Comment 0 for bug 1757565

root@ubuntu:~# lsb_release -rd
Description: Ubuntu 17.10
Release: 17.10
root@ubuntu:~# apt-cache policy linux-image-virtual
linux-image-virtual:
  Installed: 4.13.0.37.40
  Candidate: 4.13.0.37.40
  Version table:
 *** 4.13.0.37.40 500
        500 http://archive.ubuntu.com/ubuntu artful-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu artful-security/main amd64 Packages
        100 /var/lib/dpkg/status
     4.13.0.16.17 500
        500 http://archive.ubuntu.com/ubuntu artful/main amd64 Packages

3. Taring files into an archive are not truncated
4. Files included in tar are filled with NULLs

To reproduce, run an Artful system with one spare disk:

- mkfs.btrfs -f /dev/sda
- mount /dev/sda /mnt
- grep sda /proc/mounts
/dev/sda /mnt btrfs rw,relatime,space_cache,subvolid=5,subvol=/ 0 0

Then run this script which copies a 4MB binary to a btrfs filesystem, tars the directory up containing the binary; then untars to stdout and md5sum compares, showing it's different.

% SPARSE="-S"; rm -rf /mnt/tmp; md5sum /usr/bin/python3.6; mkdir -p /mnt/tmp; cp -a /usr/bin/python3.6 /mnt/tmp; tar -C /mnt/tmp $SPARSE -czf /mnt/test.tgz .; tar $SPARSE -xzf /mnt/test.tgz -O | md5sum
e4121d2f3126b8c364bfa1aaa82371a3 /usr/bin/python3.6
0ce8c4139740198926273853defcb12a -

And now without the sparse flag:

# SPARSE=""; rm -rf /mnt/tmp; md5sum /usr/bin/python3.6; mkdir -p /mnt/tmp; cp -a /usr/bin/python3.6 /mnt/tmp; tar -C /mnt/tmp $SPARSE -czf /mnt/test.tgz .; tar $SPARSE -xzf /mnt/test.tgz -O | md5sum
e4121d2f3126b8c364bfa1aaa82371a3 /usr/bin/python3.6
e4121d2f3126b8c364bfa1aaa82371a3 -

This has been reported to both gnu-tar and linux-btrfs; I'm not aware of an actual fix.
Note that Xenial 4.4 kernels do not exhibit this behavior, and Bionic 4.15 kernel appears to be fixed as well though I'm not sure what the difference is.

References:

https://patchwork.kernel.org/patch/10151037/
https://www.spinics.net/lists/linux-btrfs/msg56768.html
https://www.spinics.net/lists/linux-btrfs/msg57111.html

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: linux-image-virtual 4.13.0.37.40
ProcVersionSignature: Ubuntu 4.13.0-37.42-generic 4.13.13
Uname: Linux 4.13.0-37-generic x86_64
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Mar 21 22:55 seq
 crw-rw---- 1 root audio 116, 33 Mar 21 22:55 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay': 'aplay'
ApportVersion: 2.20.7-0ubuntu3.7
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord': 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CRDA: N/A
Date: Wed Mar 21 23:08:05 2018
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig': 'iwconfig'
Lsusb: Error: command ['lsusb'] failed with exit code 1:
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
PciMultimedia:

ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
 SHELL=/bin/bash
ProcFB:

ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.13.0-37-generic root=LABEL=cloudimg-rootfs ro console=tty1 console=ttyS0
RelatedPackageVersions:
 linux-restricted-modules-4.13.0-37-generic N/A
 linux-backports-modules-4.13.0-37-generic N/A
 linux-firmware N/A
RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: Ubuntu-1.8.2-1ubuntu1
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-xenial
dmi.modalias: dmi:bvnSeaBIOS:bvrUbuntu-1.8.2-1ubuntu1:bd04/01/2014:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-xenial:cvnQEMU:ct1:cvrpc-i440fx-xenial:
dmi.product.name: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-xenial
dmi.sys.vendor: QEMU