On 12/13/2012 11:46 AM, Brian S. Stephan wrote:
Hi,
Sorry for the direct email, this wasn't really a bug per se so I didn't
feel it appropriate to open up something on bugs.gentoo.org.
I was having a problem getting hardened-sources-3.5.4-r1 to boot on a
Linode (a Xen hypervisor VPS, if you're not familiar) using PV-GRUB, and
after some debugging I whittled it down to PV-GRUB not loading the kernel
properly on 1024 MB RAM systems --- the identical .config worked fine on
my 512 MB nodes, and merely limiting the 1024 MB node to 512 MB made that
kernel boot.
Playing around with configs, I found that the following made the kernel
boot on 1024 MB:
http://www.incorporeal.org/~bss/linode/ZZZZ-diff-between-0-and-G-plus-patch
It just sets the PHYSICAL_START and PHYSICAL_ALIGN ranges back to their
original values. I have been running with that patch for a month now with
no noticed downside.
I'm willing to chalk this all up to a PV-GRUB bug, but curiosity got the
better of me and I started looking at the patch, and was curious what
effect changing those ranges has? The patch hardly makes reference to
either config setting, other than changing the range.
Anyway, this is more just to satisfy my poking around more than anything
else. Any thoughts or info you have would certainly be interesting ---
and I'm fine with applying this patch myself in the future, so I'm not
necessarily looking for "a fix".
Thanks,
bss
Okay, I'm mostly guessing here, but I think this is right. PV-GRUB is a
multiboot loader as grub is and it expects the kernel to comply with
certain standard. For some reason (bug?) it can't handle a kernel with
a physical start address at 0x1000000. Grub, and probably PV-GRUB,
looks for a sentinel word MBOOT_HEADER_MAGIC = 0x1BADB002 and knows this
is the start of the kernel --- something like 32 bytes in is where it
places _start: PV-GRUB must fail when it has to look all the way up to
0x1000000 but succeeds in finding the sentinel by 0x100000.
If you really want to nail it, create a tiny bootable kernel using
Molloy's code at
http://www.jamesmolloy.co.uk/tutorial_html/2.-Genesis.html. Play with
the linker script link.ld and keep increasing the beginning of the .text
section. See at what point PV-GRUB fails. That'll be your answer.
Then, if you really have lots of time on your hand, read PV-GRUB code
and see why :)
On a different note, bug the linode people for hardened gentoo because I
did and they ignored me. I'll build the image for them and everything
... are their engineers really that lazy?
BTW, you can send these sorts of questions to gentoo-hardened@ there are
other people that are inerested.
--
Anthony G. Basile, Ph.D.
Gentoo Linux Developer [Hardened]
E-Mail : [email protected]
GnuPG FP : 8040 5A4D 8709 21B1 1A88 33CE 979C AF40 D045 5535
GnuPG ID : D0455535
