fs: Fix remote-revalidate cache policy.

When revalidating a Dirent, if the inode id is the same, then we don't need to
throw away the entire Dirent. We can just update the unstable attributes in
place.

If the inode id has changed, then the remote file has been deleted or moved,
and we have no choice but to throw away the dirent we have a look up another.
In this case, we may still end up losing a mounted dirent that is a child of
the revalidated dirent. However, that seems appropriate here because the entire
mount point has been pulled out from underneath us.

Because gVisor's overlay is at the Inode level rather than the Dirent level, we
must pass the parent Inode and name along with the Inode that is being
revalidated.

PiperOrigin-RevId: 210431270
Change-Id: I705caef9c68900234972d5aac4ae3a78c61c7d42

1 files changed, 8 insertions, 5 deletions

diff --git a/pkg/sentry/fs/mount.go b/pkg/sentry/fs/mount.go
index 455f5b35c..8345876fc 100644
--- a/pkg/sentry/fs/mount.go
+++ b/pkg/sentry/fs/mount.go
@@ -27,10 +27,13 @@ import (
 // DirentOperations provide file systems greater control over how long a Dirent stays pinned
 // in core. Implementations must not take Dirent.mu.
 type DirentOperations interface {
-	// Revalidate returns true if the Inode is stale and its
-	// InodeOperations needs to be reloaded. Revalidate will never be
-	// called on a Inode that is mounted.
-	Revalidate(ctx context.Context, inode *Inode) bool
+	// Revalidate is called during lookup each time we encounter a Dirent
+	// in the cache. Implementations may update stale properties of the
+	// child Inode. If Revalidate returns true, then the entire Inode will
+	// be reloaded.
+	//
+	// Revalidate will never be called on a Inode that is mounted.
+	Revalidate(ctx context.Context, name string, parent, child *Inode) bool
 
 	// Keep returns true if the Dirent should be kept in memory for as long
 	// as possible beyond any active references.
@@ -281,7 +284,7 @@ type SimpleMountSourceOperations struct {
 }
 
 // Revalidate implements MountSourceOperations.Revalidate.
-func (smo *SimpleMountSourceOperations) Revalidate(context.Context, *Inode) bool {
+func (smo *SimpleMountSourceOperations) Revalidate(context.Context, string, *Inode, *Inode) bool {
 	return smo.revalidate
 }