Re: possible bug in i386 boot.S floppy probing code

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 10/26/2010 03:08 PM, Mirko Parthey wrote:
> Hello all,
>
> while reading through grub-core/boot/i386/pc/boot.S in the Bazaar version
> of Grub 2, I noticed that the floppy probing code calls
> int $0x13, %ah===$0x02  /* disk read sector */
> without initializing the %es register, and with %bx holding a value
> that was likely meant to go into %es. (boot.S:465)
>
>   
You're right patch applied
> My proposed change is included below. The two additional instructions have 
> been
> copied from further above in boot.S, where the same BIOS function is
> used correctly. The resulting machine code is enlarged by 4 bytes, but
> apparently still fits within the 512-byte limit.
>
> Please note that I found this by inspection, not by observing undesired
> behaviour.  The patch passes "make" and "make check", but I'm not sure if the
> code will actually be exercised by "make check".
> I hope you find it useful.
>
> Mirko
>
> === modified file 'grub-core/boot/i386/pc/boot.S'
> --- grub-core/boot/i386/pc/boot.S     2010-09-19 22:06:45 +0000
> +++ grub-core/boot/i386/pc/boot.S     2010-10-26 09:51:29 +0000
> @@ -459,6 +459,8 @@
>  1:
>       /* perform read */
>       movw    $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
> +     movw    %bx, %es
> +     xorw    %bx, %bx
>       movw    $0x201, %ax
>       movb    $0, %ch
>       movb    $0, %dh
>
>
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>   


-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko

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