In this line of code in real_to_prot():
DATA32 ADDR32 lgdt %cs:gdtdesc
GAS generates an absolute address for `gdtdesc' (not relative to segment),
and so for the code to work %cs must be zero. In current usage of
real_to_prot(), %cs is always zero because we jump to 0x0:0x82xx early on.
However, in other situations this is not possible. On i386-qemu, before
moving to i386 mode the code we're running is in the 0xf0000-0x100000
range, which is inaccessible from segment 0.
This patch changes this to access gdtdesc from segment 0 unconditionally.
--
Robert Millan
The DRM opt-in fallacy: "Your data belongs to us. We will decide when (and
how) you may access your data; but nobody's threatening your freedom: we
still allow you to remove your data and not access it at all."
2009-06-22 Robert Millan <rmh.g...@aybabtu.com>
* kern/i386/pc/startup.S (real_to_prot): Access `gdtdesc' using
segment 0x0 unconditionally, because the reference generated by
GAS is an absolute address.
Index: kern/i386/pc/startup.S
===================================================================
--- kern/i386/pc/startup.S (revision 2353)
+++ kern/i386/pc/startup.S (working copy)
@@ -1,6 +1,6 @@
/*
* GRUB -- GRand Unified Bootloader
- * Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2008 Free Software Foundation, Inc.
+ * Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2008,2009 Free Software Foundation, Inc.
*
* GRUB is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
@@ -313,13 +313,9 @@
cli
/* load the GDT register */
-#ifdef APPLE_CC
- mov %cs, %ax
- mov %ax, %ds
- DATA32 ADDR32 lgdt gdtdesc
-#else
- DATA32 ADDR32 lgdt %cs:gdtdesc
-#endif
+ xorw %ax, %ax
+ movw %ax, %ds
+ DATA32 ADDR32 lgdt %ds:gdtdesc
/* turn on protected mode */
movl %cr0, %eax
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