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Remount sysctl paths in Docker entrypoint #5958

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59 changes: 49 additions & 10 deletions docker-entrypoint.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,11 @@ Environment variables:
K0S_CONFIG
Optional configuration for k0s, written to /etc/k0s/config.yaml if set.

K0S_ENTRYPOINT_REMOUNT_SYSCTLS
Set to 1 to force remounting of /proc/sys in read-write mode.
Set to 0 to disable remounting.
The default is automatic detection.

K0S_ENTRYPOINT_REMOUNT_CGROUP2FS
Set to 1 to force remounting of the cgroup2 filesystem in read-write mode.
Set to 0 to disable remounting.
Expand Down Expand Up @@ -77,18 +82,11 @@ has_cap_sys_admin() {
has_effective_capability 21
}

# Checks if the cgroup2 file system is mounted at its well-known path.
is_cgroupv2() {
# Check for the magic number of the cgroup2 fs.
# https://www.kernel.org/doc/html/v5.16/admin-guide/cgroup-v2.html#mounting
[ "$(stat -f -c %t /sys/fs/cgroup)" = 63677270 ]
}

# Checks if the file system mounted at the well-known cgroup2 path is read-write.
is_cgroupfs_rw() {
# Checks if the file system mounted at the given path is read-write.
is_mount_rw() {
local _device mountpoint _fstype opts _rest
while read -r _device mountpoint _fstype opts _rest; do
if [ "$mountpoint" = /sys/fs/cgroup ]; then
if [ "$mountpoint" = "$1" ]; then
case "$opts" in
rw* | *,rw | *,rw,*) return 0 ;;
esac
Expand All @@ -98,6 +96,44 @@ is_cgroupfs_rw() {
return 1
}

# Remounts the sysctl path in read-write mode, if necessary.
remount_sysctls() {
case "${K0S_ENTRYPOINT_REMOUNT_SYSCTLS-}" in
0) return ;; # disabled
1) ;; # enabled
*) # auto detect
if is_mount_rw /proc/sys; then
return
fi
has_cap_sys_admin || {
if [ $? -eq 1 ]; then
echo "$0: won't remount /proc/sys without CAP_SYS_ADMIN (disable with K0S_ENTRYPOINT_REMOUNT_SYSCTLS=0)" >&2
return
fi

echo "$0: failed to determine capabilities (disable with K0S_ENTRYPOINT_REMOUNT_SYSCTLS=0)" >&2
}
;;
esac

mount --make-rslave / # don't propagate mount events to other namespaces
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Why do we need this? does docker mount / as shared?

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And do we need mount --make-rslave / twice? we do it again in remount_cgroup2fs

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This has been copied over from how we remount cgroup2fs. IIRC I copied the cgroup2fs remount over from some kernel manpage example, but, of course, I fail to find the original source. The cgroup2fs comment says "don't propagate mount events to other namespaces".

mount -o remount,rw /proc/sys

[ "${K0S_ENTRYPOINT_QUIET-}" = '1' ] || echo "$0: remounted /proc/sys" >&2
}

# Checks if the cgroup2 file system is mounted at its well-known path.
is_cgroupv2() {
# Check for the magic number of the cgroup2 fs.
# https://www.kernel.org/doc/html/v5.16/admin-guide/cgroup-v2.html#mounting
[ "$(stat -f -c %t /sys/fs/cgroup)" = 63677270 ]
}

# Checks if the file system mounted at the well-known cgroup2 path is read-write.
is_cgroupfs_rw() {
is_mount_rw /sys/fs/cgroup
}

# Remounts the cgroup2 file system in read-write mode, if necessary.
remount_cgroup2fs() {
case "${K0S_ENTRYPOINT_REMOUNT_CGROUP2FS-}" in
Expand Down Expand Up @@ -298,12 +334,14 @@ main() {

controller | *)
# Disable everything that's only required when running nested containers.
: "${K0S_ENTRYPOINT_REMOUNT_SYSCTLS:=0}"
: "${K0S_ENTRYPOINT_REMOUNT_CGROUP2FS:=0}"
: "${K0S_ENTRYPOINT_ENABLE_CGROUPV2_NESTING:=0}"
: "${K0S_ENTRYPOINT_DNS_FIXUP:=0}"
;;
esac

remount_sysctls
remount_cgroup2fs
enable_cgroupv2_nesting
dns_fixup
Expand All @@ -314,6 +352,7 @@ main() {
exec env \
-u K0S_ENTRYPOINT_QUIET \
-u K0S_ENTRYPOINT_ROLE \
-u K0S_ENTRYPOINT_REMOUNT_SYSCTLS \
-u K0S_ENTRYPOINT_REMOUNT_CGROUP2FS \
-u K0S_ENTRYPOINT_ENABLE_CGROUPV2_NESTING \
-u K0S_ENTRYPOINT_DNS_FIXUP \
Expand Down