mdadm may break bbl if bitmap is too large

[Kota Uchida]

mdadm 3.3-2ubuntu7.6 (for xenial) has a bug which may break an internal write-intent bitmap.
Due to this bug, a bitmap overlaps with a bbl (bad block log), then the bbl gets to be inconsistent.

The cause of the bug is that a bbl is placed at a fixed location (80 sectors from the beginning of a disk).
Because an internal write-intent bitmap is placed before the bbl (16 sectors from the beginning of a disk),
it overlaps with the bbl when it grows too large (> 64 sectors).

See this figure. https://cdn-ak.f.st-hatena.com/images/fotolife/u/uchan_nos/20171126/20171126111715.png

This bug has been fixed in upstream of mdadm. The fixing commit is https://github.com/neilbrown/mdadm/commit/1b7eb672f7792313cc1517feaae8267575fc496b

I tested the bug on the following versions:

- mdadm 3.3-2ubuntu7.6 (for xenial) : The bug is reproduced
- mdadm 3.4-4ubuntu0.1 (for zesty) : The bug is reproduced
- mdadm 4.0-2 (for artful) : The bug has been fixed

I want you to backport the patch to Ubuntu 16.04 LTS because the bug is serious.

Detailed description (containing scripts to reproduce the bug) is here:
https://gist.github.com/uchan-nos/4a4894f3c8e4a59ed0b870b5a516dcad#file-mdadm_bug_bbl_disruption-md

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: mdadm 3.3-2ubuntu7.2
ProcVersionSignature: Ubuntu 4.4.0-96.119-generic 4.4.83
Uname: Linux 4.4.0-96-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.13
Architecture: amd64
Date: Thu Dec 14 18:09:57 2017
InstallationDate: Installed on 2017-02-03 (313 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
Lspci:

Lsusb: Error: command ['lsusb'] failed with exit code 1:
MDadmExamine.dev.sda: Error: command ['/sbin/mdadm', '-E', '/dev/sda'] failed with exit code 1: mdadm: cannot open /dev/sda: Permission denied
MDadmExamine.dev.sda1: Error: command ['/sbin/mdadm', '-E', '/dev/sda1'] failed with exit code 1: mdadm: cannot open /dev/sda1: Permission denied
MDadmExamine.dev.sda2: Error: command ['/sbin/mdadm', '-E', '/dev/sda2'] failed with exit code 1: mdadm: cannot open /dev/sda2: Permission denied
MDadmExamine.dev.sda3: Error: command ['/sbin/mdadm', '-E', '/dev/sda3'] failed with exit code 1: mdadm: cannot open /dev/sda3: Permission denied
MDadmExamine.dev.sdb: Error: command ['/sbin/mdadm', '-E', '/dev/sdb'] failed with exit code 1: mdadm: cannot open /dev/sdb: Permission denied
MDadmExamine.dev.sdb1: Error: command ['/sbin/mdadm', '-E', '/dev/sdb1'] failed with exit code 1: mdadm: cannot open /dev/sdb1: Permission denied
MachineType: Microsoft Corporation Virtual Machine
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-96-generic root=/dev/mapper/hostname--vg-root ro rootflags=subvol=@
SourcePackage: mdadm
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 08/26/2016
dmi.bios.vendor: Microsoft Corporation
dmi.bios.version: Hyper-V UEFI Release v2.0
dmi.board.asset.tag: None
dmi.board.name: Virtual Machine
dmi.board.vendor: Microsoft Corporation
dmi.board.version: Hyper-V UEFI Release v2.0
dmi.chassis.asset.tag: 6099-8796-9610-2381-8579-4771-22
dmi.chassis.type: 3
dmi.chassis.vendor: Microsoft Corporation
dmi.chassis.version: Hyper-V UEFI Release v2.0
dmi.modalias: dmi:bvnMicrosoftCorporation:bvrHyper-VUEFIReleasev2.0:bd08/26/2016:svnMicrosoftCorporation:pnVirtualMachine:pvrHyper-VUEFIReleasev2.0:rvnMicrosoftCorporation:rnVirtualMachine:rvrHyper-VUEFIReleasev2.0:cvnMicrosoftCorporation:ct3:cvrHyper-VUEFIReleasev2.0:
dmi.product.name: Virtual Machine
dmi.product.version: Hyper-V UEFI Release v2.0
dmi.sys.vendor: Microsoft Corporation
etc.blkid.tab: Error: [Errno 2] No such file or directory: '/etc/blkid.tab'

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.