1 files changed, 92 insertions, 5 deletions

diff --git a/internal/namespace/pinning.go b/internal/namespace/pinning.go
index cd81a38..7976937 100644
--- a/internal/namespace/pinning.go
+++ b/internal/namespace/pinning.go
@@ -1,26 +1,42 @@
+//go:build linux
+
 package namespace
 
 import (
 	"fmt"
 	"os"
+	"path/filepath"
+	"strconv"
+	"syscall"
 
 	"git.theodohertyfamily.com/tools/wg-wrap/internal/paths"
+	"golang.org/x/sys/unix"
 )
 
+// PinNamespace touches the namespace path to indicate it is pinned/active.
+func PinNamespace(pm *paths.PathManager, profile string) error {
+	nsPath := GetProfileNamespacePath(pm, profile)
+	profilesDir := filepath.Dir(nsPath)
+	if err := os.MkdirAll(profilesDir, 0755); err != nil {
+		return fmt.Errorf("failed to create profiles directory: %w", err)
+	}
+
+	// We write a placeholder file to indicate the profile namespace is pinned.
+	if err := os.WriteFile(nsPath, []byte("active"), 0644); err != nil {
+		return fmt.Errorf("failed to create namespace pin file: %w", err)
+	}
+	return nil
+}
+
 // UnpinNamespace removes the pinned namespace file from the filesystem.
 // This allows the namespace to be destroyed once the last process exits.
 func UnpinNamespace(pm *paths.PathManager, profile string) error {
 	nsPath := GetProfileNamespacePath(pm, profile)
 
-	// We only want to unpin if there are no more active processes.
-	// The caller (cli.ExecuteCommand) is responsible for calling this
-	// when IsLastProcess returns true.
-
 	if _, err := os.Stat(nsPath); os.IsNotExist(err) {
 		return nil
 	}
 
-	// We also want to remove the pids directory if it's empty.
 	pidsDir := GetPidsDirPath(pm, profile)
 
 	// Unlink the namespace file
@@ -33,3 +49,74 @@ func UnpinNamespace(pm *paths.PathManager, profile string) error {
 
 	return nil
 }
+
+// JoinExistingNamespace attempts to join the namespaces (user, mount, net)
+// of an already active process running under the same profile.
+// Returns true if a namespace was successfully joined, false if no active namespace exists.
+func JoinExistingNamespace(pm *paths.PathManager, profile string) (bool, error) {
+	if err := PruneStalePids(pm, profile); err != nil {
+		return false, fmt.Errorf("failed to prune stale pids: %w", err)
+	}
+
+	pidsDir := GetPidsDirPath(pm, profile)
+	files, err := os.ReadDir(pidsDir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, fmt.Errorf("failed to read pids dir: %w", err)
+	}
+
+	var activePid int
+	for _, file := range files {
+		pid, err := strconv.Atoi(file.Name())
+		if err != nil {
+			continue
+		}
+		// Since we already pruned stale pids, the first file we find is an active pid!
+		activePid = pid
+		break
+	}
+
+	if activePid == 0 {
+		return false, nil
+	}
+
+	// Join User Namespace first
+	userNsPath := fmt.Sprintf("/proc/%d/ns/user", activePid)
+	userFd, err := os.Open(userNsPath)
+	if err != nil {
+		return false, fmt.Errorf("failed to open user namespace: %w", err)
+	}
+	defer func() { _ = userFd.Close() }()
+
+	if err := unix.Setns(int(userFd.Fd()), syscall.CLONE_NEWUSER); err != nil {
+		return false, fmt.Errorf("failed to join user namespace: %w", err)
+	}
+
+	// Join Mount Namespace
+	mntNsPath := fmt.Sprintf("/proc/%d/ns/mnt", activePid)
+	mntFd, err := os.Open(mntNsPath)
+	if err != nil {
+		return false, fmt.Errorf("failed to open mount namespace: %w", err)
+	}
+	defer func() { _ = mntFd.Close() }()
+
+	if err := unix.Setns(int(mntFd.Fd()), syscall.CLONE_NEWNS); err != nil {
+		return false, fmt.Errorf("failed to join mount namespace: %w", err)
+	}
+
+	// Join Network Namespace
+	netNsPath := fmt.Sprintf("/proc/%d/ns/net", activePid)
+	netFd, err := os.Open(netNsPath)
+	if err != nil {
+		return false, fmt.Errorf("failed to open network namespace: %w", err)
+	}
+	defer func() { _ = netFd.Close() }()
+
+	if err := unix.Setns(int(netFd.Fd()), syscall.CLONE_NEWNET); err != nil {
+		return false, fmt.Errorf("failed to join network namespace: %w", err)
+	}
+
+	return true, nil
+}