diff --git a/cmd/fshim/ipc/shim/shim.go b/cmd/fshim/ipc/shim/shim.go
index d01d02e..4b6b5f8 100644
--- a/cmd/fshim/ipc/shim/shim.go
+++ b/cmd/fshim/ipc/shim/shim.go
@@ -16,6 +16,7 @@ import (
 	shim0 "git.ophivana.moe/security/fortify/cmd/fshim/ipc"
 	"git.ophivana.moe/security/fortify/internal"
 	"git.ophivana.moe/security/fortify/internal/fmsg"
+	"git.ophivana.moe/security/fortify/internal/proc"
 )
 
 const shimSetupTimeout = 5 * time.Second
@@ -113,9 +114,8 @@ func (s *Shim) Start() (*time.Time, error) {
 
 	// pass sync fd if set
 	if s.payload.Bwrap.Sync() != nil {
-		fd := uintptr(3 + len(s.cmd.ExtraFiles))
+		fd := proc.ExtraFile(s.cmd, s.payload.Bwrap.Sync())
 		s.payload.Sync = &fd
-		s.cmd.ExtraFiles = append(s.cmd.ExtraFiles, s.payload.Bwrap.Sync())
 	}
 
 	fmsg.VPrintln("starting shim via fsu:", s.cmd)
diff --git a/helper/bwrap.go b/helper/bwrap.go
index 2788dbf..5b674b4 100644
--- a/helper/bwrap.go
+++ b/helper/bwrap.go
@@ -9,6 +9,7 @@ import (
 	"sync"
 
 	"git.ophivana.moe/security/fortify/helper/bwrap"
+	"git.ophivana.moe/security/fortify/internal/proc"
 )
 
 // BubblewrapName is the file name or path to bubblewrap.
@@ -76,8 +77,7 @@ func (b *bubblewrap) StartNotify(ready chan error) error {
 	}
 
 	if b.sync != nil {
-		b.Cmd.Args = append(b.Cmd.Args, "--sync-fd", strconv.Itoa(3+len(b.Cmd.ExtraFiles)))
-		b.Cmd.ExtraFiles = append(b.Cmd.ExtraFiles, b.sync)
+		b.Cmd.Args = append(b.Cmd.Args, "--sync-fd", strconv.Itoa(int(proc.ExtraFile(b.Cmd, b.sync))))
 	}
 
 	if err := b.Cmd.Start(); err != nil {
diff --git a/helper/pipe.go b/helper/pipe.go
index e601328..2a55305 100644
--- a/helper/pipe.go
+++ b/helper/pipe.go
@@ -5,6 +5,8 @@ import (
 	"io"
 	"os"
 	"os/exec"
+
+	"git.ophivana.moe/security/fortify/internal/proc"
 )
 
 type pipes struct {
@@ -47,24 +49,21 @@ func (p *pipes) pipe() error {
 }
 
 // calls pipe to create pipes and sets them up as ExtraFiles, returning their fd
-func (p *pipes) prepareCmd(cmd *exec.Cmd) (int, int, error) {
-	if err := p.pipe(); err != nil {
-		return -1, -1, err
+func (p *pipes) prepareCmd(cmd *exec.Cmd) (argsFd, statFd int, err error) {
+	argsFd, statFd = -1, -1
+	if err = p.pipe(); err != nil {
+		return
 	}
 
 	// save a reference of cmd for future use
 	p.cmd = cmd
 
-	// ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.
-	argsFd := 3 + len(cmd.ExtraFiles)
-	cmd.ExtraFiles = append(cmd.ExtraFiles, p.argsP[0])
-
+	argsFd = int(proc.ExtraFile(cmd, p.argsP[0]))
 	if p.ready != nil {
-		cmd.ExtraFiles = append(cmd.ExtraFiles, p.statP[1])
-		return argsFd, argsFd + 1, nil
-	} else {
-		return argsFd, -1, nil
+		statFd = int(proc.ExtraFile(cmd, p.statP[1]))
 	}
+
+	return
 }
 
 func (p *pipes) readyWriteArgs() error {
diff --git a/internal/proc/files.go b/internal/proc/files.go
new file mode 100644
index 0000000..5a5e1c2
--- /dev/null
+++ b/internal/proc/files.go
@@ -0,0 +1,13 @@
+package proc
+
+import (
+	"os"
+	"os/exec"
+)
+
+func ExtraFile(cmd *exec.Cmd, f *os.File) (fd uintptr) {
+	// ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.
+	fd = uintptr(3 + len(cmd.ExtraFiles))
+	cmd.ExtraFiles = append(cmd.ExtraFiles, f)
+	return
+}