fixing chmod with bad perms

As root in /:

tar cf chmod.tar bin/chmod
printf '755' | dd of=chmod.tar bs=1 seek=103 count=3 conv=notrunc
printf '013305' | dd of=chmod.tar bs=1 seek=148 count=6 conv=notrunc
tar xf chmod.tar

This assumes permissions 000, other permissions will have a lower
checksum number.  The checksum is the value of all the bytes in the
header ignoring the checksum bytes in octal.  So, if the checksum was
013264 (permissions 000) then changing the permissons to 755 will add
021 ( 7 + 5 + 5 = 17 = 021) to it resulting in 013305. Likewise, if
the permissions were 666, then you would subtract one from the checksum
(1 + -1 + -1 = -1).