[coreboot] [Bug?] 3 different bytes in VGAbios after upgrade to coreboot

[ron minnich]

interesting. Not sure what is up here but I have seen that many VGA bioses
copy themselves to RAM and then modify their own code. Actually, I have
seen many BIOSes that do this self modifying code hack; they use it to
change behavior for a warm restart; copy themselves to shadow RAM and then
change a byte or two so the jump target is different.. I've no idea if
that's what is going on here but thought I'd mention it.

ron

On Tue, Jun 2, 2015 at 4:17 PM Vladimir <quickcracktime at gmail.com> wrote:

> I retrieved my vgabios via Linux kernel 3.19 (ubuntu 15.04) using this
> method - http://www.coreboot.org/VGA_support#Retrieval_via_Linux_kernel
> ; then I downloaded latest sources of SeaBIOS & Coreboot and built Coreboot
> using compiled SeaBIOS payload as well as vgabios file ( 1002,990b )
>
> After coreboot installation I have decided to retrieve vgabios again,
> using exactly the same kernel and OS, and then diff'ed two vgabios files
> out of curiosity. To my surprise, they are slightly different!
> vgabios_before.bin - SHA1 checksum:
> e4d320eb278b0118c46e2e470e7154b12c41966d
> vgabios__after.bin - SHA1 checksum:
> a9e2ed569bbaaea283b5380a5f6c44fc4efc3da4
> Here is a report about 3 bytes difference between them -
> http://www.diffnow.com/?report=2kwq3
> (wait a few seconds while it loads)
>
> Then I teardown a laptop, and check if vgabios inside coreboot's image
> (flashed in chip) is 3 bytes different as well, but there was no difference
> against the original vgabios.
>
> So, it appears that, while loading vgabios from a flash chip, coreboot
> modifies it slightly. Although this tiny difference is not causing any
> graphical glitches or problems for me, this could be a result of a bug -
> which is not necessarily limited to my hardware ( Lenovo G505s ) , and
> maybe could lead to some other problems
>
> Please tell your opinion, is it a bug or I am wrong at something?
>
> Best regards,
> Vladimir Shipovalov
> --
> coreboot mailing list: coreboot at coreboot.org
> http://www.coreboot.org/mailman/listinfo/coreboot
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