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Striking gold in binutils

Striking gold in binutils

Posted Apr 3, 2008 19:03 UTC (Thu) by stock (guest, #5849)
In reply to: Striking gold in binutils by nix
Parent article: Striking gold in binutils

" Digging the commands out of the makefile by hand is madness."  
  
You bet, but IF you would do such a thing, you would understand that on a  
64-bit platform we are actually booting a 32bit binary :  
  
ld -m elf_i386  -Ttext 0x100000 |\  
        -e startup_32 -m elf_i386 |\  
        arch/x86_64/boot/compressed/head.o |\  
        arch/x86_64/boot/compressed/misc.o |\  
        arch/x86_64/boot/compressed/piggy.o |\  
        -o arch/x86_64/boot/compressed/vmlinux  
  
Apparently we are actually booting a 32bit binary on our x86_64 platform.  
But that doesn't matter as long as we call our real kernel piggy.o :  
  
ld -m elf_i386  -r --format binary --oformat elf32-i386 |\  
        -T arch/x86_64/boot/compressed/vmlinux.scr |\  
        arch/x86_64/boot/compressed/vmlinux.bin.gz |\  
        -o arch/x86_64/boot/compressed/piggy.o  
  
where vmlinux.bin.gz is created by the following command :  
  
gzip -f -9 < arch/x86_64/boot/compressed/vmlinux.bin > |\  
        arch/x86_64/boot/compressed/vmlinux.bin.gz  
  
where vmlinux.bin is created by running a custom tool called objcopy :  
  
objcopy -O binary -R .note -R .comment -S  vmlinux |\  
         arch/x86_64/boot/compressed/vmlinux.bin  
  
where finally vmlinux is that ELF 64-bit LSB executable :  
  
ld -m elf_x86_64 -e stext -T arch/x86_64/kernel/vmlinux.lds.s |\  
     arch/x86_64/kernel/head.o arch/x86_64/kernel/head64.o |\  
     arch/x86_64/kernel/init_task.o   init/built-in.o |\  
     --start-group  usr/built-in.o  arch/x86_64/kernel/built-in.o |\  
     arch/x86_64/mm/built-in.o  arch/x86_64/ia32/built-in.o |\  
     kernel/built-in.o  mm/built-in.o  fs/built-in.o  ipc/built-in.o |\  
     security/built-in.o  crypto/built-in.o  lib/lib.a |\  
     arch/x86_64/lib/lib.a  lib/built-in.o  arch/x86_64/lib/built-in.o |\  
     drivers/built-in.o  sound/built-in.o  arch/x86_64/pci/built-in.o |\  
     net/built-in.o --end-group .tmp_kallsyms2.o -o vmlinux  
  
   -rwxr-xr-x  1 root root 7478601 Nov 21 14:48 vmlinux  
  
   vmlinux: ELF 64-bit LSB executable, AMD x86-64, version 1 (SYSV),  
            statically linked, not stripped  
  
Cheers,  
  
Robert  
--   
Robert M. Stockmann - RHCE  
Network Engineer - UNIX/Linux Specialist  
crashrecovery.org  stock@stokkie.net  


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Striking gold in binutils

Posted Apr 3, 2008 20:31 UTC (Thu) by nix (subscriber, #2304) [Link]

IF you would do such a thing, you would understand that on a 64-bit platform we are actually booting a 32bit binary
Actually, the only things that differ between the elf_x86-64 and elf_i386 emulations are details of the GOT and PLT (which don't exist for the kernel normally), details of relocation processing (which isn't carried out for the kernel), and the start address (which the kernel overrides anyway in its custom linker script). So since they do the same thing as far as the kernel's concerned, let's pick the one that everyone's got because it's present even in older x86 binutils and always present in newer ones, and that's elf_i386.

(At least that's my understanding.)

where vmlinux.bin is created by running a custom tool called objcopy :
Not only is objcopy not a 'custom' tool, but part of the GNU binutils, but it predates Linux (although before about 1993 it was called 'copy'). (The renaming was done by, oh look, Ian Lance Taylor. He's been involved in this area for a long, long time. ;} )

(The objcopy run is anyway hardly crucial: like most of the stuff before the final link, it's just a size optimization.)


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