Real-world-use case: running ephemeral, containerized instances of an
app:
There may be multiple instances of the app, each running
in a separate ephemeral container with a unique uid_map.
The app's configuration directory is permanently stored on the host,
and is mounted in each container using overlayfs.
Permission for each container to access the configuration directory is granted
via
either 'chmod a+rwX' or 'setfacl -m -u:container_uid:rwX'.
In certain situations it is desirable for the upper layer of overlayfs to be
tmpfs:
-read-only host filesystem
-the host's disk is writeable but very slow (eg thumb-drives, nfs)
-the host filesystem is btrfs
(cannot be used as the upper layer with the 4.4 kernel. fixed in 4.7)
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https://bugs.launchpad.net/bugs/1617388
Title:
When using overlayfs with kernel 4.4, some files cannot be deleted.
Status in linux package in Ubuntu:
Triaged
Bug description:
#!/bin/bash
# ---------------------------------------------------------------------
# This script exhibits a bug in overlayfs in kernel 4.4.
# The bug is not present in kernel 4.2.
# The bug can be reproduced in an x86_64 virtual-machine;
# 32-bit has not been tested.
#
# With kernel 4.2, the script output ends with:
# "script completed without encountering a kernel bug"
#
# With kernel 4.4, the script output ends with:
# "rm: cannot remove ‘mnt_ovl/sub/sub.txt’:
# Value too large for defined data type"
#
# The script depends upon lxc-usernsexec (part of the lxc1 package) to
# create a user-namespace.
#
# The script should be run as a normal user (not root), in a directory where
# the user has write-permission:
# ./script
# --------------------------------------------------------------------
cleanup()
{
[[ -d "$storedir" ]] || exit 1
cd "$storedir" || exit 1
[[ -d "$tmpdir" ]] || exit 1
lxc-usernsexec -m b:0:1000:1 -m b:100000:100000:1 -- rm -rf "$tmpdir"
}
trap cleanup EXIT
set -e
storedir="$(pwd)"
# create tmpdir
tmpdir="$(mktemp -d --tmpdir=.)"
cd "$tmpdir"
# create lowerdir for overlay
mkdir -p lower/sub
touch lower/lower.txt lower/sub/sub.txt
cd ..
chmod -R a+rwX "$tmpdir"
# run a script in a user namepace
lxc-usernsexec -m b:0:100000:65534 -- bash << EOF
set -e
cd "$tmpdir"
# create tmpfs
mkdir mnt_tmpfs
mount -t tmpfs tmpfs mnt_tmpfs
# create upperdir and workdir for overlay
mkdir mnt_tmpfs/{upper,work}
# mount overlay
mkdir mnt_ovl
mount -t overlay \
-o lowerdir=lower,upperdir=mnt_tmpfs/upper,workdir=mnt_tmpfs/work \
overlay mnt_ovl
echo 'overlay directory listing'
ls -RF mnt_ovl
echo ''
set -x
rm mnt_ovl/lower.txt # always succeeds
rm mnt_ovl/sub/sub.txt # fails with kernel 4.4+
set +x
echo 'script completed without encountering a kernel bug'
EOF
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