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[int13] Zero all possible registers when jumping to a boot sector

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At least one boot sector (the DUET boot sector used for bootstrapping
EFI from a non-EFI system) fails to initialise the high words of
registers before using them in calculations, leading to undefined
behaviour.

Work around such broken boot sectors by explicitly zeroing the
contents of all registers apart from %cs:%ip and %ss:%sp.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown 2012-09-14 15:42:54 +01:00
parent 8509dbf86a
commit 73eb3f17db
1 changed files with 14 additions and 1 deletions
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15
src/arch/i386/image/bootsector.c
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@ -80,9 +80,22 @@ int call_bootsector ( unsigned int segment, unsigned int offset,
"movw %%ss, %%ax\n\t"
"movw %%ax, %%cs:saved_ss\n\t"
"movw %%sp, %%cs:saved_sp\n\t"
/* Jump to boot sector */
/* Prepare jump to boot sector */
"pushw %%bx\n\t"
"pushw %%di\n\t"
/* Clear all registers */
"xorl %%eax, %%eax\n\t"
"xorl %%ebx, %%ebx\n\t"
"xorl %%ecx, %%ecx\n\t"
"xorl %%edx, %%edx\n\t"
"xorl %%esi, %%esi\n\t"
"xorl %%edi, %%edi\n\t"
"xorl %%ebp, %%ebp\n\t"
"movw %%ax, %%ds\n\t"
"movw %%ax, %%es\n\t"
"movw %%ax, %%fs\n\t"
"movw %%ax, %%gs\n\t"
/* Jump to boot sector */
"sti\n\t"
"lret\n\t"
/* Preserved variables */