[Pkg-xen-devel] Bug#745419: xen-utils-4.1: Pygrub fails to boot from LVM LV when something installed in the volume boot record

[Sjors Gielen]

Package: xen-utils-4.1
Version: 4.1.4-3+deb7u1
Severity: important

When an LVM LV that serves as the root disk for a Xen DomU contains a boot
loader (or possibly other data) in its volume boot record, pygrub fails to boot
it, printing "Error: boot loader didn't return any data" before exiting.

I think this is because of the function "is_disk_image" on line 45 of
/usr/lib/xen-4.1/bin/pygrub: it should return false on LVM LV's because they
are not disk images but virtual devices, but it returns true because of certain
bytes in the first block of 512 bytes being set. This causes
get_partition_offsets to seek fruitlessly for a partition instead of simply
returning [0], causing the bootloader not to find the right partition.

Detailed steps to reproduce:
We have a domU called blup.kassala.de, it is running Jessie, its root disk is
a LVM LV at /dev/vpsdisks/blup.kassala.de-disk. The first 512 bytes are null:

# dd if=/dev/vpsdisks/blup.kassala.de-disk bs=1 count=512 | hexdump -C
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200

"xm create -c blup.kassala.de.cfg" shows the machine boots fine. After boot, we
log in and "apt-get install grub2". During the installation, we say to install
the bootloader to /dev/xvda2, the root disk. We then halt the system. The first
512 bytes are now non-null:

# dd if=/dev/vpsdisks/blup.kassala.de-disk bs=1 count=512 | hexdump -C
00000000  eb 63 90 00 00 00 00 00  00 00 00 00 00 00 00 00  |.c..............|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000050  00 00 00 00 00 00 00 00  00 00 00 80 80 12 46 00  |..............F.|
00000060  00 00 00 00 ff fa 90 90  f6 c2 80 74 05 f6 c2 70  |...........t...p|
[snip]
00000180  7d e8 2e 00 cd 18 eb fe  47 52 55 42 20 00 47 65  |}.......GRUB .Ge|
00000190  6f 6d 00 48 61 72 64 20  44 69 73 6b 00 52 65 61  |om.Hard Disk.Rea|
000001a0  64 00 20 45 72 72 6f 72  0d 0a 00 bb 01 00 b4 0e  |d. Error........|
000001b0  cd 10 ac 3c 00 75 f4 c3  00 00 00 00 00 00 00 00  |...<.u..........|
000001c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

We try to boot the DomU again:

# xm create -c blup.kassala.de.cfg 
Using config file "./blup.kassala.de.cfg".
Error: Boot loader didn't return any data!

We write 512 nullbytes to the disk and retry booting the domU:

# dd if=/dev/zero of=/dev/vpsdisks/blup.kassala.de-disk bs=1 count=512
# xm create -c blup.kassala.de.cfg

The pyGRUB menu appears normally and the machine boots.

-- System Information:
Debian Release: 7.4
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-4-amd64 (SMP w/8 CPU cores)
Locale: LANG=nl_NL.UTF-8, LC_CTYPE=nl_NL.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages xen-utils-4.1 depends on:
ii  e2fslibs          1.42.5-1.1
ii  libc6             2.13-38+deb7u1
ii  libgnutls26       2.12.20-8+deb7u1
ii  libncurses5       5.9-10
ii  libpci3           1:3.1.9-6
ii  libtinfo5         5.9-10
ii  libuuid1          2.20.1-5.3
ii  libxen-4.1        4.1.4-3+deb7u1
ii  libxenstore3.0    4.1.4-3+deb7u1
ii  python            2.7.3-4+deb7u1
ii  python2.7         2.7.3-6+deb7u2
ii  xen-utils-common  4.1.4-3+deb7u1
ii  zlib1g            1:1.2.7.dfsg-13

Versions of packages xen-utils-4.1 recommends:
ii  bridge-utils                                   1.5-6
ii  qemu-keymaps                                   1.1.2+dfsg-6a+deb7u1
ii  qemu-utils                                     1.1.2+dfsg-6a+deb7u1
ii  xen-hypervisor-4.1-amd64 [xen-hypervisor-4.1]  4.1.4-3+deb7u1

Versions of packages xen-utils-4.1 suggests:
pn  xen-docs-4.1  <none>

-- no debconf information