pkg/sys: add GetUsernsForNamespace interface

Signed-off-by: Wei Fu <fuweid89@gmail.com>

pkg/sys/namespace_linux.go

@@ -0,0 +1,38 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sys
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// GetUsernsForNamespace returns a file descriptor that refers to the owning
+// user namespace for the namespace referred to by fd.
+//
+// REF: https://man7.org/linux/man-pages/man2/ioctl_ns.2.html
+func GetUsernsForNamespace(fd uintptr) (*os.File, error) {
+	fd, _, errno := unix.Syscall(syscall.SYS_IOCTL, fd, uintptr(unix.NS_GET_USERNS), 0)
+	if errno != 0 {
+		return nil, fmt.Errorf("failed to get user namespace fd: %w", errno)
+	}
+
+	return os.NewFile(fd, fmt.Sprintf("/proc/%d/fd/%d", os.Getpid(), fd)), nil
+}

pkg/sys/namespace_linux_test.go

@@ -0,0 +1,106 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sys
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+	"testing"
+
+	kernel "github.com/containerd/containerd/v2/pkg/kernelversion"
+	"github.com/containerd/continuity/testutil"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/unix"
+)
+
+func TestGetUsernsForNamespace(t *testing.T) {
+	testutil.RequiresRoot(t)
+
+	t.Parallel()
+
+	k409 := kernel.KernelVersion{Kernel: 4, Major: 9}
+	ok, err := kernel.GreaterEqualThan(k409)
+	require.NoError(t, err)
+	if !ok {
+		t.Skip("Requires kernel >= 4.9")
+	}
+
+	tmpDir := t.TempDir()
+
+	f, err := os.CreateTemp(tmpDir, "netns")
+	require.NoError(t, err)
+
+	netnsPath := f.Name()
+	f.Close()
+
+	defer testutil.Unmount(t, netnsPath)
+
+	currentUsernsIno, err := getNamespaceInode(os.Getpid(), "user")
+	require.NoError(t, err)
+
+	usernsIno := uint64(0)
+	uerr := UnshareAfterEnterUserns("0:1000:10", "0:1000:10", syscall.CLONE_NEWNET, func(pid int) error {
+		err := unix.Mount(
+			fmt.Sprintf("/proc/%d/ns/net", pid),
+			netnsPath,
+			"",
+			unix.MS_BIND|unix.MS_RDONLY,
+			"",
+		)
+		if err != nil {
+			return err
+		}
+
+		usernsIno, err = getNamespaceInode(pid, "user")
+		if err != nil {
+			return err
+		}
+		return nil
+	})
+	require.NoError(t, uerr)
+
+	require.NotEqual(t, currentUsernsIno, usernsIno)
+	t.Logf("Current user namespace [%d], new user namespace [%d]", currentUsernsIno, usernsIno)
+
+	netnsFd, err := os.Open(netnsPath)
+	require.NoError(t, err)
+	defer netnsFd.Close()
+
+	usernsFd, err := GetUsernsForNamespace(netnsFd.Fd())
+	require.NoError(t, err)
+	defer usernsFd.Close()
+
+	usernsInoFromNetnsFd := getInode(t, usernsFd)
+
+	t.Logf("Fetch netns namespace %s' user namespace owner %d", netnsPath, usernsInoFromNetnsFd)
+	require.Equal(t, usernsIno, usernsInoFromNetnsFd)
+
+	parentUsernsFd, err := GetUsernsForNamespace(usernsFd.Fd())
+	require.NoError(t, err)
+	defer parentUsernsFd.Close()
+
+	parentUsernsIno := getInode(t, parentUsernsFd)
+	t.Logf("User namespace %d's parent %d", usernsInoFromNetnsFd, parentUsernsIno)
+	require.Equal(t, currentUsernsIno, parentUsernsIno)
+}
+
+func getInode(t *testing.T, f *os.File) uint64 {
+	info, err := f.Stat()
+	require.NoError(t, err)
+	return info.Sys().(*syscall.Stat_t).Ino
+}