2 files changed, 29 insertions, 20 deletions
diff --git a/pkg/sentry/fsimpl/overlay/copy_up.go b/pkg/sentry/fsimpl/overlay/copy_up.go
index 469f3a33d..27b00cf6f 100644
--- a/pkg/sentry/fsimpl/overlay/copy_up.go
+++ b/pkg/sentry/fsimpl/overlay/copy_up.go
@@ -16,7 +16,6 @@ package overlay
 
 import (
 	"fmt"
-	"io"
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
@@ -129,25 +128,9 @@ func (d *dentry) copyUpLocked(ctx context.Context) error {
 			return err
 		}
 		defer newFD.DecRef(ctx)
-		bufIOSeq := usermem.BytesIOSequence(make([]byte, 32*1024)) // arbitrary buffer size
-		for {
-			readN, readErr := oldFD.Read(ctx, bufIOSeq, vfs.ReadOptions{})
-			if readErr != nil && readErr != io.EOF {
-				cleanupUndoCopyUp()
-				return readErr
-			}
-			total := int64(0)
-			for total < readN {
-				writeN, writeErr := newFD.Write(ctx, bufIOSeq.DropFirst64(total), vfs.WriteOptions{})
-				total += writeN
-				if writeErr != nil {
-					cleanupUndoCopyUp()
-					return writeErr
-				}
-			}
-			if readErr == io.EOF {
-				break
-			}
+		if _, err := vfs.CopyRegularFileData(ctx, newFD, oldFD); err != nil {
+			cleanupUndoCopyUp()
+			return err
 		}
 		d.mapsMu.Lock()
 		defer d.mapsMu.Unlock()
diff --git a/pkg/sentry/vfs/file_description.go b/pkg/sentry/vfs/file_description.go
index 2153382e9..5321ac80a 100644
--- a/pkg/sentry/vfs/file_description.go
+++ b/pkg/sentry/vfs/file_description.go
@@ -15,6 +15,7 @@
 package vfs
 
 import (
+	"io"
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
@@ -838,3 +839,28 @@ func (fd *FileDescription) SetAsyncHandler(newHandler func() FileAsync) FileAsyn
 	}
 	return fd.asyncHandler
 }
+
+// CopyRegularFileData copies data from srcFD to dstFD until reading from srcFD
+// returns EOF or an error. It returns the number of bytes copied.
+func CopyRegularFileData(ctx context.Context, dstFD, srcFD *FileDescription) (int64, error) {
+	done := int64(0)
+	buf := usermem.BytesIOSequence(make([]byte, 32*1024)) // arbitrary buffer size
+	for {
+		readN, readErr := srcFD.Read(ctx, buf, ReadOptions{})
+		if readErr != nil && readErr != io.EOF {
+			return done, readErr
+		}
+		src := buf.TakeFirst64(readN)
+		for src.NumBytes() != 0 {
+			writeN, writeErr := dstFD.Write(ctx, src, WriteOptions{})
+			done += writeN
+			src = src.DropFirst64(writeN)
+			if writeErr != nil {
+				return done, writeErr
+			}
+		}
+		if readErr == io.EOF {
+			return done, nil
+		}
+	}
+}