Add support to mount pod shared tmpfs mounts

Parse annotations containing 'gvisor.dev/spec/mount' that gives
hints about how mounts are shared between containers inside a
pod. This information can be used to better inform how to mount
these volumes inside gVisor. For example, a volume that is shared
between containers inside a pod can be bind mounted inside the
sandbox, instead of being two independent mounts.

For now, this information is used to allow the same tmpfs mounts
to be shared between containers which wasn't possible before.

PiperOrigin-RevId: 252704037

1 files changed, 22 insertions, 3 deletions

diff --git a/pkg/sentry/fs/tmpfs/fs.go b/pkg/sentry/fs/tmpfs/fs.go
index b7c29a4d1..83e1bf247 100644
--- a/pkg/sentry/fs/tmpfs/fs.go
+++ b/pkg/sentry/fs/tmpfs/fs.go
@@ -34,6 +34,16 @@ const (
 	// GID for the root directory.
 	rootGIDKey = "gid"
 
+	// cacheKey sets the caching policy for the mount.
+	cacheKey = "cache"
+
+	// cacheAll uses the virtual file system cache for everything (default).
+	cacheAll = "cache"
+
+	// cacheRevalidate allows dirents to be cached, but revalidates them on each
+	// lookup.
+	cacheRevalidate = "revalidate"
+
 	// TODO(edahlgren/mpratt): support a tmpfs size limit.
 	// size = "size"
 
@@ -122,15 +132,24 @@ func (f *Filesystem) Mount(ctx context.Context, device string, flags fs.MountSou
 		delete(options, rootGIDKey)
 	}
 
+	// Construct a mount which will follow the cache options provided.
+	var msrc *fs.MountSource
+	switch options[cacheKey] {
+	case "", cacheAll:
+		msrc = fs.NewCachingMountSource(f, flags)
+	case cacheRevalidate:
+		msrc = fs.NewRevalidatingMountSource(f, flags)
+	default:
+		return nil, fmt.Errorf("invalid cache policy option %q", options[cacheKey])
+	}
+	delete(options, cacheKey)
+
 	// Fail if the caller passed us more options than we can parse. They may be
 	// expecting us to set something we can't set.
 	if len(options) > 0 {
 		return nil, fmt.Errorf("unsupported mount options: %v", options)
 	}
 
-	// Construct a mount which will cache dirents.
-	msrc := fs.NewCachingMountSource(f, flags)
-
 	// Construct the tmpfs root.
 	return NewDir(ctx, nil, owner, perms, msrc), nil
 }