version-512 # pointing to kernel version 
string
                                        # above section of header is 
compatible
                                        # with loadlin-1.5 (header 
v1.5). Don't
                                        # change it.

The second .byte is what generates the R_386_PC8 relocation.

	-hpa