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Fix/cgroup2 bind mount shared cgroupns #5267

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35a174e
fix(libcontainer): avoid fresh cgroup2 mounts without private cgroupns
xujihui1985 May 3, 2026
e8edba3
fix(libcontainer): bats test for cgroupfs mount in host cgroupns
xujihui1985 May 3, 2026
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20 changes: 14 additions & 6 deletions libcontainer/rootfs_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -397,11 +397,13 @@ func mountCgroupV1(m mountEntry, c *mountConfig) error {
}

func mountCgroupV2(m mountEntry, c *mountConfig) error {
err := utils.WithProcfdFile(m.dstFile, func(dstFd string) error {
return mountViaFds(m.Source, nil, m.Destination, dstFd, "cgroup2", uintptr(m.Flags), m.Data)
})
if err == nil || (!errors.Is(err, unix.EPERM) && !errors.Is(err, unix.EBUSY)) {
return err
if c.cgroupns {
err := utils.WithProcfdFile(m.dstFile, func(dstFd string) error {
return mountViaFds(m.Source, nil, m.Destination, dstFd, "cgroup2", uintptr(m.Flags), m.Data)
})
if err == nil || (!errors.Is(err, unix.EPERM) && !errors.Is(err, unix.EBUSY)) {
return err
}
}

// When we are in UserNS but CgroupNS is not unshared, we cannot mount
Expand All @@ -418,8 +420,14 @@ func mountCgroupV2(m mountEntry, c *mountConfig) error {
// rather than the whole /sys/fs/cgroup.
bindM.Source = c.cgroup2Path
}
if !c.cgroupns {
// With host cgroup namespace, bind-mount the existing cgroup2
// hierarchy instead of mounting a fresh cgroup2 instance, and detach
// it from host-side propagation.
bindM.PropagationFlags = append(bindM.PropagationFlags, unix.MS_PRIVATE)
}
// mountToRootfs() handles remounting for MS_RDONLY.
err = mountToRootfs(c, mountEntry{Mount: bindM})
err := mountToRootfs(c, mountEntry{Mount: bindM})
if c.rootlessCgroups && errors.Is(err, unix.ENOENT) {
// ENOENT (for `src = c.cgroup2Path`) happens when rootless runc is being executed
// outside the userns+mountns.
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10 changes: 10 additions & 0 deletions tests/integration/mounts.bats
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Original file line number Diff line number Diff line change
Expand Up @@ -163,6 +163,10 @@ test_mount_target() {
mv "$old_config" "./config.json"
}

function get_cgroup_mount_flags() {
awk '$5 == "/sys/fs/cgroup" {print $10; exit}' /proc/self/mountinfo
}

# https://github.com/opencontainers/runc/issues/3991
@test "runc run [tmpcopyup]" {
mkdir -p rootfs/dir1/dir2
Expand Down Expand Up @@ -354,7 +358,13 @@ test_mount_target() {
@test "runc run [ro /sys/fs/cgroup mounts]" {
# Without cgroup namespace.
update_config '.linux.namespaces -= [{"type": "cgroup"}]'
# Save the host cgroup mount flags.
CG_MNT=$(get_cgroup_mount_flags)
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xujihui1985 marked this conversation as resolved.
test_ro_cgroup_mount
# Verify that the cgroup mount flags after container creation are the same as before container creation.
if [ "$CG_MNT" != "$(get_cgroup_mount_flags)" ]; then
fail "Host cgroup mount flags changed"
fi
}

@test "runc run [ro /sys/fs/cgroup mounts + cgroupns]" {
Expand Down
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