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[modrom] Avoid clobbering near jump with checksum 

A jump instruction starts at the third byte of an option ROM image, and
it is required that the bytes in the whole image add up to zero. To
achieve this, a checksum byte is usually placed after the jump. The jump
can be either a short jump (2 bytes, EB xx) or a near jump (3 bytes,
E9 xx xx). gPXE's romprefix.S uses a near jump, but modrom.pl assumed
a short jump, and clobbered the high byte of the offset. This caused
modrom-modified gPXE ROM images to crash the system during POST.

Fix by making modrom.pl place the checksum at byte 6, like makerom.pl does.

Signed-off-by: Marty Connor <mdc@etherboot.org>
This commit is contained in:
Joshua Oreman 2009-10-07 16:40:49 -04:00 committed by Marty Connor
parent 3fa2779208
commit b0b0b8f65c
1 changed files with 2 additions and 2 deletions
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4
src/util/modrom.pl
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@ -131,9 +131,9 @@ sub writerom ($$) {
sub checksum ($) {
my ($romref) = @_;
substr($$romref, 5, 1) = "\x00";
substr($$romref, 6, 1) = "\x00";
my $sum = unpack('%8C*', $$romref);
substr($$romref, 5, 1) = chr(256 - $sum);
substr($$romref, 6, 1) = chr(256 - $sum);
# Double check
$sum = unpack('%8C*', $$romref);
if ($sum != 0) {