Re: Windows,grub and grub2

[Viswesh S <viswesh_vichu@yahoo.com>]

Hi,



----- Original Message ----
> From: Bean <bean123ch@gmail.com>
> To: The development of GRUB 2 <grub-devel@gnu.org>
> Sent: Monday, 29 September, 2008 4:20:57 PM
> Subject: Re: Windows,grub and grub2
> 
> On Mon, Sep 29, 2008 at 1:18 PM, Viswesh S wrote:
> > Hi,
> >
> >
> >
> > ----- Original Message ----
> >> From: Bean 
> >> To: The development of GRUB 2 
> >> Sent: Tuesday, 23 September, 2008 7:20:49 PM
> >> Subject: Re: Windows,grub and grub2
> >>
> >> On Tue, Sep 23, 2008 at 4:23 PM, Viswesh S wrote:
> >> >
> >> >
> >> >
> >> >
> >> > ----- Original Message ----
> >> >> From: Bean
> >> >> To: The development of GRUB 2
> >> >> Sent: Monday, 22 September, 2008 9:10:26 AM
> >> >> Subject: Re: Windows,grub and grub2
> >> >>
> >> >> On Tue, Sep 9, 2008 at 2:00 PM, Viswesh S wrote:
> >> >> > Below is the dump of screen output while chainloading the ntfsnew file.
> >> >> > ***************************************
> >> >> > DI=CFF0 SI=07EE BP=1FF0 SP=1FE8 BX=0000 DX=0000 CX=0000 AX=0000
> >> >> > CS=0000 SS=0000 DS=0000 ES=0000 FG=0246 IP=7C57
> >> >> >
> >> >> > DI=7FF0 SI=07EE BP=1FF0 SP=7BF4 BX=55AA DX=0000 CX=0000 AX=0100 CX=07C0
> >> >> > DS=07C0 ES=0000 FG=0007 IP=0082
> >> >> > ******************************************
> >> >> > Could you please let me know the way to disassemble the binary file 
> without
> >> >> > any header.The way in which you decoded the boot record.
> >> >> >
> >> >> > Also one more thing to let you know is that,
> >> >> >
> >> >> > with the grub-1.96 ( without the chainloader patch of disk->dev->read() 
> ) ,
> >> >> > with windows2003 in partition 1 and linux in partition 3, when we
> >> chainload,
> >> >> > if we look at the partition table passed to another bootloader ie 
> location
> >> >> > 0x7be - we can see that it is junk, but the surprising point is that, in
> >> >> > this case as I have mentioned in my first mail, windows boots up from
> >> >> > grub2.So it is that the partition table is not required for the 
> chainloader
> >> >> > thing and just the boot record is sufficient
> >> >>
> >> >> Hi,
> >> >>
> >> >> Oh, sorry for another long delay. I disassemble the file with ida,
> >> >> which is an amazing tool. I don't know if there is open source
> >> >> alternative, please let me know if you find one.
> >> >>
> >> >> The output from ida is in masm format, I modify it a bit so that it
> >> >> can be compiled using nasm. Please note that nasm doesn't generate the
> >> >> same binary file as original one, but you can get an idea what it
> >> >> does.
> >> >>
> >> >> From the output, the program fails at the second int 13 call, int
> >> >> 13/ah = 48h. Although I notice that DL=0, which is not supposed to
> >> >> happen. Perhaps you can add a grub_printf in grub_chainloader_boot to
> >> >> show the value of boot drive:
> >> >>
> >> >> static grub_err_t
> >> >> grub_chainloader_boot (void)
> >> >> {
> >> >>   grub_printf ("boot_drive=%d\n", boot_drive);
> >> >>   grub_chainloader_real_boot (boot_drive, boot_part_addr);
> >> >>
> >> >>   /* Never reach here.  */
> >> >>   return GRUB_ERR_NONE;
> >> >> }
> >> >>
> >> >> --
> >> >> Bean
> >> >>
> >> >>
> >> >
> >> > Hi,
> >> >
> >> > The value of boot drive is 0x80.
> >> >
> >> > This was the same value in disk->drive also.
> >>
> >> Hi,
> >>
> >> Interesting, perhaps %dx is changed somewhere. Please try the
> >> following patch, it dumps the value of %dx just before jumping to the
> >> boot sector.
> >>
> >> --
> >> Bean
> >
> > The patch works and now Windows is booting perfectly fine from Grub2.
> >
> > I will go through the assembly and try to understand what modifications you 
> have done.So there is a problem in Grub2 code, which needs to be fixed ?
> >
> > Till this point, I was chainloading grub from Grub2 and then chainloading 
> Windows2008 from it.
> >
> > Thanks for the consistent help till this point and for the future also.
> 
> Hi,
> 
> That's strange, the patch doesn't do anything except output the value of dx:
> 
>     /* set up to pass boot drive */
>     popl    %edx
> +    movl    %edx, %edi
> 
>     /* ESI must point to a partition table entry */
>     popl    %esi
> 
>     call    prot_to_real
>     .code16
> +
> +    push    %dx
> +    call    hex_out
> +    push    %di
> +    call    hex_out
> +
>     ljmp    $0, $GRUB_MEMORY_MACHINE_BOOT_LOADER_ADDR
> +
> +hex_out:
> +    pushw    %bp
> +    movw    %sp, %bp
> +    pushaw
> +    movb    $0xE, %ah
> +     movw    $7, %bx
> +    movw    $4, %cx
> +    movw    4(%bp), %dx
> +1:
> +    rol    $4, %dx
> +    movb    %dl, %al
> +    andb    $0xF, %al
> +    cmpb    $10, %al
> +    jb    2f
> +    subb    $('0'-'A'+10), %al
> +2:
> +    addb    $'0', %al
> +    int    $0x10
> +    loop    1b
> +    movb    $' ', %al
> +    int    $0x10
> +    popaw
> +    popw    %bp
> +    ret    $2
>     .code32
> 
> #include "../loader.S"
> 
> Perhaps you can try:
> 
> 1, %edi is used as backup register in case %edx is changed by
> prot_to_real, you can remove "movl %edx, %edi", "push %di", "call
> hex_out" and see if it still works.
> 
> 2, It's possible that the bug is position related, replace "push %dx",
> "call hex_out" with equal number of nop and see what happens.
> 
> -- 
> Bean
> 

We can remove the patch completely, without putting nop also, but just comment out the following code

/*
    xorl    %eax, %eax
    call    EXT_C(grub_gate_a20)
*/

This is the only difference and when I comment out this, Windows boots up from Grub2.

Regards,
Viswesh


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