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[Patch] Fix VESA VBE BIOS support in GRUB Legacy

From: Martin Decky
Subject: [Patch] Fix VESA VBE BIOS support in GRUB Legacy
Date: Sat, 20 Nov 2010 19:54:49 +0100
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Hello folks,

GRUB Legacy has two builtin commands "vbeprobe" and "testvbe" which are intended to test VESA VBE BIOS graphics modes (e.g. for manually testing the correct Linux VESA framebuffer mode before setting the vga= Linux kernel command line value).

However, on many recent graphics cards which are VESA 3.0 compliant these commands do not work. The reason is a wrong definition of the vbe_mode structure in stage2/shared.h. The VBE mode info structure should be 256 bytes long, but here it is actually only 255 bytes long, the array "unsigned char reserved3[189]" should be actually declared as "unsigned char reserved3[190]" (frankly, the same mistake is also present in the official VESA VBE 3.0 specification document).

As the vbe_mode structure is allocated on stack in vbeprobe_func() and testvbe_func() (stage2/builtins.c), if the VESA BIOS actually stores all 256 bytes in the 0x4f01 call (Get Mode Info), the last byte overruns the structure and ends in a different variable on the stack (mode_number in case of vbeprobe_func()), thus rendering both routines non-functional.

Please see a trivial patch attached to this email which solves the issue. Thanks for committing it to the mainline of GRUB Legacy -- I understand that GRUB Legacy is no longer in development, but this is a stack overrun type of bug which should be fixed. I have not checked GRUB 2 whether it contains the same mistake, but somebody might have a look on it.

Best regards

Martin Decky
    Mgr. Martin Decky
     Department of Distributed and Dependable Systems,
     Faculty of Mathematics and Physics,
     Charles University in Prague,
     Czech Republic

    address@hidden                              address@hidden
    http://d3s.mff.cuni.cz/~decky/                 http://www.decky.cz

Attachment: grub-0.97-vbe.patch
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