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Package: linux-image-4.5.0-0.bpo.2-amd64
Version: 4.5.4-1~bpo8+1
Severity: normal
kernels 4.5 and 4.6 breaks qemu-nbd from mapping partitions
qemu-nbd can map partitions from VM images and raw disk images to
/dev/nbdNpM nodes, but proper module options have to be loaded prior so
that qemu-nbd performs these additional mappings (the general consensus
around mapping image files has users issuing "kpartx -a /dev/<device>",
but kpartx is not needed at all after issuing qemu-nbd)
nothing is specially set up other than a module option file and that's
pretty much it -- there's just 2 or 3 commands for the testing --- all
which indicates the issue is more towards kernel than it is with qemu-nbd.
after a modprobe nbd is issued, there is no "/dev/nbd0p1"-like devices,
so the main problem with kernels 4.5/4.6 is that qemu-nbd is not being
allowed to generate any device nodes. ("partition" device nodes that is
-- "/dev/nbd0" and "/dev/nbd1" nodes are generated by modprobe nbd)
here in nbd(module) options, there is a hint for max_part that needs to
be specified in order for qemu-nbd to later work in generating the
partition devices /dev/nbdNpM.
(nbds_max=2 just means for modprobe to create "/dev/nbd0" and "/dev/nbd1")
/etc/modprobe.d/mynbd.conf
"
options nbd nbds_max=2 max_part=10
"
basically just a raw disk image containing one partition
dd if=/dev/zero of=1.bin bs=10M count=10
,creates a raw diskimage called 1.bin
any tool to create a dosmbr table containing at least 1 partition
(qemu-nbd can also work with GPT tables but it is not used)
cfdisk 1.bin , and exit the partition tool
no mkfs is needed, so one can immediately try to map the partitions from
the disk image 1.bin just by using qemu-nbd (here again "kpartx" is not
used at all. If one uses "kpartx", then it would rather create partition
device nodes in /dev/mapper. Using qemu-nbd -d [/dev/nbdN] also unmaps
partition nodes.. so forgetting to issue kpartx -d /dev/nbd0 prior
qemu-nbd -d, would leave a mess of broken device links in /dev/mapper)
qemu-nbd should map 1.bin to /dev/nbd0 and its first partition to
"/dev/nbd0p1". this partition device node does not exist yet but is
something that qemu-nbd would generate
"qemu-nbd -c /dev/nbd0 1.bin"
there is an option "-P "(qemu-nbd command) for specifying an exposure to
a single partition, but this has null effect
" -P, --partition=num
only expose partition I<num>"
it is always "qemu-nbd -c /dev/nbd0 [IMAGE FILE]" that works (and after
max_part is used with "modprobe nbd") -- the /dev/nbd0p1 node then gets
generated... so something is up with the later kernels in breaking this
particular behaviour feature on behalf of qemu-nbd
Here the qemu-utils was updated and still no success...dmesg shows no
errors... there's also no errors from qemu-nbd's output but there is a
new notice about using "-f raw" for more safety for the end-user.
... the outcome is still the same (with or without "-f raw") -- the end
result is the image file is still treated as a "raw disk" which is
correct but there is no generation of partition devices
some output for the report,
modprobe -vvv nbd
insmod /lib/modules/4.5.0-2-grsec-amd64/kernel/drivers/block/nbd.ko
nbds_max=20 max_part=10
modprobe: DEBUG: ../libkmod/libkmod-module.c:728 kmod_module_get_path()
name='nbd'
path='/lib/modules/4.5.0-2-grsec-amd64/kernel/drivers/block/nbd.ko'
dpkg -l 'linux-image*'
ii linux-image-4.5.0-0.bpo.2-amd64 4.5.4-1~bpo8+1 amd64 Linux 4.5 for
64-bit PCs
ii linux-image-4.5.0-2-grsec-amd64 4.5.7-1+grsec201606222150+1~bpo8+1
amd64 Linux 4.5 for 64-bit PCs, Grsecurity protection
ii linux-image-4.6.0-0.bpo.1-amd64 4.6.1-1~bpo8+1 amd64 Linux 4.6 for
64-bit PCs
apt-cache policy qemu-utils
qemu-utils:
Installed: 1:2.1+dfsg-12+deb8u6
Candidate: 1:2.1+dfsg-12+deb8u6
Version table:
1:2.5+dfsg-4~bpo8+1 0
100 http://debian.bhs.mirrors.ovh.net/debian/
jessie-backports/main amd64 Packages
*** 1:2.1+dfsg-12+deb8u6 0
500 http://debian.bhs.mirrors.ovh.net/debian/ jessie/main amd64
Packages
500 http://security.debian.org/ jessie/updates/main amd64 Packages
100 /var/lib/dpkg/status
linux-image-4.3.0-0.bpo.1-amd64 4.3.5-1~bpo8+1
latest kernel that works is 4.3, though i don't have access to a 4.4
kernel, i can't tell when between 4.3 and 4.5 qemu-nbd started to fail
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