Ensure sync is called for readonly file

Calling sync on a readonly file flushes metadata that
may have been modified, like last access time.

Updates #1198

PiperOrigin-RevId: 319888290

2 files changed, 17 insertions, 13 deletions

diff --git a/pkg/sentry/fsimpl/gofer/handle.go b/pkg/sentry/fsimpl/gofer/handle.go
index 724a3f1f7..8792ca4f2 100644
--- a/pkg/sentry/fsimpl/gofer/handle.go
+++ b/pkg/sentry/fsimpl/gofer/handle.go
@@ -126,11 +126,16 @@ func (h *handle) writeFromBlocksAt(ctx context.Context, srcs safemem.BlockSeq, o
 }
 
 func (h *handle) sync(ctx context.Context) error {
+	// Handle most common case first.
 	if h.fd >= 0 {
 		ctx.UninterruptibleSleepStart(false)
 		err := syscall.Fsync(int(h.fd))
 		ctx.UninterruptibleSleepFinish(false)
 		return err
 	}
+	if h.file.isNil() {
+		// File hasn't been touched, there is nothing to sync.
+		return nil
+	}
 	return h.file.fsync(ctx)
 }
diff --git a/pkg/sentry/fsimpl/gofer/regular_file.go b/pkg/sentry/fsimpl/gofer/regular_file.go
index 3d2d3530a..a2f02d9c7 100644
--- a/pkg/sentry/fsimpl/gofer/regular_file.go
+++ b/pkg/sentry/fsimpl/gofer/regular_file.go
@@ -582,20 +582,19 @@ func (fd *regularFileFD) Sync(ctx context.Context) error {
 
 func (d *dentry) syncSharedHandle(ctx context.Context) error {
 	d.handleMu.RLock()
-	if !d.handleWritable {
-		d.handleMu.RUnlock()
-		return nil
-	}
-	d.dataMu.Lock()
-	// Write dirty cached data to the remote file.
-	err := fsutil.SyncDirtyAll(ctx, &d.cache, &d.dirty, d.size, d.fs.mfp.MemoryFile(), d.handle.writeFromBlocksAt)
-	d.dataMu.Unlock()
-	if err == nil {
-		// Sync the remote file.
-		err = d.handle.sync(ctx)
+	defer d.handleMu.RUnlock()
+
+	if d.handleWritable {
+		d.dataMu.Lock()
+		// Write dirty cached data to the remote file.
+		err := fsutil.SyncDirtyAll(ctx, &d.cache, &d.dirty, d.size, d.fs.mfp.MemoryFile(), d.handle.writeFromBlocksAt)
+		d.dataMu.Unlock()
+		if err != nil {
+			return err
+		}
 	}
-	d.handleMu.RUnlock()
-	return err
+	// Sync the remote file.
+	return d.handle.sync(ctx)
 }
 
 // ConfigureMMap implements vfs.FileDescriptionImpl.ConfigureMMap.